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STRUCTURAL INTEGRITY HANDBOOK
ISSUE 1.1
APR 2015
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Contents
Contents.................................................................................................................. 2
Figures .................................................................................................................... 3
Foreword ................................................................................................................. 4
The need for SI ....................................................................................................... 5
History of SI in the UK military ................................................................................ 6
Key definitions......................................................................................................... 7
Threats .................................................................................................................... 7
ESVRE .................................................................................................................. 10
Timing ................................................................................................................... 12
Detail and process ....................................................................................................12
SSIs and classification of structure ....................................................................... 12
Evidence record .................................................................................................... 13
Structural Examination Programme ...................................................................... 15
Statement of Operating Intent/and Usage............................................................. 16
Usage Monitoring .................................................................................................. 17
Structural configuration control ............................................................................. 19
EDPC .................................................................................................................... 20
Extensions............................................................................................................. 20
Operational Loads and Usage Validation – including OLM/ODR.......................... 21
MDRE.................................................................................................................... 23
Structural Sampling Programme ........................................................................... 23
Teardown .............................................................................................................. 25
Maintenance Schedule review .............................................................................. 25
Ageing Aircraft Programme................................................................................... 26
Damage................................................................................................................. 26
Lost usage data..................................................................................................... 27
Recovery ............................................................................................................... 27
Changes to clearances due to Validating activities............................................... 27
Pre-emptive reinforcement.................................................................................... 28
Fatigue conservation............................................................................................. 28
Cleared life ............................................................................................................ 29
Use of other evidence ........................................................................................... 29
Risk analysis ......................................................................................................... 29
Probabilistic SI management ................................................................................ 29
Practical management ..............................................................................................29
SISD .................................................................................................................... 29
SIWG .................................................................................................................... 30
Supporting information ..............................................................................................31
Suggested SI training............................................................................................ 31
References and further reading ............................................................................ 31
ANNEX A
SIWG AIDE-MEMOIRE Version 1.1 ....................................... 34
ANNEX B
GLOSSARY AND ABBREVIATIONS ..................................... 41
2
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Figures
Figure 1 SI Document Hierarchy .................................................................................... 4
Figure 2 Maintenance Error.......................................................................................... 10
Figure 3 ESVRE concept ............................................................................................. 11
Figure 4 ESVRE in the context of Air Safety ................................................................ 12
Figure 5 Flow chart – Understanding actual usage ...................................................... 22
Figure 6 SI document hierarchy ................................................................................... 30
Cover image shows a severed front spar from a 1994 incident involving a Red Arrows
Hawk
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Foreword
Expanded
detail
Peer-reviewed
research
The techniques and processes described in
this handbook have been refined by the
collected experience of many years. The
contents are not themselves mandatory, but
may instead be considered as a resource of
proven and trustworthy methods for
achieving compliance with the higher level
criteria set by the Regulation and elaborated
within its Acceptable Means of Compliance
(AMC) and Guidance Material.
Regulation
This handbook supports RA 5720 1 , providing additional information on the theory and
practical implementation of Structural Integrity (SI) Management and its relation to the
rest of the MAA Regulatory Publications (MRP). While it may be used as a repository
of, and guide to, further reading on the
processes mandated in RA 5720 (Figure 1),
the handbook is presented in a format that
also permits its use as introductory reading
RA 5720
on SI for all staff .
SI Handbook
MASAAG Papers
Figure 1 SI Document Hierarchy
No single approach will suit every aircraft and RA 5720 explicitly requires individuals to
act on an informed assessment of the risks under consideration for their aircraft. Much
of this handbook expands on the AMC in the RA but, where this is not the case or
where there is a choice between different approaches, it may be found that a less
complex, or even completely novel, technique can achieve compliance with less cost or
disruption than initially thought. Advice is to be sought from MAA Cert S&ADS if
alternative approaches are considered. It is therefore proposed that an appreciation of
the contents of this handbook will provide the individuals concerned with the foundation
of understanding necessary to successfully and efficiently manage SI.
This handbook is offered without prejudice to the MRP.
Suggestions for improvement may be sent by e-mail to:
DSA-MAA-Cert-Structures@mod.uk
Or by post to:
MAA Cert S&ADS
#5003
MOD Abbey Wood North
Bristol
BS34 8QW
1
Refer to RA 5720 Structural Integrity Management
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The need for SI
Risk to Life.
1.
The consequences of structural failure are often catastrophic. Lessons learned
from decades of experience – as well as some high profile accidents – have led the
MOD to prescribe a dedicated framework to manage the risks associated with the
through-life management of aircraft structures. The mandated approach, ESVRE,
which has developed over many years, cuts across both Type and Continuing
Airworthiness, and today's RA 5720 references several essential SI-related activities
now covered by other, dedicated, RAs at the same time as providing overarching
regulation for SI Management.
2.
The MOD, like the civil world, regulates the design and maintenance of aircraft
and establishes limits within which they can safely be operated. Yet experience has
shown that military aircraft are particularly likely to experience changes in usage and
operating environment and to be operated for longer than was anticipated at the design
stage. Numerous case studies illustrate how failure to adequately manage all of the
variables involved can exacerbate risks to air safety.
Constraint on service life
3.
SI often dictates useful life. The relative ease of incorporating improvements to
aircraft systems means that the operational life of an aircraft is usually determined by
SI considerations rather than by equipment obsolescence.
Cost
4.
Structural inspections and monitoring can be the most costly element of a
maintenance programme. Moreover, as aircraft age, the need for additional directed
structural inspections, sampling, teardown, and life-extending modifications will
increase. Through-life cost is therefore sensitive to how well SI is managed.
Integrity
5.
The technical and organisational uncertainties associated with military aviation
contribute to a complex range of hazards and have, over time, led the MOD to develop
a dedicated framework for their management through-life. At root, this concerns
nothing more than the efficient management of a variety of interdependent activities
related to design, maintenance and operation. In the context of airworthiness, integrity
may therefore be thought of as a measure of risk reduction specific to these activities.
The overall activity called SI is regulated in the MRP (RA 5720) and is defined in
MAA02 2 :
Structural Integrity: The ability of an aircraft structure to retain its
strength, function and shape within acceptable limits, without failure when
subjected to the loads imposed throughout the aircraft’s service life by
operation within the limitations of Release To Service (RTS) and to the
usage described in the Statement of Operating Intent (SOI) or the
Statement of Operating Intent and Usage (SOIU).
6.
Similar frameworks have now been adopted for the management of aircraft
systems and propulsion systems.
7.
While support from various stakeholders is needed to manage SI, overall
responsibility is assigned to the Type Airworthiness Authority (TAA) as the individual
responsible for the Type Airworthiness of the air system.
2
MAA02 MAA Master Glossary
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History of SI in the UK military
SIGNIFICANT TURNING POINTS IN SI
Buccaneer Incident – Ex Red Flag 1980
1954
BOAC DEHAVILLAND
COMET ► FAIL SAFE AND FATIGUE TESTING
1969
USAF F111
► DAMAGE TOLERANCE
1977
DAN AIR BOEING 707
► GERIATRIC (AGEING) AIRCRAFT
1979/80
RAF BUCCANEERS
► USAGE VALIDATION (MILITARY)
ALOHA BOEING 737
► WIDESPREAD FATIGUE
DAMAGE/MULTIPLE SITE DAMAGE
1988
SIGNIFICANT SERVICE INCIDENTS
1974 HARRIER FLAP DRIVE SHAFT
1976 PHANTOM WING FOLD LOCK LUG
Chinook 1989 – loss of
rotor sync
1980 HARRIER ROLL REACTION CONTROL
VALVE ROD
1989 TRISTAR WING SPAR FAILURE
1989 CHINOOK LOSS OF ROTOR
SYNCHRONISATION
1993 WESSEX LOSS OF TAIL ROTOR DRIVE
1995 TUCANO PROPELLER BLADE FAILURE
1996 HARRIER T10 WHEEL UNIT
1999 C130K FUSELAGE ROOF
2001 GAZELLE ROTOR TIE-BAR
2001 WESSEX TAIL ROTOR DRIVE
2003 HARRIER T10 WHEEL UNIT
2004 GROB TUTOR PROPELLER DETACHED
2004 HARRIER GR7 CANOPY
2004 TORNADO UPLOCK DAMPER BOLT FAILURE
2007 VC10 MULTI-ELEMENT FATIGUE DAMAGE
2010 DOMINIE WIDESPREAD EXFOLIATION CORROSION
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Key definitions
8.
The following key definitions, taken from MAA02, apply2:
a. Structure: Aircraft structure consists of all load-carrying members
including wings, fuselage (including some transparencies), empennage, engine
mountings, landing gear, flight control surfaces and related points of
attachment, control rods, propellers and propeller hubs if applicable and, for
helicopters: rotor blades, rotor heads and associated transmission systems.
The actuating portion of items such as landing gear, flight controls and doors
are subject to System Integrity Management regulation (RA 5721) as well as SI
Management regulation.
b. Structural Airworthiness: This is the ability of an aircraft to maintain SI
without significant hazard to the aircrew, ground crew, passengers (where
relevant), or to the general public over whom the aircraft is flown.
Threats
9.
Compromised SI may exacerbate risks to airworthiness. Any one, or
combination of, the following threats can compromise SI:
a.
Overload.
b.
Fatigue, fretting and wear.
c.
Accidental Damage (AD) and Environmental Damage (ED).
d.
Procedural (Design, Manufacturing, Maintenance or Supply) error.
Overload
10.
An aircraft encounters overload when subjected to forces that are above the
Design Limit Load (DLL) for the structure. The DLL is either the maximum and most
critical combination of loads and environmental conditions likely to occur during the life
of the aircraft, or standard airworthiness specification cases that are unlikely ever to be
exceeded in service. This may occur in extreme atmospheric conditions (gusts and air
turbulence), during heavy landings, violent manoeuvring or divergent flutter. Overload
may also arise because the crew or flight control system exceed RTS limitations, or if
there has been an error in defining the RTS limits themselves, and more subtly, where
the design and application of repairs or modifications bring about changes to load
paths or where in-service loads are not fully understood. Aircraft structures are
designed so that there will be no permanent deformation, loss of function or necessity
for repair when loaded up to the Design Proof Load (DPL). The DPL is the product of
DLL and the proof factor, which can range from 1 for a large civil type aircraft to 1.125
for a combat aircraft. In the case of an aircraft with a proof factor greater than 1, an
overload event may not result in permanent deformation or structural failure; however,
the overload event is to be fully investigated.
11.
Aircraft are subject to fluctuating loads that can be categorised as either high or
low cycle. Sources of these repeated loads include:
a.
High Cycle: vibration caused by acoustic loading, non-divergent flutter,
manoeuvre buffet, blade passing frequencies and rotating components in
aircraft dynamic systems.
b.
Low Cycle: manoeuvres, gusts, the ‘ground-air-ground’ cycle, cockpit or
cabin pressurisations, landings, retractions, taxiing, rotor start/stop, changes in
engine power, thermal changes, hydraulic and fuel system pressurisations and
arrester hook use.
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12.
Fluctuating loads can pose a threat to SI if not adequately accounted for in
design. The following paragraphs describe how fatigue, fretting and wear manifest
themselves from this threat.
Fatigue
13.
This is a process of progressive, permanent structural change occurring in a
material that is subjected to fluctuating loads below the static yield strength of the
material. Fatigue damage nucleates and grows on a microscopic scale until it manifests
itself as cracking, the growth of which depends on material properties and geometry,
the level, amplitude and frequency of fluctuating loads and the number of load cycles
applied. Fatigue damage culminates in cracks that reduce the residual strength of the
cracked structure and cause fracture if the residual strength falls below the applied
load. Fatigue is applicable to both metallic and composite structures (for further
guidance on composite materials, see AP 101A-0601-1 3 . Fatigue damage is the
predominant cause of catastrophic structural failure in metallic aircraft structures. It can
also develop into Widespread Fatigue Damage (WFD), which is defined as the
simultaneous presence of cracks at multiple structural details that are of sufficient size
and density whereby the structure will no longer meet its residual strength
requirements. Sources of WFD are:
a.
Multiple Site damage (MSD): the simultaneous presence of multiple
fatigue cracks in the same structural element.
b.
Multiple Element Damage (MED): the simultaneous presence of fatigue
cracks in similar adjacent structural elements.
Fretting
14.
Fretting is a special wear process that occurs at the contact area between two
materials under load and subject to minute relative motion by vibration or some other
force. In the presence of an aggressive environment, the contact movement causes
wear and material transfer at the surface, often followed by oxidation of the debris and
the freshly exposed surface. The oxidised debris can further act as an abrasive and
such degradation is termed fretting corrosion. Fretting increases the threat of initiating
fatigue cracking. This can result in so-called fretting fatigue.
Wear
15.
Wear is the undesired cumulative change in dimensions brought about by the
gradual removal of discrete particles from contacting surfaces in relative motion,
usually sliding, predominantly as a result of mechanical action. Wear is not a single
process, but a number of different processes that can take place independently or in a
combination, resulting in material removal from contacting surfaces through a complex
combination of local shearing, ploughing, gouging, welding, tearing and other actions.
The outcome of wear may result in the cumulative loss of material, and ultimately
structural failure, when the residual strength of the structure falls below the applied
load.
AD
16.
This is the physical alteration of an item (or its surface protection where
applicable) caused by contact, impact or interaction with an object which is not a part of
the aircraft, or by human error during manufacture, operation or maintenance of the
aircraft. Typically AD may be caused by external impact in the air (for example: mid-air
collision, bird strike, wire strike, lightning strike, severe hail, weapons release selfdamage or ricochet damage) or on the ground (such as Foreign Object Damage,
maintenance activities, ground handling, freight loading or vehicle movements). In
addition, battle damage or sabotage, although not strictly accidental, may also be
3
AP 101A-0601-1 – Employment and Repair of Aircraft Composite Materials
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considered within this category, as the effects are comparable. Moreover, less obvious
internal AD may arise from occupant/maintainer accidents or from overheating metallic
or composite structure. AD may manifest itself as distorted, torn, punctured or
otherwise distressed structure or surface protection, or as delamination or disbonding,
or in less visible forms such as a change in metal heat treatment condition or ‘barely
visible impact damage’ to composite materials such as fibre-reinforced plastics.
ED
17.
ED is the physical degradation of structural material properties as a result of
their interaction with the climate or environment. Structurally significant ED is normally
caused by chemical interaction, erosion, fluid/gas absorption, thermal cycling or
electro-magnetic radiation. ED may manifest itself as corrosion, stress-corrosion
cracking, loss of surface finish, softening of composite material matrices (including
adhesives used in laminated wood), delamination, or disbonding resulting in
degradation of static, fatigue or impact strength properties.
Procedural errors can be the result of design, manufacturing, maintenance or supply
errors.
18.
Design error can result from failure to adhere to recognised design standards,
design best practice and qualification evidence methodology. Examples of design error
include:
a.
Underestimating local loads or overestimating of material properties.
b.
Not addressing the potential for incorrect assembly.
c.
Specifying inappropriate material and manufacturing processes.
19.
Manufacturing error applies to structure or an individual structural component
that fails to meet the design specification. Factors leading to manufacturing error
include failure to adhere to specified manufacturing drawing requirements and
processes, such as:
a.
Use of incorrect material.
b.
Application of incorrect heat treatment.
c.
Incorrect dimensioning and feature (such as holes) location.
d.
Use of unauthorised or inappropriate jigs, fixtures and tooling.
20.
Maintenance error (Figure 2) describes an unsound maintenance process on
an aircraft structure. Factors leading to maintenance errors that threaten SI may
include:
a.
Inadequate training or supervision.
b.
Inadequate resources.
c.
Incorrect technical information.
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Nonround
holes
Figure 2 Maintenance Error
21.
Supply error describes the supply of a component or product that does not
meet the aircraft current structural design specification. Factors leading to supply errors
may include non-conforming components or products, those from an unknown
pedigree, those from unapproved suppliers and those that are incorrectly identified or
codified.
ESVRE
22.
The acronym, ESVRE, stands for Establish-Sustain-Validate-Recover-Exploit –
the five activities that completely cover the process of SI Management (Figure 3) – and
represents the framework identified by the MOD as best suiting the variety of aircraft
and procurement models represented on the Military Aircraft Register. ESVRE is
intended to be as applicable to a future fighter bought from the US as to a home-grown
Remotely Piloted Air Vehicle or a historic helicopter 4 . It is not a purely chronological
framework (like CADMID is, for example) and aspects of each ESVRE section may
require action at various stages of an aircraft's life.
23.
RA 5720, which is aligned with ESVRE, is the descendant of a long series of
publications and guidance that developed alongside the UK military's growing
knowledge and experience in dealing with structures issues – first fatigue monitoring,
then corrosion control, Operational Loads and Usage Validation, structural sampling,
ageing, and so on.
24.
A great deal of this experience is also embodied in the set of standards for
design and airworthiness known as Def Stan 00-970 5 , which appeared in its earliest
form in 1916. Applicants for a Military Type Certificate are required 6 to demonstrate
compliance of Type Designs to this, or to an appropriate and equivalent, standard.
Much of the initial evidence for SI is obtained during the development phase of a
project, usually to support certification and qualification.
25.
Fatigue testing may continue for some time after the aircraft enters service, to
achieve full structural clearances. During the development phase, the nature and
frequency of structural inspections and monitoring that will be necessary to maintain
the fleet through life will also be determined.
26.
Once the aircraft is in service, the operating conditions, usage and configuration
on which the Designer based their decisions during development may change. The
4
5
6
‘Developing Issues in Aircraft Structural Integrity’, Wg Cdr Ron Eckersley
Refer to Def Stan 00-970 Design and Airworthiness Requirements for Service Aircraft
RA 1500 Certification of UK Military Registered Air Systems
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implications of any changes are assessed and SI evidence (usually kept in the static
and fatigue Type Records) kept updated throughout the life of an aircraft type.
27.
The work to gather initial SI evidence is classed as an Establishing SI activity.
The actions to maintain SI through life and act on any changes to usage and operation
are Sustaining and Validating activities, respectively.
Figure 3 ESVRE concept
28.
Structural damage can be caused by any of the threats to SI or combinations
thereof. More generally, failure to carry out adequate SI Management, eg Structural
Configuration Control (SCC) or Usage Monitoring may undermine confidence in SI.
Recovering activity, which may range from simple component exchange to a full
Design Organisation (DO) repair or the retirement of the aircraft, will be required.
29.
Occasionally, additional activity is needed to safely Exploit the inherent
capabilities of the structure, to extend the Out-of-Service Date (OSD), for example.
30.
The crucial role played by the DO at each stage of ESVRE is explicit in
RA 5720. It is not, however, limited to generating, maintaining and updating the
evidence record through life. It is essential that contractual arrangements allow for the
DO’s participation in the exchange of information that is fundamental to in-service SI
Management, such as attendance at working groups, upkeep of Usage Monitoring
systems or reassessment of component lives in response to changes in aircraft usage.
31.
It was stated earlier that the TAA is ultimately responsible for ensuring that all of
these activities are carried out – this is reflected in the wording of RA 5720 – but Figure
4 illustrates the importance of contributions from other stakeholders to achieving SI.
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Figure 4 ESVRE in the context of Air Safety
Timing
32.
Adequate funding is key to supporting the aircraft’s SI Strategy throughout the
anticipated service life of the aircraft.
33.
Measures to assure SI are to be planned in advance and included in the aircraft
Through Life Management Plan (TLMP), and also in any project whole life cost
forecasts.
Detail and process
SSIs and classification of structure
34.
Aircraft structure comprises assemblies, components and features (elements
and details) that have traditionally been classified according to their relative
contribution to the function or residual strength of their higher assembly. Various terms
such as Primary Structure, Class 1 Structure, Vital Parts or ‘Grade A’ Parts, Principal
Structural Element (PSE), Airworthiness Limitation Items, Structural Significant Items
(SSIs) and Safety of Flight Structure have been used. There is no universally accepted
method for selecting or identifying important structure, or commonly accepted
interpretation of ‘critical’ or ‘significant’ in this context. Nevertheless, SI can be
established only when all items whose failure would have an unacceptable impact on
the structural airworthiness of the airframe as a whole are identified, assessed and
recorded in documents that are maintained and accessible to SI stakeholders. Such
items of structure are defined in the following paragraphs.
35.
An SSI is defined in MAA022 as:
a.
SSI: Any detail, element or assembly, which contributes significantly to
carrying flight, ground, pressure or control loads and whose failure could affect
the SI necessary for the continued safe and controlled flight of the aircraft.
36.
Selection of SSIs is pivotal to the derivation of the Preventive Maintenance
programme by such methods as the Operator/Manufacturer Maintenance Development
Processes, as developed by the Maintenance Steering Group Task Force (currently
known as MSG-3) and published by the Air Transport Association of America and
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Reliability Centred Maintenance (RCM) published within JAP (D) 100C-22 7 . The
characteristics of SSIs have to be assessed in terms of their sensitivity and vulnerability
to the threats to SI. The characterisation of Safe Life or Damage Tolerant SSIs is
dependent upon a fatigue damage assessment.
SSI classification by design philosophy
37.
Safe Life (specifically, either Stress Life or Strain Life) is a design philosophy
that establishes a finite service life within which the probability of fatigue cracks
developing and compromising residual strength is acceptably low. Safe Life SSIs are
those items of structure designed to have a finite fatigue life, or those where application
of a Damage Tolerant approach is not possible. The Safe Life is qualified by
calculation, often supported by test as in the case of the full scale fatigue test required
by Def Stan 00-970. Safe Life SSIs are subject to an analysis of their vulnerability to
AD and ED during development of a Preventive Maintenance programme. This
analysis is essential because the life of an SSI can be adversely affected by both AD
and ED and thus occurrence of these threats is detected by an appropriate inspection
regime.
38.
Damage Tolerance is a design philosophy which leads to a structure that can
retain the required residual strength for a period of use after the structure has
sustained specific levels of detectable fatigue damage, AD or ED. Airworthiness of
Damage Tolerant structure is therefore assured by a specified inspection regime.
Damage-tolerant SSIs are either structure that was designed and demonstrated to be
Damage Tolerant or fail safe, or structure which was designed to be Safe Life but
which has subsequently been re-categorised as Damage Tolerant because of failure
on test or in service, or life extension beyond the designed Safe Life limit where SI can
be assured using an inspection-based regime. Damage Tolerant SSIs are subject to a
fatigue assessment as well as AD and ED analyses.
Evidence record
39.
Aircraft accepted into UK military service may be designed to satisfy one of a
number of different design requirements or standards, such as Def Stan 00-970, or
international military or civil standards. Notwithstanding the wealth of evidence required
for certification and qualification of the aircraft structure, the minimum evidence
required to sustain the management of the aircraft throughout its service life is usually
summarised in the static and fatigue evidence document set.
40.
Static qualification evidence is usually found in the form of a Static Type Record
(STR), or equivalent document. Whether an aircraft type is procured in accordance with
the 5000 series RAs or some alternative acquisition management standard or
procurement model, a Type Record is the ideal vehicle for the collation and summary
of static strength evidence.
41.
An STR or equivalent document comprises a general arrangement and
description of the aircraft, a summary of static design assumptions and criteria, a
summary of critical loading, shear force, bending moment, torque and mass
distributions, and a summary of static reserve factors. The static evidence contains all
the relevant supporting stress and test reports and calculation files. For modern
designs, evidence may have been obtained from aerodynamic and loads models (such
as Finite Element Models) and so it is necessary that these models should be
maintained and updated through life, in the same manner as the static evidence
document itself. The scope of the static evidence document, in terms of the structural
components involved, includes all SSIs.
7
Refer to JAP (D) 100C-22 Guide to Developing and Sustaining Preventive Maintenance Programmes, available on the
MAA website
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42.
Fatigue qualification evidence, usually in the form of a Fatigue Type Record
(FTR) or equivalent document, is mandated for aircraft designed to Def Stan 00-970
and is recommended by RA 53098. Notwithstanding how an aircraft type has been
procured, an FTR or equivalent document is the ideal vehicle for the collation and
summary of fatigue qualification evidence to support the DH-facing Safety Assessment
for RTS and sustain SI.
43.
For multi-national or off-the-shelf designs, for both the static evidence and
fatigue evidence, it may be possible to develop a multi-national fatigue evidence
document or to adapt existing evidence into an acceptable format. However, in due
course it will be necessary to develop a national fatigue evidence document to reflect
the UK military as-flown usage and configuration as it develops and diverges from that
of other operators.
44.
The initial issue of an FTR or equivalent document generally consists of a single
Part 1 (see RA 5309(1) 8 ) that comprises a statement of the fatigue principles used and
a summary of fatigue and/or Damage Tolerance analyses for each item of structure. As
with the static evidence document, the FTR also lists all relevant reports and may be
supported by computer models.
45.
Furthermore, because both usage and fatigue evidence evolve significantly
over the life of a type, the fatigue evidence is to incorporate additional sections, in
accordance with RA 5309:
a.
A historical record of the fatigue substantiation for the aircraft.
b.
A reassessment in light of service usage and fatigue test results.
c.
(b).
A reassessment of inspection requirements shown to be necessary by
d.
A life extension document (as required).
46.
SI requirements must be considered during certification activities so that
applicable evidence can be captured within the Structural Integrity Strategy Document
(SISD) (see Page 29).
Loads models, including computer modelling, are to be substantiated by test
47.
evidence and verified by an Independent Technical Evaluator (ITE) as part of
certification and qualification activities.
48.
Resourcing and tasking of DO support is needed to support certification (in
accordance with the Military Air Systems Certification Process (MACP)) and throughlife SI management. RA 15006 contains regulation concerning the MACP.
49.
Adequate arrangements are to be made with appropriate organisations to
support computer models used to generate qualification evidence. These are then
available and maintained in a usable state for the life of the platform.
50.
Review of structural qualification evidence: Both the static and fatigue
qualification evidence will initially have been compiled during the design and
qualification of an aircraft type, as SI Establishing activities. However, as the as-flown
configuration and usage diverge over time from that assumed during design, and as
information becomes available from any ongoing testing and SOIU review/Operational
Loads and Usage Validation, it will become necessary, progressively, to update the
qualification evidence documents.
51.
Repairs, modifications or changes in usage or configuration may invalidate the
original qualification. In order to assure the continued airworthiness of the fleet, there is
an ongoing requirement to validate and update the structural qualification evidence
against measured loads data and other relevant information. The Project Team (PT)
8
Refer to RA 5309 Fatigue Type Record for Aircraft
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ensures that the DO is tasked to review and update the qualification evidence
whenever it may have been invalidated.
Structural Examination Programme
52.
Once an aircraft enters service, its structure is subjected to the four main
groups of threat to SI: overload, fatigue, AD/ED and procedural error (design,
manufacturing, maintenance and supply). For all SSIs, with the current state of
technology, the only counter to AD/ED is to examine for and repair any damage.
Furthermore, safe lives and Damage Tolerance examination thresholds and intervals
are based on analysis and/or testing of structure that has not been exposed to AD/ED.
Therefore, there is a need to examine for AD/ED to ensure that the occurrence of
AD/ED does not invalidate fatigue and Damage Tolerance clearances. In addition to
the examinations for AD/ED that may be necessary for both Safe Life and Damage
Tolerant structures for these two reasons, Damage Tolerant structures rely on
examinations for cracks to counter the threat posed by fatigue (in contrast to Safe Life
structures, which are not examined for fatigue cracking per se). For all structures,
therefore, a Structural Examination Programme (SEP) is required.
53.
The SEP is accomplished through the structures-related parts of flight servicing
and scheduled maintenance procedures, and through structural sampling and
teardown. In the context of the ESVRE framework, the structural elements of flight
servicing and scheduled maintenance are Sustaining activities. Structural sampling and
teardown are Validating activities; see RA 5720(4). Furthermore, as aircraft age, the
likelihood of interaction between the different threats to SI increases. PTs ensure that
procedures are in place to identify the interaction of these threats in order to enable
appropriate action to be taken – see RA 5723 9 . An effective SEP, supported by
structural sampling, is a key element that enables the early identification of likely
threats.
54.
Each SSI can be characterised in terms of its fatigue or Damage Tolerance
clearance, which will depend on the original design philosophy. It will either be Safe
Life or Damage Tolerant. For Safe Life components the life is determined so that the
component is retired before fatigue cracks are likely to have compromised residual
strength. For Damage Tolerant components, testing will have shown when a fatigue
crack is likely to develop and how long the crack will take to reach a critical length. The
likely time of crack initiation will determine when in the aircraft’s life the Damage
Tolerance inspections start (the examination threshold), while the rate of crack
propagation determines the inspection periods (examination interval).
55.
Maintenance Schedules: In parallel with the above characterisation, each SSI
is also assessed for its vulnerability and susceptibility to AD/ED. This is normally
carried out as part of the Maintenance Schedule development process. However,
typical Maintenance Schedule development processes, such as RCM or MSG-3,
commonly only deal with AD or ED mechanisms acting independently. It is therefore
necessary to consider the vulnerability and susceptibility of each SSI to the interaction
of AD and ED (for example, the onset of corrosion following impact damage causing
the breach of a corrosion protection coating) and to the interaction of AD and ED with
fatigue or static loading (for example, stress-corrosion cracking or corrosionaccelerated fatigue), even though this may not be accounted for by the RCM process.
The result of the assessment of vulnerability and susceptibility to AD/ED is to designate
each item as either At Risk (AR) or Not At Risk (NAR) of AD/ED.
56.
The Maintenance Schedule review process identifies which SSIs are AR. These
items are then included in the Master Maintenance Schedule (MMS) and marked as
SSIs to ensure they are examined at a suitable frequency to detect AD or ED before it
becomes a hazard to SI.
9
Refer to RA 5723 Ageing Aircraft Audit
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57.
For Safe Life items that are NAR, and Damage Tolerant items with a long
threshold before inspections begin, it is possible that they may not have been included
in the MMS. Therefore, to maintain SI for those items, it is necessary to validate the
assumptions that led to determination of the item being NAR and determination of the
Safe Life or the Damage Tolerance examination threshold. This is achieved by carrying
out sample checks of the items before they reach the end of their Safe Life or first
Damage Tolerance inspection. Consequently, PTs ensure that the SSI list is crossreferred to the MMS and that SSIs that are not included in the MMS are subject to
structural sampling to confirm that they do not suffer from AD, ED or fatigue damage
earlier than expected.
58.
Structural Examination – Reporting: The results of the Maintenance
Schedule review process are promulgated in a Topic 5V (Sampling Requirements and
Procedures) or equivalent, which lists all SSIs (both AR and NAR) and states the
means by which they are to be examined. The list of SSIs is supported by the use of
diagrams, drawings or photographs as appropriate. The Topic 5V also specifies the
method for recording the result of each examination and feeding back this information
to the PT; positive and negative reporting is always required to enable the PT to ensure
SI is maintained. The use of omnibus reporting for inspected SSIs where no structural
problems are found may be considered, with detailed reporting only when a structural
arising is found. The PT ensures that the inspection results are analysed to determine
whether SI or availability is being compromised and whether Recovering action is
required. Furthermore, the results are utilised in Maintenance Schedule reviews and
Continued Airworthiness activities. The Topic 5V examinations are called up in the
Topic 5A1 or equivalent Maintenance Schedule as SSI-coded activities and crossreferred or hyperlinked to the Topic 5V entry.
59.
For Safe Life structures, the threat posed to SI by fatigue is addressed by
retiring individual airframes or major components before their probability of failure
reaches an unacceptable level (typically higher than 1 in 1000). The Safe Life is
derived by applying a statistical scatter factor to the predicted life (from testing or
analysis), hence Safe Life methods result in the majority of structures being retired
before any fatigue cracks are likely to have reached the point where the residual
strength is compromised.
60.
For Damage Tolerant (examination-dependent) structures, the threat posed to
SI by fatigue is addressed by examining the structure for cracks. These examinations
commence at the point in the aircraft’s life (the examination threshold) at which the
crack is predicted to be detectable by the inspection technique and are repeated at
every inspection interval.
Statement of Operating Intent/and Usage
61.
The Statement of Operating Intent (SOI) is the means by which the UK
Services’ future operating intent for a particular aircraft type and major mark are
conveyed formally to the DO. The SOIU replaces the SOI when the aircraft has
accumulated sufficient representative in-service usage data. The SOI is part of the
Aircraft Document Set (ADS) and its production is mandated for all UK military aircraft.
The SOI is published as the Topic 15S within the ADS. The DO may use SOI
information as the basis for deriving a Design Usage Spectrum for underpinning fatigue
and Damage Tolerance clearances and associated inspections.
62.
SI can be highly sensitive to changes in aircraft usage. Continued Airworthiness
assurance therefore requires regular reviews of aircraft usage.
63.
The SOIU describes and quantifies actual usage over a period of time and
describes intended future aircraft use. The SOIU formally conveys this data to the DO
so that it can be analysed in comparison with original or extant usage assumptions and
the implications fed into revised fatigue and damage-tolerance inspection thresholds.
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64.
The SOIU must be reviewed annually to check its continued validity and
triennially to carry out a quantitative update, including fleet fatigue and usage data
derived from IAT (including MDRE, HUMS, FDR, ADR, etc).
65.
The Aircraft Usage Validation Process document10 is the primary source of
further detailed guidance on SOI and SOIU creation and the SOIU review cycle.
Usage Monitoring
66.
For both Safe Life and Damage Tolerant structures, Usage Monitoring through
Individual Aircraft Tracking (IAT) is necessary to manage the timing of either the
retirement or examination of individual airframes, as appropriate. Usage Monitoring
systems have therefore been developed to identify when SSIs are to be retired or
examined, as appropriate. These systems vary in the metrics used and their complexity
and may include:
a.
Structural Health Monitoring.
b.
Health and Usage Monitoring Systems.
c.
Fatigue meter and fatigue meter formula (FMF).
67.
In this context, it is important to note that Usage Monitoring for IAT is primarily
used only to address the threat to SI posed by fatigue. Some usage monitoring
systems do have the additional ability to monitor flight parameters for potential overload
conditions.
68.
IAT can comprise direct or parametric measurement techniques at both fleet
and individual aircraft level. Depending on the type of structure and the predominant
fatigue loading action, metrics may be any one of (or a combination of) Fatigue Index
(FI), flying hours (which may be factored to account for differences in role severity),
landings or other relevant data.
69.
FI is defined as a measure of the fatigue usage of a structure in comparison
with the qualified usage, at which examination, modification or withdrawal from service
is necessary. The design Safe Life of a structure will nominally be 100 FI, though this
figure may be increased or decreased for in-service release as a result of fatigue
testing, loads monitoring or in-service arisings. For many legacy fixed-wing aircraft, FI
is calculated using an FMF, incorporating normal acceleration data (captured by a
‘fatigue meter’ counting accelerometer) plus mass, configuration and other relevant
usage parameters. Normal acceleration is defined as normal (at 90°) to the plan of the
aircraft in the direction conventionally annotated ‘z’. This is usually caused by a turning
manoeuvre, pitching manoeuvre or gust and often measured in multiples of
acceleration due to gravity, ‘g’.
70.
Fatigue and usage data will normally be captured post-sortie by type-specific
means such as MOD Form 724 (Flying Log and Equipment Running Log), MOD Form
725 (Flying Log and Fatigue Data Sheet), Technical Logs, electronic equivalents such
as LITS/Gold post-sortie data feedback, or by automatic data download from an onboard system.
71.
The fatigue damage accrual for some structural features on both helicopters
(eg, transmission components) and fixed-wing aircraft (eg, fin and tailplane for agile
combat and trainer aircraft) can be dominated by high-frequency, cyclic stresses in
structure. It increases with the magnitude of the stress cycle and accumulates with
each application, eventually leading to initiation and growth of cracks. The severity of
individual manoeuvres directly affects the fatigue consumption of components. Gentle
handling should be encouraged, unless operational circumstances dictate otherwise.
Excessive vibration can cause an increase in fatigue damage rates and necessitate
10
see the MAA website under Certification
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structural repairs. Fatigue damage to helicopter rotors can be reduced by avoiding a
complete shutdown of the rotor during sorties.
72.
Certain operational environments and roles can cause high rates of fatigue
damage in excess of those suffered by the majority of a fleet. Air Staff and PTs would
normally consider the regular transfer of aircraft between various roles and
environments to avoid high rates of fatigue damage being concentrated upon a limited
number of aircraft.
73.
Good practice requires systems are to be in place for usage data capture and
computation (either by internal agencies or external contractors), for managing overall
fleet usage, for monitoring individual aircraft usage against safe lives or examination
thresholds and intervals, and for sponsoring changes to Usage Monitoring, data
capture and computation systems as and when required. Data is to be entered
promptly onto electronic systems or forwarded to the appropriate agency, as directed
by the PT.
74.
The output of IAT systems is flight by-flight and component-by-component
cumulative lifing data, which is used to manage retirement, replacement, modification
or examination as appropriate to the individual aircraft/component and fatigue design
philosophy. This data is therefore essential to fleet management and to allow
interchangeability of lifed components. For a Safe Life airframe, the rate of
consumption of fatigue life will determine the ability of the fleet to reach its OSD and
the need for a life extension programme (LEP). For a Damage Tolerant design, the rate
of consumption of fatigue will determine the examination burden; this may necessitate
supplemental structural examinations for very high-life aircraft and will determine the
economic life of the fleet.
75.
It is important for the PT to manage the consumption of fatigue to achieve the
defined OSD. Occasionally, therefore, it may be appropriate to determine and
promulgate a usage budget to individual aircraft in conjunction with the RTSA and the
Duty Holder (DH), where differences in operation from the rest of the fleet are
significant, eg dual control or specially instrumented aircraft used for trials or training
exercises.
76.
Modification or refurbishment programmes to mitigate fatigue can be initiated
where necessary to maintain capability, achieve target OSD, enhance capability or
extend life (see RA 5724 11 and RA 5725 12 ).
77.
The DO responsibilities are:
a.
Specify for each SSI the usage parameter to be used and either the Safe
Life or the Damage Tolerance examination threshold and interval, in terms of
the appropriate usage metric.
b.
Provide the Usage Monitoring system and implement changes to it as
required.
78.
Most Helicopters are fitted with Health and Usage Monitoring Systems (HUMS)
in accordance with RA 4500 13 and there are opportunities for exploiting data from
these systems in the IAT role. Helicopters which spend more time in unusually biased
or severe flying conditions (eg heavy lift operations or mountain flying) may also load
parts of the structure or rotating components considerably more severely than in the
Designer’s assumptions. Unmonitored, such unusual usage could lead to structural
failure of a rotating component before the predicted Safe Life has been reached. For
this reason, fleet managers at all levels are to ensure that wherever possible, individual
aircraft experience as wide a range of roles as possible. Should it be necessary,
11
12
13
Refer to RA 5724 Life Extension Programme
Refer to RA 5725 Out of Service Date Extension Programme
Refer to RA 4500 Health and Usage Monitoring
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through modification or location, to limit an airframe to one type of flying only, details
are to be brought to the attention of the Designer in order that the fatigue implications
can be assessed.
79.
Any period of unmonitored operation or change in usage of the aircraft will
threaten SI, as either can lead to aircraft being operated beyond the assumptions
underpinning SI. Specific threats include:
a.
Inadequate validation of sortie profile codes (SPCs) leading to incorrect
recording of aircraft usage and incorrect critical part lifing assumptions.
b.
Lack of monitoring or analysis of aircraft usage on an individual aircraft
basis leading to inappropriate sortie profile mix and incorrect critical part lifing
assumptions.
c.
Change of sortie profiles.
d.
Differing environments.
e.
New working practices or changes to maintenance processes.
f.
New operating practices.
80.
Where fatigue and usage data is lost or compromised, either due to
unserviceability of aircraft systems or failure of data capture or computation, fill-in data
will generally be derived from mean fatigue usage data. However, this attracts a
penalty factor to account for uncertainty. Since this represents a loss of available Safe
Life or time to next examination, it is important that such occurrences are minimised.
Structural configuration control
81.
Throughout the life of an aircraft there are risks to SI from changes in structural
configuration. The structural configuration of aircraft at the point of delivery can be
expected to vary from the baseline design due to differing build standards and build
concessions. Furthermore, the structural configuration of in-service aircraft is likely to
diverge from the as-built configuration as individual aircraft are subject to damage,
repairs, modifications and Special Instruction (Technical) (SI(T)). These deviations or
changes may affect aircraft mass, centre of gravity (C of G), internal structural layout or
local structural stiffness, and may therefore alter internal load distributions. Such
deviations or changes may compromise static strength, fatigue and Damage Tolerance
clearances, and may jeopardise future repair, modification and life extension. Static
and fatigue strength may be threatened if repairs are not adequately assessed or
qualified (including static and fatigue qualification), and if the potential interaction of
adjacent structural repairs is not addressed. The migration of lifed structure between
aircraft further complicates the problem. Furthermore, if the DO is unaware of the
structural configuration of an aircraft, then recommendations for modification, repairs
and life extension will be based upon a presumed configuration usually based on the
information held in the aircraft master drawing set. To mitigate these risks, expensive
and time-consuming whole-fleet aircraft surveys may be required if SCC has not been
maintained throughout the life of the platform. SCC is a process that assists the PT to
identify the cumulative risk to SI arising from deviations or changes from the build
standard, and to avoid additional costs and logistics risks in the future.
82.
A PT is required to take appropriate measures to ensure that a suitable system
is put in place to maintain overall SCC of each aircraft within the fleet. Accurately
recording structural arisings will assist decision-making and trend analysis for targeting
appropriate husbandry or maintenance, and will be further aided if the SCC system can
handle graphical material. Ideally a single database should be used to maintain SCC,
although it may utilise data from a range of different sources eg as a minimum,
structural concessions, repairs, modifications, accidental and environmental damage
for all aircraft. For legacy aircraft it may be necessary to carry out a structural survey of
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the whole fleet to establish a baseline. The database includes all relevant arisings to
individual aircraft and is maintained for the life of the aircraft. The data should permit a
fleet-wide assessment of structural health.
83.
Key elements of the database are:
a.
Build concessions.
b.
The extent of damage (such as AD and ED) present before repair.
c.
The condition of the material after the repair has been carried out (such
as the thickness remaining after blending).
d.
Repairs, which may originate from the Topic 6/Structural Repair Manual,
from an approved DO or from an approved Aircraft Repair Organisation.
e.
Un-repaired damage (damage within authorised limits), so as to provide a
complete picture alongside embodied repairs and any further damage that may
occur.
f.
Modifications.
(1)
DO-produced.
(2)
Service-produced.
84.
The regulation, including responsibilities, for technical data exploitation can be
found in RA 1140 14 .
85.
For safe and effective operation of aircraft, the weight and C of G must remain
at all times within the limits specified in the RTS. If these conditions are not satisfied,
the consequences may range from an increased consumption of fatigue life that will
threaten SI, to failure to maintain adequate control and stability, to loss of the aircraft. If
weight and C of G are deliberately changed through modification or change of role,
then it may be necessary for the PT to initiate a review of static and fatigue qualification
in the light of the cumulative effects of the changes. Regulation and guidance on
aircraft weight and C of G can be found in RA 4256 15 .
EDPC
86.
As aircraft age, ED becomes more widespread and is more likely to occur
concurrently with other forms of damage such as fatigue cracking. Similarly, aircraft
structures will suffer from ED mechanisms such as erosion and degradation of
composite material properties and surface finish. ED degrades SI and, if uncontrolled,
will reduce the inherent ability of the structure to sustain loads in the presence of other
forms of damage. Establishment and promotion of an EDPC Programme to minimise
environmental damage to the aircraft structure should be in place. The EDPC
Programme defines the minimum requirements for preventing and controlling safetyrelated ED problems within the fleet (see RA 4507 16 ).
Extensions
87.
Fatigue safe lives and Damage Tolerance examination thresholds and intervals
will normally be determined by the DO, on the basis of analysis and testing in
accordance with a suitable design and qualification standard, and promulgated in the
FTR. Where there is an anticipated need to extend lives, thresholds or intervals, this
can be achieved through the DO via a life extension programme. However, in
exceptional cases there may be an urgent need for a temporary extension of lives, or
examination thresholds and intervals, of SSIs for a limited number of aircraft. In such
cases, the PT may consider an extension.
14
15
16
Refer to RA 1140 Military Air System Technical Data Exploitation
Refer to RA 4256 Aircraft Weighing
Refer to RA 4507 Aircraft Environmental Damage Prevention and Control
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88.
Any temporary extension beyond the cleared fatigue Safe Life or Damage
Tolerance examination threshold or interval, for a single airframe or component,
required in exceptional circumstances, is authorised by an appropriate PT engineer
with the necessary delegated airworthiness authority and with the agreement of the
relevant 2-Star Cluster Leader (see RA 1003 17 and RA 1006 18 ).
Operational Loads and Usage Validation – including OLM/ODR
89.
The structural design, test and qualification and validation methodology for an
aircraft requires the DO to make assumptions about the aircraft’s intended usage and
the associated loads acting upon the structure during its service life, as well as their
frequency and magnitude of application. As an aircraft’s role, operating patterns
(including operational theatre and individual manoeuvres or SPCs) or configuration
change, the relationships between flight parameters and local structural stresses may
also change. Therefore, the actual in-service aircraft’s usage and the loads associated
with this usage can be determined at regular periods throughout the aircraft’s life, for
comparison with the aircraft’s design usage assumptions and the fatigue spectra used
in test and qualification. Usually, the SOIU does not provide the level of detail required
to validate all design usage and loads assumptions. Similarly, the use of simple
parametric usage monitoring systems, such as those that rely simply on flying hours or
‘g’ counts coupled with FMF, may not provide the required level of detail. Furthermore,
FMF accuracy relies upon a known mix of sortie profiles and confirmed relationships
between flight parameters and structural loads.
90.
Operational Loads and Usage Validation is the preferred term for the direct or
indirect measurement of representative in-service data for comparison with the
assumptions supporting aircraft and component clearances and structural lives (see
Figure 5). This activity, which builds upon the loads validation or flight loads survey
activity undertaken during the design process, is commonly achieved using OLM, for
fixed-wing aircraft, and Operational Data Recording (ODR), for rotary-wing aircraft, but
the complexity of the solution chosen will be determined by the aims of the programme
and existing confidence not only in actual usage, but also in the relationship between
usage and the loads experienced by the airframe. The TAA, in consultation with the
DO, may make use of routine Usage Monitoring systems as well as parametric data,
and instrumentation systems developed especially for the task. Data from less complex
systems may confirm a need to progress to a more complex one.
91.
The main aim of Operational Loads and Usage Validation is the substantiation
of assumptions made during design, qualification and test regarding usage and
associated loads. This process may comprise:
a. The validation of the fatigue usage spectrum and the assumptions used
during the design, structural qualification and test of the aircraft, including the
review of fatigue clearances as well as maintenance and inspection
periodicities.
b. The validation of the aircraft’s Usage Monitoring system, including any
lifing, damage or FI algorithms.
17
18
Refer to RA 1003 Delegation of Airworthiness Authority and Notification of Air Safety Responsibility (DE&S)
Refer to RA1006 Delegation of Engineering Authorizations
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SOI
(Initially)
SOIU
Operational Loads
& Usage Validation
Evidence
record
Lifing
clearances
Usage Monitoring
Actual usage
Figure 5 Flow chart – Understanding actual usage
92.
The benefit of considering the Operational Loads and Usage Validation
requirement during platform development is that onboard systems that meet the
requirement can be included in the design (see MASAAG Paper 109 19 Chapter 7). This
reduces the need to install the necessary instrumentation through expensive retrofits
once in service, although this may be required for legacy fleets whose IAT systems fall
short of the full requirement or where component/assembly-specific validation
programmes (eg undercarriage or tail rotor) are required. The precise instrumentation
solution, such as strain gauges or parametric systems, would then be agreed with the
DO. Once installed, a permanent installation can be activated either continuously or as
and when required.
93.
The PT will normally task the DO to define the usage validation requirements
and proposed methodolgy for meeting these requirements. Depending on when this
task is initiated, it may be possible to obtain benefit from the knowledge and evidence
gathered by the DO or Original Equipment Manufacturer during the structural design
and qualification phase.
94.
The complexity of Operational Loads and Usage Validation programmes and
their management is such that PTs may appoint a project manager and form a
dedicated working group including appropriate DO representation. Similarly, the Front
Line Command may appoint the technical and operating unit project officers
responsible for facilitating the programme on unit on a day-to-day basis.
95.
Limited guidance on the specification of an Operational Loads and Usage
Validation system can be found within Defence Standard 00-970, Part 1, Section 3,
19
Refer to MASAAG Paper 109 Guidance for Aircraft Operational Loads Measurement Programmes
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Leaflet 38. Comprehensive guidance for planning and undertaking an OLM/ODR
programme can be found within MASAAG 20 Papers 10919 and 120 21 .
MDRE
96.
Where the method of IAT cannot adequately characterise aircraft usage, a
Manual Data Recording Exercise (MDRE) can be employed to record additional flight
condition parameters during a representative sample of sorties. An MDRE involves an
additional crewmember recording the occurrence and duration of all relevant flight
conditions throughout each sortie.
97.
The MDRE includes a sufficient number of sorties of each kind flown by the
fleet to obtain statistically significant results. In lieu of an MDRE a suitable, automatic,
means of recording usage data may be employed.
98.
It is important that the F724 already includes an SPC column, so that the record
of usage in-service can be validated by the results of MDRE and Operational Loads
and Usage Validation.
Structural Sampling Programme
99.
The Structural Sampling Programme (SSP) is part of the SEP (the derivation of
the SEP is described on Page 15). A properly implemented Structural Sampling
Programme (SSP) helps to mitigate the unquantified risk of critical structural failure
from unanticipated causes. Sampling is applicable to all SSIs that are not otherwise
examined during flight servicing or scheduled maintenance. Predominantly these will
be items that have been sub-classified as ‘Not At Risk’ from AD/ED and demonstrated,
by test or analysis, to be free from fatigue cracking during the expected service life of
the aircraft. However, where there are also ‘At Risk’ SSIs and parts of AR structure that
are not normally accessible for inspection, such as the inside surfaces of hollow flying
control rods or torsion boxes, it is most important that these are be sampled regularly
through the life of the fleet. Structural sampling, combined with the feedback from
scheduled examinations, validates the scheduled examination element of the SEP.
Furthermore, by examining the actual condition of all critical structure, the SEP
validates the assumptions used to forecast, for the lifetime of the aircraft, the effects of
usage and exposure to AD/ED, including the interaction of the different threats to SI. If
structural sampling uncovers unexpected damage, Recovering, and/or Exploiting
activities are employed. Unexpected damage may also indicate a lack of understanding
of local in-service loading that could be addressed by OLM/ODR including targeted
instrumentation to improve understanding and inform the development of effective
repair or modification action. Non-SSI components are considered for sampling where
failure would result in unacceptable economic or operational impact.
100. To achieve the necessary SI Validating objectives, the PT actively manages
structural sampling so that the necessary sampling information is gathered and
interpreted and any necessary follow-up action completed. Policy for sampling
programmes is detailed in the Support Policy Statement of the aircraft Topic 2(N/A/R)1
and sampling activities, plus the resulting corporate knowledge gained from sampling,
is to be captured in the SISD and the aircraft’s Topic 5V or equivalent. Where the SSP
highlights significant deviation between expected and actual in-service damage, this
information is to be fed back to the DO.
101. Structural sampling is planned so that by the time the fleet leader reaches 80%
of its original design life, or revised life if less, examples of all parts of all SSIs have
been examined either through scheduled maintenance or sampling. Where it is not
possible to examine all parts of all SSIs, alternative approaches can be explored in
conjunction with the DO to assess the condition of the SSIs. For example, if it is
20
21
Military Aircraft Structural Airworthiness Advisory Group, a subgroup of the Joint Air Safety Committee hierarchy
Guidance for Helicopter Operational Data Recording Programmes
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physically impossible to access and inspect an SSI, the DO may be tasked to assess
the risk associated with not being able to carry out inspection and propose alternative
analyses or mitigations, including teardown. Some of the threats to SI, if not correctly
anticipated by the RCM analysis, could result in indications early in the aircraft’s life
(such as AD and ED), whereas others would not be apparent until late in the aircraft’s
life. Therefore, it may be appropriate to sample at both points. The decision on how
much further sampling is required will need to be determined through a regular review
of the sampling results and the level of risk. Such analysis and decisions are examined
periodically by the SI Working Group (SIWG) (see Page 30). It is advisable to continue
the sampling process right up to the OSD, as the cumulative exposure to damage from
AD, ED and the probability of unexpected fatigue cracking increases towards the end
of the aircraft’s life. Creeping deferment of fleet OSDs has, in the past, also exposed
shortfalls in the lifing of, for example, older modifications, which could have been
picked up by sampling.
102. Candidate aircraft for structural sampling inspections are selected carefully to
maximise the probability of finding any unanticipated damage that could affect the fleet.
The probability of damage increases with exposure to the threats to SI. Therefore,
aircraft that lead the fleet in terms of flying hours, FI, age, maintenance time or
exposure time in corrosive or dusty environments, or environments significantly
different from that assumed during development (eg, embarked ops), or that have
experienced the worst combination of these factors, are selected for sampling. For
example, corrosion arising rates by tail number may indicate the environmental
exposure level. A statement describing how representative the chosen aircraft are of
the fleet as a whole may provide useful evidence that thought has been given to where
this relatively small sample size lies in the overall aircraft population. It is also useful to
have a number of alternative candidate aircraft on standby to take full advantage of the
available sampling opportunities. For removable components, the history and usage of
the component is considered, since it may differ from the usage of the aircraft to which
the component is currently fitted.
103. Where possible, sampling should be done within a scheduled maintenance
activity, and should include feedback directly into the RCM analysis. The following
opportunities are worth considering for implementing the structural sampling plan:
a. Planned sampling may usefully be carried out concurrently with scheduled
maintenance on the selected aircraft, when access to the structure to be
sampled is improved by the scheduled maintenance activity.
b. Opportunities for sampling inaccessible structure may arise from
modification programmes and extensive repairs. PTs can take full advantage of
those occasions when aircraft are sufficiently stripped to enable examination of
normally inaccessible structure, or when individual aircraft, assemblies or major
components are scrapped following major damage or life expiry. Although the
aircraft concerned may not be the highest lifed within the fleet, value may still
be obtained from opportunity sampling of these airframes. This may be in
addition to, or instead of, sampling on the originally selected aircraft. A simple
process to consider structural sampling requirements should be considered for
inclusion in the instructions for repair scheme applications. Structural sampling
opportunities should also be considered as part of the modification
authorisation process (MOD Form 714 / 715 or equivalent). Unless absolutely
necessary, SSP should not be undertaken in a first-line environment.
c.
If there is no suitable maintenance opportunity for the selected candidate
aircraft within the required timeframe and no emergent opportunity afforded by
modification, incident or accident, it may be necessary to undertake sampling
by directed inspections using an SI(T). Due to the large amount of preparatory
work likely to be required to access the structure to be sampled, this option is
likely to have the greatest adverse impact on aircraft availability.
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104. Finally, there is little fundamental difference between sampling of critical
structure and critical systems, and consideration can be given to harmonising these
policies where possible, to avoid duplication of effort.
Teardown
105. A Teardown (also known as sampling and forensic examination) is defined as a
progressive, detailed, controlled and destructive examination of aircraft structure. The
teardown of a full-scale fatigue test specimen airframe at the conclusion of testing is
normally required for completion of fatigue and Damage Tolerance qualification, and is
therefore an SI Establishing activity. Teardown as part of the structural sampling
programme involves teardown of the whole or selected parts of an ex-service aircraft or
teardown of removable components or sub-assemblies. This may be the only way to
achieve some of the structural sampling requirements and may be done on an
opportunity basis. However, The same level of consideration as to where to focus the
teardown is required as it is for structural sampling. Teardown may also be used to
examine in detail particular areas of SI concern on an airframe and to develop and
validate examination techniques. Teardown can also be used to provide on going
confidence in repairs by removing them and inspecting the underlying damage site for
evidence of any damage growth.
106. Teardown of a high-life aircraft is a useful tool to support life extension activities
(see RA 5724(1) & (2)11).
107. Detailed guidance on the conduct of structural sampling and teardown can be
found in MASAAG Paper 105 22 and Teardown Handbook produced by the Technical
Co-Operation Program (TTCP), and details of potential structural sampling areas,
based upon the SSI list, are detailed in the Topic 5V or equivalent.
Maintenance Schedule review
108. The structural elements of the Maintenance Schedule for an aircraft type will
normally be reviewed in the course of the Maintenance Schedule review; see
RA 4351 23 for regulation on the conduct and timing of these reviews. The structural
element of this review is an SI Validating activity. In-service SI arisings and changes of
operating intent and usage are to be taken into account when reviewing the SEP
elements of the Maintenance Schedule.
109. In reviewing the structural elements of the Maintenance Schedule, in-service SI
arisings (including those from Topic 5V or equivalent reporting) and failures, results of
sampling and teardown, plus the implications of major changes in fleet disposition,
operating environment, roles and usage, are be taken into account. It is essential that
schedule changes generated by the MOD or contracted schedule review body do not
extend examination intervals derived from fatigue and Damage Tolerance analyses,
unless supported by the DO. Moreover, it is also recognised that, for any scheduled
structural examinations driven by fatigue considerations, historical examination results
will be of very limited predictive value and therefore do not provide a sound basis for
increased examination intervals; the absence of cracking to date may not indicate that
it would be safe to adjust examination intervals. A further consideration is that schedule
review activity is to be managed to ensure continuity in trend monitoring of SI arisings
both before and after a schedule is revised. For example, Schedule Identification
Numbers (SINs) for directed examinations of individual SSIs are usually left
unchanged; however, if the SINs are changed, pre-and post-review SINs are to be
tracked.
22
23
MASAAG Paper 105 Teardown Inspections – Guidance and Best Practice
Refer to RA 4351 Production and Maintenance of Maintenance Schedules
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Ageing Aircraft Programme
110. Ageing Aircraft aspects should be considered as a through life programme with
consideration given to ageing within the SISD. See RA 5723 24 for regulation on the
Ageing Aircraft Audit. Relevant reading on the subject can be found on the MAA
website.
Damage
111. Although damage may be detected on individual aircraft, it will usually be
necessary to assess whether such occurrences have fleet-wide implications. An
evaluation of the extent to which damage to a single aircraft may indicate the potential
for similar damage to be present on, or likely to be sustained by, other aircraft in the
fleet may need to be undertaken. For example, new arisings of AD/ED may be
indicative of an emerging fleet-wide SI problem that can be addressed. For fatigue, the
occurrence of unexpected cracking in an in-service aircraft may indicate that Safe Life,
fatigue or Damage Tolerance clearances are grossly in error and a significant portion of
the fleet may be at risk. Therefore, all instances of in-service fatigue cracking and
damage to SSIs as identified in the Topic 5V or equivalent, plus previously unreported
cracking and damage to other structure, may be brought promptly to the attention of
the PT and subsequently to the DO. The DO alerts the PT to any potential read-across
damage information from other operators of the same aircraft type.
112. Suspected structural damage that is not readily apparent may have occurred
following a specific incident to an individual aircraft or operation of aircraft in an
adverse environment. Recovery actions are prompted by, and dependent upon, the
following triggering events:
a.
Reported overloads (including exceeding RTS operating limits).
b.
Reported lightning strikes.
c.
Reported bird strikes.
d.
Impacts to composite structure.
e. Exposure of structure (metal or composite) to any elevated temperatures
that could cause degradation in material properties.
f.
Exposure to corrosive substances:
(1) Caustic soda, acids, mercury, vehicle decontamination compounds,
etc (see RA 450716).
(2) Bodily fluids (see RA 4103 25 ).
(3) Exposure to a maritime environment.
g.
Operating in an environment that may invalidate the Maintenance
Schedule, such as embarked operations.
113. As far as practicable, type-specific procedures are published within the ADS,
detailing the action, including the appropriate inspections, to be taken following each of
the events listed above. However, the impact of unusual or unexpected events will
depend on the specific structure affected and specialist advice is to be sought from the
DO. If the inspections confirm the presence of damage, the principles of recovering
detected damage apply.
114.
24
25
Arisings are reported to the PT and monitored by the SIWG.
Refer to RA 5723 Ageing Aircraft Audit
Refer to RA 4103 Decontamination of Aircraft after Spillage of Body Fluids
26
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Lost usage data
115. If usage data is lost or not recorded at whole aircraft or lifed-item level, the
trigger points for retirement of Safe Life structure or examination of Damage Tolerant
structure become uncertain. Conservative assumptions are therefore to be made about
the missing usage data. IAT programmes can be compromised by failures of
monitoring equipment (such as an unserviceable fatigue meter, strain gauge, other
sensor or data logger), or loss of sortie data (such as errors in post-sortie feedback
recording, data corruption or transfer failure). Sorties where this occurs are classified
as unmetered sorties.
116. Unmetered sorties are to be positively identified within the IAT record so that
fill-in data can be applied and re-evaluated as necessary. This fill-in data is normally
based on the mean usage rate for flying of a similar nature to that for which data is
missing, multiplied by a standard scatter factor of 1.5. For example, for a fixed-wing
aircraft structure lifed in FI, the fill-in FI for a missing sortie record could be based on
the mean FI per flying hour rate for the same SPC, multiplied by 1.5. In most cases this
factor will be conservative, resulting in a significant life penalty, but where usage is
known to be highly variable, it may not be adequate. However, where more statistical
evidence is available and more accurate fill-in data is required, the fatigue consumption
rate equivalent to the 90th percentile of usage severity for similar sorties is to be used.
Since this represents a loss of usable Safe Life, or a reduction in the time to the next
Damage Tolerance examination, such loss of data is to be minimised.
117. The PT monitors the percentage of unmetered sorties and determines the
extent and impact of the lost sortie information and the reason for the loss. Recovery
action to prevent recurrence is to be carried out. Provided the data loss is not from the
same aircraft or unit, or within a single or the most fatigue damaging SPC, the following
percentages can be used as guidelines for the percentage of unmetered sorties on
which the PT is to act: <2% acceptable, 2-5% further investigation required, >5% the
structural airworthiness of the fleet is to be assessed and urgent recovery action
undertaken if it is found that structural airworthiness has been compromised. The rate
of, and recovery of, unmetered sorties is monitored by the SIWG. If the penalty fill-in
life exceeds the Safe Life of the aircraft structure or lifed item, Exploiting measures can
be employed.
Recovery
118. The need for repairs to meet extant design standards is especially relevant for
ageing aircraft and for aircraft with multiple adjacent repairs.
119. The repair of detected damage to individual aircraft will be effected by
undertaking repairs detailed within the ADS or schemes provided by either the Aircraft
Repair Organisation (see RA 4815(2) 26 ) or the DO; these processes are outside the
scope of SI management and this RA. However, in order that structural configuration
control is maintained, the extent of the damage and its repair is to be recorded on a
suitable database. Refer to RA 4403 27 and RA 4815(2).
Changes to clearances due to Validating activities
120. Validation activities such as SOIU reviews and Operational Loads and Usage
Validation may introduce changes to fatigue formulae or fatigue and Damage
Tolerance clearances that may reduce available Safe Life or render Damage Tolerance
examinations overdue. In the short term, such reductions may indicate that the risk of
fatigue failure is higher than originally thought and, in extreme cases, that safe lives
have been over-flown. Action is then required to recover SI to restore risks to As Low
as Reasonably Practicable (ALARP) levels.
26
27
Refer to RA 4815(2) Procedures for Good Maintenance Practices
Refer to RA 4403 Expedient Repair
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121. Once adverse lifing revisions have been verified, it is essential that the PT reevaluate the position of the fleet in relation to Safe Life and/or Damage Tolerance
clearances. Subject to the results of this re-evaluation, it will be necessary to quantify
risk and generate recovery options. Options for Recovering of SI may include early
retirement of the fleet or individual airframes, transfer to a Damage Tolerance
(examination dependent) regime, additional testing, modification, repair or
replacement, or the imposition of operating restrictions. These options are, as far as
possible, to be based on quantitative analyses by the DO. In all cases, it is important
that the risk presented is thoroughly assessed and the acceptance/mitigation endorsed
by the SIWG and then accepted by the appropriate Letter of Airworthiness Authority
holder. Furthermore, it will be important to address the understandable, intuitive
reaction to under-estimate risk when there is a lack of physical damage on in-service
aircraft.
Pre-emptive reinforcement
122. This section is being rewritten and has been temporarily removed. Interim
guidance on this subject may be obtained by contacting DSA-MAA-CertStructures@mod.uk.
Fatigue conservation
123. The rate of fatigue usage of an airframe will determine when the Safe Life is
reached (for retirement or embodiment of a structural modification) or, for Damage
Tolerant designs, the point at which the airframe becomes uneconomic to examine and
repair. These safe or economic lives are frequently the constraint on the useful
calendar life of a type and are therefore managed to achieve the required
airworthiness, availability and capability until the required OSD. Whereas fatigue
budgeting is an SI Sustaining measure, fatigue conservation is an SI exploitation
measure that may be required if the fatigue budget would be insufficient to achieve the
necessary OSD.
124. Fatigue-damaging loads on the life-limiting structure of fixed-wing aircraft are
driven by key parameters such as normal acceleration, ground-air-ground cycle, pitch
and roll accelerations, mass properties and time spent in buffet conditions. Life may be
prolonged by identifying and avoiding, or only undertaking when absolutely necessary,
particularly damaging manoeuvres or configurations. For a helicopter, the loading
situation is more complex, involving high-cycle loading (from the rotating assemblies)
and a low-cycle element (similar to fixed-wing manoeuvring and exposure to gusts), so
the accuracy of the SPCs is particularly important. This makes it harder to identify the
most damaging manoeuvres or types and phases of flight or configurations for a
helicopter, but it may be possible to use knowledge of design spectra or MDRE, ODR
or HUMS data to derive operating advice for fatigue conservation purposes.
125. The following procedure is to be used to implement fatigue conservation
measures on a fleet or individual aircraft:
a.
Identify shortfall in required fatigue budget.
b.
Identify the most damaging manoeuvres/operations/role fits using, for
example, the following information and techniques:
(1) Analyse Operational Loads and Usage Validation data, HUMS data
and results from MDRE, as available, to identify the most significant
contributors to structural life consumption.
c.
In consultation with the DH, identify means of reducing or eliminating
fatigue-damaging operating methods.
d.
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DH: promulgate and enforce the revised operating methods.
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e.
Review the effectiveness of the revised operating methods.
Cleared life
126. The potential requirement for a structures LEP should be identified in the SI
Strategy and Plan from their inception. A decision point is to be marked and reviewed
periodically so that any LEP work can be started and completed before the current
planned OSD. Practically, it can take ten years between starting a major LEP and
getting the evidence to give the required life clearances. The FTR and supporting
technical reports are used as evidence for this activity. RA 572411 and Def Stan 00-970
Part 1 Section 3 (Structure) Leaflet 39 (Fatigue Life Extension) provides a good
template for structures LEP activity, regardless of the design standard originally used
for the aircraft
Use of other evidence
Risk analysis
Probabilistic SI management
127. These sections are being rewritten and have been temporarily removed. Interim
guidance on these subjects may be obtained by contacting DSA-MAA-CertStructures@mod.uk.
Practical management
SISD
128. Establishing the Strategy allows all SI stakeholders to underwrite the intended
approach while maintaining visibility of the actions required. Moreover, the SI strategy
will form part of an aircraft’s Safety Assessment evidence and TLMP. It is essential that
development of a SI strategy to meet this Regulation is considered early in the
acquisition cycle and well before the In-Service Date.
129.
The SISD covers the following aspects:
a. Details of overarching strategy: what measures are considered necessary
to support the fleet throughout its life and to address ESVRE activities, and
details of any necessary deviations from MOD regulation.
b. Details of strategy implementation: an introduction, implementation of the
strategy to ESVRE principles, the SI meeting strategy including frequency and
membership, the SI Plan and a record of historic SI activities and decisions.
130. Compromised SI can affect an aircraft’s airworthiness, capability, availability
and associated whole-life costs at any point in its service life. The aim in sustaining an
SI Strategy is to maintain and promulgate the PT’s intended approach to implementing
the required acquisition cycle and through-life SI management activities for an aircraft
in terms of methodology, timescale and financial commitment. To achieve this, a PT
develops a SI Strategy to ensure that there is a clear understanding across all SI
Stakeholders of what is to be achieved and what the short and long-term objectives are
in meeting the vision. Sustaining the strategy allows all SI Stakeholders to continue to
update and underwrite the intended approach while maintaining visibility of the actions
vital to maintaining structural airworthiness. The strategy is promulgated in an SISD,
which includes an SI Plan. This communication is particularly important as it will form
part of the platform’s Through Life Management activities and ultimately the associated
Safety Assessment, and will also act as a record of the evidence and rationale behind
the SI decisions, such as termination of full-scale fatigue testing, taken throughout the
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life of the aircraft. Therefore, this document outlines the strategy and details the
activities required to implement and maintain UK MOD SI regulation.
131. The SISD will include relevant items such as design philosophy, verification and
validation approach, major modification and capability upgrade programmes,
integration of new stores, change in fleet disposition, fleet draw-down and OSD plans.
132. The SI Plan needs to be integrated within the SI Strategy Document and the
TLMP (see Figure 6). The SI Plan may incorporate all elements of the ESVRE
framework or equivalent for SI management, identifying all of the SI activities planned
to achieve the necessary Structural Airworthiness, capability, availability and cost, until
the funded OSD. It may be useful to mirror the RA headings of RA 5720 plus selected
relevant additional line items such as major modification and capability upgrade
programmes, integration of new stores, changes in fleet disposition, fleet draw-down
and OSD plans. Accordingly, the SI Plan is owned by the PT and made available to all
SI Stakeholders.
Airworthiness Strategy
Safety Case
System Integrity Strategy
Through Life Management
Plan
Propulsion Integrity
Strategy
Structural Integrity Strategy
Structural Integrity Plan
Figure 6 SI document hierarchy
133. It is essential that development of an SI Strategy is considered early in the
acquisition cycle and well before the aircraft is in service. The SISD and SI Plan are
living documents and should be reviewed periodically by structure specialists in the
SIWG. They should incorporate all elements of the ESVRE framework for SI
Management, broken down into individual programme, recurring and one-off activities,
key milestones and decision points, as appropriate to the position of the aircraft type in
the project life cycle. Detailed guidance on the content and approach required of the
SISD may be found in the MAA SISD Template 28 .
134. SI planning is a continuous activity that ensures that all SI-related activities are
captured, coordinated and adequately resourced.
SIWG
135. The SIWG forms a fundamental part of the governance of the through-life safety
management of the Air System. At the top level, the SIWG should keep in mind
questions that define its aim:
a.
Is there sufficient evidence to prove that, at current and forecast usage
rates, the platform will safely achieve its OSD? If not, what do we need to do to
address this?
28
Structural Integrity Strategy Document Template
30
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b.
How do we ensure that risks to air safety arising from the four main
threats to SI are managed, tolerable and ALARP?
136. The PT is to have processes that enable it to monitor, measure and sustain SI.
The effectiveness of these processes is ratified at the SIWG. Although executive
airworthiness responsibility rests with the TAA, there are a number of stakeholders in
SI Management who will contribute to the SI decision-making process. A SIWG is a
suitable forum for discussing a SI Strategy, structural issues and risks.
137. The SIWG is to be sufficiently ahead of higher level meetings (Project Safety
Panel, etc) to allow SI risks to be raised at these forums.
138. SIWGs should be initiated by the PT at an appropriate time prior to the InService Date and in consultation with MAA Cert S&ADS. In multi-national projects,
although multi-national forums may be established to progress SI issues common to
the partner nations, it is likely that a UK-only SI meeting structure will also be
necessary to progress national issues associated with UK configuration, usage and
operating practices and SI standards.
139. Current risks to Structural Airworthiness are to be discussed by all SI
stakeholders in sufficient detail to allow the LoAA holder to understand the current level
of risk. Risks are to be formally recorded in order to provide an audit trail for SI
decisions. Any Structural Airworthiness risk identified that is potentially generic in
nature or broad-based is to be raised by the PT at the appropriate Airworthiness
Management Group.
140.
A SIWG 'Aide-Memoire' is available at Annex A.
Supporting information
Suggested SI training
141. The following training courses may be suitable for staff involved in SI
Management. Applications can be made via the MAA website.
a.
Aircraft Structural Integrity Course (ASIC) (MAA).
b.
Airworthiness of Military Aircraft Course (DA-CMT).
References and further reading
MAA regulation and further reading on SI can be found on the MAA website at
www.gov.uk/maa
AAPWG Paper 10 – A Framework for Ageing Aircraft Audits
Aircraft Topic 15S – Statement of Operating Intent and Usage.
Aircraft Topic 5V – Sampling Requirements and Procedures.
Aircraft Usage and Validation Programme
AOF – Aquistion Operating Framework for further information on Risks and
Hazards Via the Defence Intranet
AP 101A-0601-1 – Employment and Repair of Aircraft Composite
AP 101A-1500-0 – Joint Service Aircraft Battle Damage Repair Manual.
Def Stan 00-970 Pt 1 Sect 3 Lft 36 (Structural Inspection Programme)
Def Stan 00-970 Pt 1 Sect 3 Lft 37 (Ensure Fatigue Qualification Testing covers
fleet usage to OSD)
Def Stan 00-970 Pt 1 Sect 3 Lft 38 (Conduct a continuous or periodic
OLM/ODR programme throughout aircraft life)
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Def Stan 00-970 Pt 1 Sect 3 Lft 38 (Fatigue and Usage Monitoring and HUMS.
Assess Fleet Leader status and need for budgeting)
Def Stan 00-970 Pt 1 Sect 3 Lft 38 (SOIU Review)
Def Stan 00-970 Pt 1 Sect 3 Lft 39 (Life extension measures)
Defence Standard 00-970 – Design and Airworthiness Requirements for
Service Aircraft.
Guidance for Helicopter Operational Data Recording Programmes, Terry, G. K.,
Reed, S. C., and Perrett, B. H. E.
Handbook of Best Practice in Teardown of Aircraft Structures, Second Edition,
TTCP, Sep 2011
JAPD-100C-20 – Preparation and Amendment of Maintenance Schedules.
JAPD-100C-22 – Procedures for Developing Preventive Maintenance.
JSP 886 – The Defence Logistics Support Chain Manual
MAA Structural Integrity Strategy Document Template
MAA02 MAA Master Glossary
MASAAG Paper 104 – Recommendations for the future shape of the Ageing
Aircraft Structural Audit
MASAAG Paper 105 – Teardown Inspections - Guidance and best
MASAAG Paper 106 – Repair Assessment Programme for Military Transport
Aircraft
MASAAG Paper 109 – Guidance for Aircraft Operational Loads Measurement
Programmes.
MASAAG Paper 116 – Widespread fatigue damage in military aircraft
MASAAG Paper 118 – Interaction of Corrosion and Fatigue
MASAAG Paper 123 – Development, Validation, Verification and Certification of
Structural Health Monitoring Systems for Military Aircraft
Materials.
practice.
RA 1003 – Managing the Delegation of Safety and Airworthiness
Responsibilities.
RA 1210 Ownership and Management of Operating Risk (Risk to Life)
RA 1500 Certification of UK Military Registered Air Systems
RA 4103 Decontamination of Aircraft after Spillage of Body Fluids
RA 4256 Aircraft Weighing
RA 4350 Through Life Management of Technical Information
RA 4351 Production and Maintenance of Maintenance Schedules
RA 4403 Expedient Repair
RA 4500 Health and Usage Monitoring
RA 4507 Aircraft Environmental Damage Prevention and Control
RA 4815(2) Procedures for Good Maintenance Practices
RA 5309 Fatigue Type Record for Aircraft
RA 5720 Structural Integrity Management
RA 5723 Ageing Aircraft Audit
RA 5724 Life Extension Programme
RA 5725 Out of Service Date Extension Programme
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Classic articles (available to ASIC delegates):
Managing an Ageing Aircraft Fleet, Wg Cdr Andy March
Developing Issues in Aircraft Structural Integrity
Wg Cdr Ron Eckersley, (Ex
WC SSG)
Helicopter Fatigue and Qualification Case History, M Overd, Westland
Helicopters Ltd
Helicopter Structural Integrity, Sqn Ldr D Thomas
Fibre-reinforced Polymer Composites, Dr D Bray, DSGT, RAFC Cranwell
The Hawk Life Extension Programme, Wg Cdr M J Kilshaw (adapted from
original articles)
Future Fatigue Monitoring Systems, Sqn Ldr S Armitage, (ex SM17b (RAF))
and D M Holford, DERA
Fatigue Management – Current RAF Practice, the Future and some Case
Studies, Sqn Ldr S Armitage
Tornado Structural Integrity, Sqn Ldr M A Sibley (adapted from original paper)
Eurofighter Typhoon – Production Major Airframe Fatigue Test (PMAFT), M.D.
Greenhalgh, BAE Systems
Structural Airworthiness Requirements for Aircraft Fatigue Design, Alison Mew,
QinetiQ(F)
Ageing Aircraft in Military Service, Sqn Ldr N Lea
Economic Benefits of Usage monitoring in UK Military Rotary Wing Aircraft Sqn
Ldr L Sumner
Aviation Occurrence Reporting CAL Boeing 747 Accident Report (ASC-AOR05-02-001) – Executive Summary, ASC
Repairs to Damage Tolerant Aircraft, T Swift (FAA)
Safe Life Scatter Factors – Their Origins, Applicability and Opportunities for
Exploitation, Sqn Ldr T Bleakley
The Structural Audit – A Positive Contribution To The Sustainment Of
Airworthiness And Safety, M J Duffield – Systems & Structures Airworthiness –
Structural Integrity (Large Aircraft) QinetiQ(F)
The Nimrod Review
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/2
29037/1025.pdf
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ANNEX A
1 Introduction
a. Introduction.
b. Actions arising from previous minutes
and not covered under agenda items.
SIWG AIDE-MEMOIRE Version 1.1
The SIWG forms a fundamental part of the governance of the through-life safety
management of the Air System. At the top level, the SIWG should keep in mind
2 questions that define its aim:
a.
Is there sufficient evidence to prove that, at current and forecast useage
rates, the platform will safely achieve its OSD? If not, what do we need to do to
address this?
b.
How do we ensure that risks to air safety arising from the 4 threats to
structural integrity are managed, tolerable and ALARP?
2 Fleet
Planning
a. Fleet Statistics.
Check on consumption statistics (e.g. FI/1000hr, landings/hr – whatever is important
for that type of platform) looking for significant changes. Stats need to
include historic or design assumptions for comparison. Average data can be
misleading if fleets-within-fleets issues exist.
b. Fleet Size.
Review of how many aircraft there are in the fleet and how they are operated – does
this reflect design assumptions or has Designer been made aware of variations.
c. Fleet Disposition.
3 Establishing
SI
a. SI Strategy Document and SI Plan.
b. SOI.
34
Identification of fleets-within-fleets issues. Identification of changes in bases or
operations (e.g. rough strip).
Do SI Plan and Strategy exist? Make sure you have read the Plan and Strategy. Are
they just repeats of the template or have they been thought about? Has the
Designer been consulted / included in the Strategy / Plan? Post-RTS and after the SI
Strategy and SI Plan have both been established this agenda item may be moved to
Sustaining SI, where the discussion should focus on when the Strategy and the Plan
were last updated and whether they still deliver a
regulatory compliant and suitably safe outcome (eg in light of updated or new
regulation, or in light of in-service findings necessitating a change to the strategy).
Has the SOI been produced? Alternatively, when is it planned to be published? Has
Requirements Manager been heavily involved in SOI development. Has
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the Designer been tasked to review and identify implications for lives. This
agenda item can be marked as N/A if the SOI has been updated to become an
SOIU.
35
c. Release to Service – SI aspects.
This should cover both the initial RTS, with specialist advice from the SI Advisor,
and ongoing RTS amendments (with implication for SOI/U). Looking for
significant issues or shortfalls (e.g. limited clearances or envelope restrictions –
looking for coherent plan to get to full RTS). Also are there any RTS issues that
will require validation in service? For example, probabilistic gust or birdstrike
analysis will require validation from in-service data.
d. Structural Design Certification:
What is the structural design certification basis for the aircraft (e.g. stress-life,
strain-life, damage tolerance)? Where is this defined and recorded (e.g.
designed / certified to Def Stan/EASA/FAA/BCAR/JSGS etc)?
(1)
Classification of Structure &
review of SSI list.
Does the platform have a clearly identified SSI list (or eqivalent) that has
credibility and has been reviewed, involving Designer? If not, what plans are in
place to develop list? SSIs are required for developing RCM-based
maintenance schedules.
(2)
Static Type Record.
Does the platform have an up-to-date and credible STR? If not, what
documents constitute the equivalent and are they identified and controlled in the
same way. Are processes in place to ensure that modifications likely to affect
the STR or equivalent are identified and STR amendments actioned?
(3)
Fatigue Type Record.
Does the platform have an up-to-date and credible FTR? If not, what
documents constitute the equivalent and are they identified and controlled in the
same way. Are processes in place to ensure that modifications likely to affect
the FTR or equivalent are identified and FTR amendments actioned?
What plans are in place for incorporating OLM/ODR/IAT results into the FTR (or
equivalent) to move from design assumptions to actual usage for lifing.
e. Fatigue Test Clearances.
What are the fatigue clearances for the fleet? Where are they promulgated and
how are they managed in service. What plans are in place to take interim
fatigue clearances to meet the life-time requirements. Are adequate plans in
place for ongoing fatigue tests or tests likely to be needed to generate lifing
evidence?
f.
Red/Amber/Yellow/Green. Review previous meeting’s score (and the reasons
for that score if not green) to ensure consistency and to monitor progress of
long-running issues.
SARC Assessment.
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4 Sustaining SI
36
a. Usage Monitoring – method of Individual
Aircraft Tracking (IAT); fatigue
monitoring/budgeting/formulae; structural
and transmission Health and Usage
Monitoring Systems.
Does platform have a clear plan for usage monitoring / IAT/ HUMS? This has
been an area of weakness on many platforms. Often too much focus on ‘fitting
systems’ and not enough on what is going to be done with the data and through
life management of the system, data and results and what are the aims of
IAT/HUMS ? Where FMF are used how are they validated and reviewed? This
should include measures to identify when the IAT has been compromised (e.g.
excessive un-metered rates or system failures) and plans to rectify the situation
if this occurs. Is there sufficient fatigue life (however it is calculated) remaining
to see the fleet through to its OSD?
b. Structural Examination Programme (SEP)
(to include discussion on structural
examination to counter threat posed by
hazardous incidents, corrosion, erosion
and other environmental degradation).
How has the SEP been developed? Was RCM used in conjunction with an upto-date SSI list? How is the PT going to review the SEP and ensure that it is fit
for purpose? – reliance on F765s alone is not sufficient.
(1)
SEP Reporting Process
(Topic 5V).
Does a Topic 5V exist? If not are there plans in place to develop one, in
conjunction with Designer? How will it be done and how does the PT intend to
exploit the 5V to support SI (e.g. sampling opportunities)? Also did the 5V
development identify any shortfall in existing SEP (e.g. hard to inspect or uninspectable at risk SSIs)?
(2)
Scheduled maintenance plan
(Topic 5A1).
How has the maintenance schedule been developed? Has the Designer been
fully involved in its development? Also ensure that mechanism is in place to
ensure that structural inspections are undertaken by authorised tradesmen
(MAP-B473).
c. Structural Configuration Control.
Historically this has been a challenging issue. What measures are in place to
retain config control and how are these measures going to be validated to
ensure they are working (e.g. periodic mod and repair checks at deep
maintenance)?
d. OSD.
What is the declared OSD for the platform, how does this relate to the Design
Service Goal (DSG) and when will any potential LEP requirements be identified?
Creeping life extension should be avoided.
e. Obsolescence Management Plan
Are there any structural materials or coatings used on the ac type that are
obsolete/obsolescent? If so, how are repairs to structural items made from
obsolete/obsolescent materials going to be performed and what plans are in
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place for identifying and approving new coatings.
f.
5 Validating SI
SIWG Frequency and Composition
Does the SIWG take place twice yearly? Are the key stakeholders noted in
RA5720(3) present? Does the Chairperson’s LOAA allow him/her to chair
SIWGs?
g. SARC Assessment.
Red/Amber/Yellow/Green. Also review previous meeting’s score (and the
reasons for that score if not green) to ensure consistency in the scoring and to
monitor progress of long-running issues.
a. SOIU Review.
When were the last annual and triennial SOIU reviews started and, therefore,
when are the next reviews due to begin? What plans are in place for the next
reviews? Are adequate data collection and analysis tools in place to collate the
data for 3-yearly review? What calendar period does the useage data come
from? Does the review also include updating the ‘Intent’ element? What
mechanisms are in place for Designer to analyse the revised SOIU and report
back on lifing implications in context of the DSG?
b. Operational Loads and Usage Validation.
What OLM/ODR/MDRE is planned or underway? This section should include
reports from OLM/ODR/MDRE Working Groups. Encourage identification of
likely OLM/ODR/MDRE validation requirements early in the programme.
Additional OLM/ODR/MDRE requirements may still be required for ac with strain
gauge-based monitoring systems due to incomplete coverage (e.g. landing
gear). May also require validation of RTS assumptions if not possible by other
means (e.g. time at flight-level for birdstrike or gust risks).
c. Structural Sampling.
What sampling has been done and what is planned? Opportunity sampling can
prove extremely valuable and does not have to be restricted to Cat5 aircraft.
What can be done alongside mod programmes? Use Topic 5A1 to identify
candidates. Particular focus should be on any ‘at risk’ SSIs that are not
inspectable (e.g. inside flying control rods).
d. Teardown.
As per structural sampling. Invaluable particularly for validation of corrosion
protection / prevention. Successful programme requires careful thought as to
targeted areas. Requires Designer involvement and also benefitted by
involvement from other users (particularly if older fleets exist elsewhere).
e. Maintenance schedule review (Including
Topic 5V returns).
37
What plans are in place for review of the schedule? How will the schedule
review be done? What data are available and is the data considered for
purpose? Is the Designer heavily involved in this process and are fatigue-life
related schedule elements clearly identified.
Structural Integrity Working Group – Aide-Memoire v1.1
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f.
6 Recovering SI
38
Review of Type Records.
What plans are in place for the review of type records (or documents identified
as meeting these requirements)? Should be linked in with fatigue test
completion, OLM/ODR programmes, mod programmes etc. STR sometimes
forgotten as in-service focus is on fatigue – care needs to be taken over mass
growth (often incremental).
g. NDT Technique Review
NDT review should be about both whether the techniques are still required, and
whether they are still appropriate. Techniques are often introduced rapidly in
response to an in-service or fatigue test arising and should be reviewed if they
are going to be part of a longer-term SI measure.
h. Ageing Aircraft Measures.
When was previous AAA started and when is next planned? How are issues
identified by AAA managed within the platform and what measures are in place
to ensure the completion of remedial measures is endorsed by the SIWG?
Again important that the Designer is involved in this process (see RA5723 for
background information).
i.
Red/Amber/Yellow/Green. Also review previous meeting’s score (and the
reasons for that score if not green) to ensure consistency in the scoring and to
monitor progress of long-running issues.
SARC Assessment.
a. Structural Problems.
(1)
Review Contents of Structural
Arising Database.
Identification of structural issues and recovery plans.
(2)
Review EDPC WG/Database.
Report from EDPC WG. Should include issues taken on by SIWG and
directives back to EDPC WG. Confirm that there is an ED Prevention and
Control Programme in place and that this has been reviewed by SMEs at SIWG
(including Designer) and that issues and progress are tracked by SIWG.
b. Modifications with SI implications.
Review of recent modifications to confirm that SI implications have been
identified and addressed (e.g. mass growth (global or local), changes in flight
control system, reduced access for maintenance etc).
c. Special Instructions Technical SI(T) with
SI implications.
Review of tech instructions issued to ensure SI implications identified and
addressed (e.g. removal of surface finish, making access for inspections).
d. Repair Assessment Programme
Where applicable, what RAP surveys have been undertaken and what is
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(MASAAG 106)(mandatory for large ac
only).
7 Exploiting SI
39
planned? How is the information being preserved and updated? Has the
Designer reviewed the RAP reports and have remedial actions been
implemented (e.g. fatigue analysis of repairs undertaken and inadequate repairs
replaced with adequate repairs)? What measures have been implemented to
prevent reoccurrences in the future (e.g. revised procedures, cancellation of
inadequate repair instructions or removal of inadequate repairs from the Topic
6).
e. Compromised IAT.
It is likely that an IAT system will be compomised at some time during the life of
the fleet, particularly (but not exclusively) with modern electronic structural
health monitoring systems. What is the percentage of compromised IAT data?
Are there any trends that could identify a significant issue for collecting IAT data
(eg large percentage of compromised IAT data from a single airframe)? What is
the gap-filling method and does it comply with the regulation?
f.
Red/Amber/Yellow/Green. Also review previous meeting’s score (and the
reasons for that score if not green) to ensure consistency in the scoring and to
monitor progress of long-running issues.
SARC Assessment.
a. Review of Structural Hazards or Risks.
Identify if any of the discussions during the meeting invalidate the structural
hazards/risks that already comprise the loss model, meaning that those
hazards/risks need to be reconsidered in light of new evidence, or whether new
hazards need to be considered.
b. Fatigue Conservation Measures.
If it has been identified that there is insufficient fatigue life (however it is
calculated) remaining until the OSD, what measures could be introduced to
ameliorate the situation? For example, VC10 adjusted the aileron upset angle to
reduce wing bending. RW platforms could extend cleared safe lives of some
rotables by reducing the time spent in hover. Historically measures have been
introduced too late and they have been less successful than anticipated.
Therefore this should include methods of monitoring fatigue conservation
measures.
c. Exploiting In-service experience.
Should include any relavant experience from other users or similar types.
Reports from User Groups on SI issues can be highly relevant.
d. Life/OSD Extension Programme.
Identification of plans for LEP. Historically often identifed too late. Significant
LEP should be idenfied 10 years before OSD. Deadlines for decisions should
be identified in Strategy/Plan. Often an issue for elevation above SIWG.
Structural Integrity Working Group – Aide-Memoire v1.1
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40
e. Exploitation of OLM/ODR/HUMS/SHM
/MDRE data.
Data can be essential in meeting OSD or in LEP. Plans to exploit data to
convert from design assumed usage to actual usage should be identified.
8 Ac Disposal
Plan
9 AOB
10 SARC Score
a. Aircraft Disposal Plan
Potential source of structural sampling / teardown to support remaining fleet.
11 New SI Risks
12 Upward
Reporting
a. Consideration of new SI Risks that should
be registered within Risk Management
system
a. Actions from and reports to higher
fora.
13 Next Meeting
a. Arrangements for next meeting
a. AOB
a. Overall SARC Assessment.
Red/Amber/Yellow/Green. Also review previous meeting’s score (and the
reasons for that score if not green) to ensure consistency in the scoring and to
monitor progress of long-running issues.
Formal pull through of SI issues identified as structural issues in the SIWG into
the risk register (where not already identifed as entries)
Should include significant risks (technical and programme (e.g. LEP)) and
significant non compliances with RA
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ANNEX B
GLOSSARY AND ABBREVIATIONS
Readers are advised to consult MAA02 for the most current definitions relating to Air
Safety. The following list is offered as an additional resource.
Term
Abbreviation
Definition
Naval Service Modification
(N)SM
First Line
1st Line
(MAA02 Issue 5: The obsolescent RN terminology for a Service
Modification. Regulations for NSMs are given in AP100N-0140 Ch
18.)
Obsolete. Refer to MAA02.
Second Line
2nd Line
Obsolete. Refer to MAA02.
Third Line
3rd Line
Obsolete. Refer to MAA02.
Fourth Line
4th Line
Obsolete. Refer to MAA02.
724/725 (MOD Forms 724/725)
Hard copy data collection forms used to record sortie details
including mass, fuel, stores, duration, landings and counts from
fatigue meter (qv) if fitted. The data recorded on the F725 was the
original basis for fatigue calculation in an MOD central computer.
Armament and Explosive
Regulations
Ageing Aircraft Audit
A & ERs
Aircraft Armament Electrical
Installation
Aircraft Assisted Escape System
AAEI
provides assurance that the Structural Integrity, System Integrity
and Propulsion Integrity, and hence the airworthiness risks, of a
fleet’s aircraft are being managed appropriately from the
perspective of ageing.
Refer to MAA02.
AAES
Refer to MAA02.
Aircraft Armament System
AAS
Refer to MAA02.
Ageing Aircraft Structural Audit
AASA
Aircraft Armament Suspension
Equipment
Aircraft Charge Certificate
AASE
Subdivision of an AAA, considering the failure modes and
airworthiness consequences related to aircraft structure.
Refer to MAA02.
ACC
Refer to MAA02.
Aircraft Control System
ACS
In specific circumstances, an occurence that is not repairable by
the aircraft maintenance staff at the aircraft operating location.
(MAA02 Issue 5: An occurrence which results in: a person being
killed or suffering a major injury or an aircraft sustaining Category
4 or 5 damage. With respect to Aviation Risk Management an
accident is the realization of the Hazard becoming a harmful
outcome.)
Refer to MAA02.
Aircraft Cross-Servicing
ACS
Refer to MAA02.
Active Control of Structural
Response
Accidental Damage
ACSR
AAA
Accident
Assistant Directorate Engineering
Policy
Assistant Directorate Forward
Support
Assistant Director/Airworthiness,
Design Requirements and
Procedures
Accidental Damage/Environmental
Damage
Automatic Direct Exchange
Scheme
Anti-deterioration maintenance
Aircraft Document Set
SI Glossary Version 1.1
AD
AD Eng Pol
The physical alteration of an item (or its surface protection where
applicable) caused by contact, impact or interaction with an
object, which is not a part of the aircraft, or by the result of human
error during manufacture, operation or maintenance of the
aircraft. Often used with ED (qv) when making assessment to
determine suitable design approach or maintenance policy for
specific component.
Obsolete. Refer to MAA02.
AD FS
AD/ADRP
AD/ED
See Accidental Damage; Environmental Damage.
ADES
Refer to MAA02.
ADM
Refer to MAA02.
ADS
The documents that have a prime airworthiness function for each
aircraft type. They include the Release To Service (RTS), Air
System Safety Case, Aircraft Maintenance Manual (AMM),
Operating Data Manual(ODM), Flight Reference Cards (FRCs),
Support Policy Statement, Engineering Air Publications (including
the Flight Test Schedule (FTS)) and the Statement of Operating
41
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Term
Air Engineer Officer
Abbreviation
Definition
AEO
Intent and Usage(SOIU). The documents comprising the ADS
may be held electronically. (MAA02 Issue 5)
Refer to MAA02.
Aeroelastic (effect)
Process where aerodynamic forces on a structure cause it to twist
or bend thereby modifying the forces and, in turn, the twisting or
bending. Resultant effects may be aeroelastic divergence (qv),
steady aeroelastic distortion of the structure, or oscillation. The
oscillations may be damped and stable, neutrally stabile (Limit
Cycle Oscillation, LCO qv) or divergent (Flutter qv.).
Specific case of aeroelastic (qv) deflection of the structure when
the deflection caused by aerodynamic forces acts to increase
those forces, leading to increasing deflection of the structure until
it fails.
Aeroelastic Divergence
Automatic Flight Control System
AFCS
Aerospace Ground Equipment
AGE
Aircraft Ground Engineer
AGE
Above Ground Level
AGL
Aircraft Integrity Monitoring
Equipment Committee
AIMEC
Airframe
Airworthiness
As Low As Reasonably
Practicable
Airworthiness Management Group
ALARP
AMG
Aircraft Maintenance Manual
AMM
Artificial Neural Network
ANN
Air Navigation Order
ANO
(MAA02 Issue 5: The committee responsible for provisioning
AIME.)
Structure qv that defines and supports the aerodynamic profile of
an aircraft.
(MAA02 Issue 5: The ability of an aircraft or other airborne
equipment or system to be operated in flight and on the ground
without significant hazard to aircrew, ground crew, passengers or
to third parties; it is a technical attribute of materiel throughout its
lifecycle.)
As Low as Reasonably Practicable (Risk). The principle for Safety
management in the MoD. Refer to MAA02 for a detailed definition.
6-monthly meetings, structured around Combat Air, Air Support
and the Helicopter Clusters, attended by PTLs and specialists to
discuss platform-specific and cross platform airworthiness issues.
An Artificial Neural Network (ANN) is an information processing
paradigm that is inspired by the way biological nervous systems,
such as the brain, process information. The key element of this
paradigm is the novel structure of the information processing
system. It is composed of a number of highly interconnected
processing elements (neurones) working in unison to solve
specific problems. ANNs, like people, learn by example. An ANN
is configured for a specific application, such as pattern recognition
or data classification, through a learning process.
Statuary Rules for air navigation of civil aircraft in UK.
Aircraft Operating Authority
AOA
Refer to MAA02.
Aircraft Post Crash Management
APCM
Refer to MAA02.
Auxiliary Power Unit
APU
Refer to MAA02.
At-Risk
AR
A term used in the assessment of structure in its vulnerability and
susceptibility to AD/ED.
Refer to MAA02.
Armament Role Change Harness
ARCH
Airborne Radio Installation
ARI
Refer to MAA02.
Aircraft Repair Organization
ARO
Refer to MAA02.
Aviation Safety Implementation
Group
ASIG
Aircraft Structural Integrity
Program
ASIP
Consists of representatives from all aspects of military aviation
across the 3 services, DLO and DPA who have authority to
implement policy.
As used by the US Department of Defense, an organized and
disciplined approach to ensure the achievement of the desired
level of structural safety, performance, durability, and
supportability with the least possible burden throughout the
aircraft’s design service life (refer to MIL-STD-1530C(USAF)).
Aircraft are generally symmetric (left and right sides) and the
structural usage on each side is normally balanced. However,
some manoeuvres are asymmetric (rolling, turning, yawing) and
loading depends on the direction of motion. Bias towards one
loading direction may arise.
Asymmetric Usage
Audit Team Leader
ATL
Airworthiness Flying Limitation
AWFL
Bending Moment
42
Document that contains flight limitations, to which the aircraft is
flown. It is normally issued by the Designer and may give the full
intended clearance rather than something less. It is used to give
clearance for flight trials so that an RTS can be issued.
The reaction induced in a piece of structure subject to bending,
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Term
Abbreviation
Definition
measured as a force multiplied by a distance. For example, the lift
forces on a wing create a bending moment that is greatest at the
wing root.
Alleviation of aerodynamic or inertia-induced tensile stresses in a
structure by opposing inertia or aerodynamic forces. Sometimes
employed as a fatigue reduction measure when outboard wing
stores or aerodynamic control devices are fitted/used to reduce
fatigue stresses in wing root/centre fuselage structure.
Bending Moment Relief
Built-In Test Equipment
BITE
Basic Level Budget
BLB
Brinelling
Engineering term for the indentation of raceways by ball/roller
bearings that are overloaded causing plastic deformation. Note
“False Brinelling” which has the appearance of true brinelling but
is caused by wear.
Failure Fracture of a material with little or no prior deformation.
Brittle
Buffet
Buzz
Barely Visible Impact Damage
BVID
Centre of Gravity
C of G or CG
When the airflow over a structure is no longer smooth but
becomes unstable, often characterised by random vortices, the
forces on the structure start to fluctuate. Often occurs at high
angles of attack or when airbrake, spoilers etc are deployed.
A high frequency alternating loading, including oscillating shock
waves, on a strut, etc that can cause it to resonate at its natural
frequency and may cause fatigue failure.
Term used in relation to composite structure where little or no
damage is visible to the eye, but significant sub-surface damage
exists.
The point of equilibrium. Normally, only the fore and aft
(longitudinal) location of the centre of gravity is important for
symmetrically loaded aircraft. If the lateral, or vertical position of
the centre of gravity is likely to have any material effect, reference
to it will be made in the Weight and Balance Data of the Air
Publication concerned and, when applicable, on the Trim Sheet.
(MAA02 Issue 5)
Refer to MAA02.
Civil Aviation Authority
CAA
Controller Aircraft Release
CAR
Condition Based Monitoring
CBM
Configuration Control
CC
Refer to MAA02.
Crew Escape System
CES
A theoretical point where the aerodynamic force is considered to
act on an aircraft, wing, etc at a specific speed and configuration.
Point will move backward or forwards in different situations.
Refer to MAA02.
Carbon Fibre Reinforced Plastic
CFRP
Communications and Information
Systems
Clearance
CIS
Refer to MAA02.
CLS
The limitation in terms of percentage maximum load for static
loading or life expressed in either flying hours, landings or FI for
fatigue loading, imposed on a structure pending further
analysis/testing to prove full load or life. Full clearances with no
limitations are issue don successful completion of all
testing/analysis.
Interpretation of qualified life for a particular aircraft or group of
aircraft, taking into account actual service usage and variability
effects.
Refer to MAA02.
Centre of Pressure
Cleared Life
Contractor Logistic Support
Obsolete term used to indicate that an aircraft had been cleared
for service use within specified limits, comparable with the civil
Type Certificate (qv). Replaced in 1996 by term MCofA but
quickly changed to Military Aircraft Release (MA Release) to
prevent confusion with civil terms.
Configuration Management
CM
Refer to MAA02.
Configuration Management Plan
CMP
Refer to MAA02.
Contractor-Owned ContractorOperated
Cold Working of Holes
COCO
Refer to MAA02.
Civil-Owned Military Aircraft
COMA
Creation of residual compressive stresses in surrounding parent
material rendering externally applied tensile load cycles less
damaging in fatigue. May be used to enhance structural features
subject to fatigue.
Obsolete. Refer to MAA02.
COMR
The CLR is part of Topic 5A1, the Master Maintenance Schedule.
The CLR is a list of components that are replaced out-of-phase
with scheduled maintenance.
Obsolete. Refer to MAA02.
Component Life Register
Civil-Owned Military-Registered
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43
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Term
Abbreviation
Definition
Communications Security
COMSEC
Refer to MAA02.
COSHH
(Sometimes called a Build Concession or, in the US, an
Engineering Disposition). A departure in production, modification
or repair from that laid down in the design standard which is
subsequently underwritten by the Design Organization's quality
organization and the customer. Categorized as Major or Minor or
Cat 1, 2 or 3. Major Concessions are granted on MOD Form 77
(UK only) and could affect safety, fatigue, etc. A Minor
Concession may be recorded by the manufacturer, providing the
affected part complies with static strength, fatigue life, fit, form
and function, and interchangeability requirements. Concessions
may have to be reconsidered if fatigue lives are extended.
(MAA02 Issue 5: The functional and physical characteristics of
materiel as described in technical documents.)
The deterioration of a material or its properties due to the reaction
of that material with its chemical environment. Usually associated
with the chemical degradation of metals as they invariably attempt
to revert to an ore-like, oxide state at a lower energy level.
Method of tracking corrosion history of individual aircraft and
entire fleets by recording of the pre- and post-corrosion-removal
states in a book, file or database. The contents of the register are
then subject to periodic review by the PT/Service Provider.
Failure of a metallic component caused by the simultaneous
effect of fatigue and corrosion. Not to be confused with Stress
Corrosion Cracking (qv).
Refer to MAA02.
Customer Supplier Agreement
CSA
A hole drilled in the skin of a structure at the tip of a crack,
intended to blunt the crack, reduce the stress concentration and
thereby reduce the tendency of the crack to grow.
Excessive deformation or distortion of a material at high
temperatures, without fracture occurring.
The crack size associated with the onset of rapid, unstable crack
growth under application of load. This is further defined within Def
Stan 00-970, Pt1, Sect 3, Lft 36 – ‘Fatigue Inspection Based
Substantiation’ as: The crack size associated with the onset of
rapid, unstable crack growth under application of load
corresponding to the residual strength requirement. (MAA02 Issue
5: The length of a crack for a given stress at which unstable crack
growth will occur.)
Refer to MAA02.
Configuration Status Record
CSR
Refer to MAA02.
Contingency Maintenance
CTM
Refer to MAA02.
Corporate Technical Services
CTS
Clearly Visible Impact Damage
CVID
Cockpit Voice Recorder
CVR
Convergence Working Group
CWG
Concession
Configuration
Corrosion
Corrosion Control Register
Corrosion Fatigue
Control of Substances Hazardous
to Health
Crack Stopper Hole
Creep Failure
Critical crack size
Term used to describe damage to composite structure that can
be identified with the naked eye.
Contractor's Working Party
CWP
Design Authority
DA
Obsolete. Refer to MAA02.
Design Approved Organisation
Scheme
DAOS
Defence Aviation Repair Agency
DARA
A system whereby organizations that have been assessed as
competent to design and supply equipment to a satisfactory
standard are approved by the MAA. (MAA02 Issue 5)
Obsolete. Refer to MAA02.
DASB
Obsolete. Refer to MAA02.
Defence Aviation Safety Centre
DASC
Defence Aviation Safety
Management System
Design Change Proposal
DASMS
Declaration of Design and
Performance
Debonding
DDP
Def Stan 00-970
Delamination
44
DCP
Tri-Service MOD Organisation that co-ordinates UK Military Flight
Safety policy, awareness and records, for all UK registered
military aircraft.
– Has a policy that risks must be identified and their likelihood and
severity evaluated before their tolerability can be judged.
Usually follows a Production Permit (qv).
This document is the design certificate for the Tornado. Can be
for the Equipment, Major Assembly or the Aircraft.
Failure of a glue bond between structures that are made up
separately and subsequently joined.
Design and Airworthiness Requirements for Service Aircraft,
which determines the UK build, test and qualification
requirements for military aircraft.
Failure of the matrix between layers of composite materials
resulting in layers separating.
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Term
Abbreviation
Definition
Dangerous Engineering
Substance
Defence Environment and Safety
Board
Designer
DES
Refer to MAA02.
DESB
Responsible to SofS for Defence. for all matters of this nature.
Refer to MAA02.
Detail
Design office term describing part of a structure, normally where
there is a joint or change in geometry and where formerly, in hard
copy drawing days, a specific drawing of that area would be
called for and local stresses may have been assessed.
An approach in which parameters are given a single value to
simplify mathematical treatment and allowance for uncertainty is
made by applying (safety) factors to results. (cf Probabilistic).
See Debonding.
Deterministic
Disbonding
Divergent Oscillation
Design Limit Load
DLL
Designer Modification
DM
Design Organization
DO
Describes motion of whole aircraft or part of structure when the
forces causing oscillation are undamped. Oscillation increases in
amplitude usually, leading to catastrophic failure.
See Limit Load.
Date of Segregation
DoS
The organization appointed by the PTL to be responsible for the
design or design change of an airborne system or its associated
equipment, and for certifying the airworthiness of the design by
issue of a Certificate of Design. (MAA02 Issue 5)
Refer to MAA02.
Defence Ordnance Safety Group
DOSG
Refer to MAA02.
Design Proof Load
DPL
Data Reporting, Analysis and
Corrective Action System.
Common System
DRACAS
Defence Systems Approach to
Training
Defence Storage and Distribution
Centre
Design Service Goal
DSAT
Used in industry, it encompasses all aspects of tests and trials
and includes performance data. DRACAS is the preferred system
for MOD contracts (DEF STAN 00-40 refers). (MAA02 Issue 5: A
data reporting and analysis system to record safety
problems and use this data to improve safety management.)
Refer to MAA02.
DSDC
Refer to MAA02.
Defence Science and Technology
Laboratory
Damage Tolerance
DSG
Dstl
Refer to MAA02.
DT
Engineering Authority
EA
Approach developed by USAF which differs from the Safe Life
philosophy in that it assumes cracks to exist in the structure
already at the very first load cycle. Damage tolerance is an
attribute of a structure that permits it to retain a level of residual
strength for a period of unrepaired usage after the structure has
sustained specific levels of fatigue, corrosion, accidental or
environmental damage and/or discrete source damage. A
distinction is made between inspectable and non-inspectable
structures, and areas classified either as Slow Crack Growth, Fail
Safe Multiple Load Path, or Fail Safe Crack Arrest structures. This
concept has been extended to include ”Inspection Dependent”
structure where cracks are assumed to exist upon expiry of the
Safe Life and is an approach that may be used during life
extension activities.
Fracture of a material with significant permanent (plastic)
deformation.
May be considered as a quantitative measure of the structure's
resistance to the threats to SI (qv), especially fatigue cracking
under specified service conditions. This means that the economic
lifetime, including all inspections, replacements or repairs, should
exceed or at least equal the design life based on damage
tolerance. The traditional means to achieve durability has been
through conventional fatigue testing and analysis. However,
recently, a fracture mechanics philosophy, combining a
probabilistic format with a deterministic crack growth approach
has been advised.
Test in which high frequency loading is simulated such as that
experienced in buffet, whereas the conventional fatigue test
applies lower frequency loads.
Refer to MAA02.
European Aviation Safety Agency
EASA
Refer to MAA02.
Ductile Failure
Durability
Dynamic Fatigue Test
Electronic Counter Measure
ECM
Refer to MAA02.
Environmental Damage
ED
The physical degradation of material properties as a direct result
of interaction with the climate or the environment.
SI Glossary Version 1.1
45
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Term
Abbreviation
Definition
Engineering Development and
Investigation Team
EDIT
Refer to MAA02.
Environmental Damage
Prevention and Control
EDPC
Electro-Explosive Device
EED
An EDPC Programme is a comprehensive and systematic
approach to managing the risk to airworthiness, capability,
availability and costs arising from ED throughout the life of an
aircraft.
Refer to MAA02.
Electrical Firing Circuit
EFC
Refer to MAA02.
Early Failure Detection Cell
EFDC
Refer to MAA02.
Engine Health and Usage
Monitoring Systems
Electro-Magnetic Compatibility
EHUMS
Refer to MAA02.
EMC
Refer to MAA02.
Endurance Limit
The alternating stress level in a material below which it will not fail
in fatigue no matter how many times the stress is applied. The
item effectively has an infinite fatigue life below this level of
stress.
see Concession.
Engineering Disposition
External Quality Audit
EQA
Refer to MAA02.
Expedient Repair
ER
Refer to MAA02.
Engineering Record Card
ERC
Refer to MAA02.
Engineering Sciences Data Unit
ESDU
ESVRE
ESVRE
Electronic Technical Information
ETI
Now known simply as ESDU International plc. Providers of
engineering data such as properties of alloys, etc, based on
empirical and theoretical research, for use by industry and
research establishments.
UK MOD approach to managing SI for in-service aircraft in which
the 5 areas of Establishing, Sustaining, Validating, Recovering
and Exploiting SI comprehensively cover all activities needed to
economically manage structural airworthiness risks.
Refer to MAA02.
Engine Usage Condition
Monitoring and Management
System
Federal Aviation Administration
EUCAMS
Refer to MAA02.
FAA
Refer to MAA02.
Fleet Air Arm
FAA
Factor
Fail Safe
Federal Aviation Requirement
Fastener
Fatigue Allowable
Fatigue Failure.
Fatigue Index
Fatigue Life
Fatigue Meter
46
FAR
The term factor is used in several applications on aircraft
structures, ie usage factor, scatter factor, safety factor, reserve
factor and stress factors, given as a number.
The Fail Safe design philosophy was a forerunner to the Damage
Tolerance philosophy and relies on continued Structural Integrity
being predicated on the presence of alternate loads paths or
crack arrest features and the ability to detect the failure. These
can however only withstand the redirected loads for a limited time
before further failure. Therefore, SI is maintained through suitably
timed inspections aimed at determining whether failure of the
primary load path has occurred.
Certification standards published by the FAA.
Generic name for single rivet, nut/bolt, screw or latch used to join
or attach aircraft structure. A latch (such as a Quick Release
Fastener) may itself be attached by fasteners (rivets).
The maximum stress a detail could experience when the Limit
Load was applied so that the detail’s fatigue life when subjected
to the expected loading spectrum would coincide with specified
aircraft life.
Fracture of a material due to cyclical application of a load.
Fracture usually occurs by a three-step proce: 1. Nucleation (qv)
of the crack. 2. Slow, cyclic propagation of the crack. 3.
Catastrophic failure of the metal.
The non-dimensional measure of fatigue usage of an aircraft or
specific items of its structure such as fuselage frames or
empennage. For an aircraft/component in which there are no
premature failures during Full Scale Fatigue Testing (qv), the Safe
Life (qv) will usually be set at 100 Fatigue Index (FI).
Normally used to mean the point at which the risk of failure from
fatigue of an item subjected to repeated stresses is predicted to
reach an unacceptably high level, at which point the item is
withdrawn from service. Can also be used to mean the proportion
of this life consumed to date.
A basic aircraft instrument, which records exceedances of pre-set
levels of acceleration normal to the aircraft longitudinal axis (in the
aircraft z direction) during a sortie. Separate counters indicate the
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Term
Abbreviation
Definition
Flight Data Recorder
FDR
number of exceedances above a specified level but additional
counts will only register if, in the intervening interval, the
acceleration falls below the priming (threshold) value for that
level. Used only on some fixed wing platforms. Now being
superseded by load monitoring/measuring systems to give a more
accurate picture of aircraft usage. See also FMCS.
The maximum cyclic stress a material can withstand for a
specified number of cycles before failure occurs.
Refer to MAA02.
Finite Element
FE
Flying Hours
fg hr
Front Line Command
FLC
Fatigue Strength
Flutter
Refer to MAA02.
Refer to MAA02.
Refer to MAA02.
Fatigue Monitoring and Computing
System
FMCS
Failure Modes Effects and
Criticality Analysis
Fatigue Meter Formula
FMECA
FMF
Foreign Object Damage
FOD
Fatigue Quality Factor
FQF
Failure Reporting, Analysis and
Corrective Action System.
Common System
FRACAS
Fatigue Research Advisory Group
FRAG
Fatigue Research Database
FREDA
Flight Servicing Competency
Check
Full Scale Fatigue Test
FSCC
FSFT
Functionally Significant Item
FSI
Fatigue Type Record
FTR
Flight Test Schedule
FTS
Fatigue monitoring system previously used on limited number of
Harrier GR9 and all T12 ac. Relies on 16 strain-gauged
monitoring points, on-board rainflow cycle counting and direct
computation of fatigue damage for downloading via hand-held
solid-state unit to PC-based ground station.
Refer to MAA02.
A formula which predicts the proportion of total fatigue life
consumed by an aircraft or component during a sortie. The
formula may include mass, configuration, landing or
pressurisation data in addition to normal acceleration
exceedances obtained from the Fatigue Meter (qv).
Refer to MAA02.
The ratio of the Fatigue Allowable (stress) to the actual stress in a
component at Limit Load. A sort of “Fatigue Reserve Factor”.
Used in industry, is essentially similar to DRACAS but limited in
scope. It is only concerned with failure data, unlike DRACAS,
which encompasses all aspects of tests and trials and includes
performance data. DRACAS is the preferred system for MOD
contracts. (MAA02 Issue 5)
A meeting group of fatigue specialists from Industry and MOD
which discusses and exchanges information on fatigue research
as well as directing modest joint projects.
A 'M-Vision' database of fatigue information established and
maintained by the organisations involved in its development.
Refer to MAA02.
(Often known as Main Airframe Fatigue Test or MAFT qv). A
programme lasting several years in which a representative
development, production, or preproduction, airframe is subjected
to repetitive, simulated flight and landing loads to establish the
crack free period of Safe Life designs or the inspection free period
for Damage Tolerant (qv) aircraft. For UK and UK-collaborative
designs, the loading spectrum is that anticipated in-Service and
the test is run for 3-5 time the Design Service Goal of the aircraft.
Other nations may use more severe loads than those expected in
service to accelerate testing so that only twice the equivalent
flying hours are required. Damage tolerant designs are usually
tested to twice the equivalent inspection free period, following
which the test is normally used to validate the damage tolerance
analysis by monitoring the development of both natural and
artificially induced cracks.
Refer to MAA02.
A document used to record the method by which the fatigue life of
all Structural Significant Items were substantiated.
Refer to MAA02.
Fixed Wing
FW
Ground Air Ground
GAG
Refer to MAA02.
Generic Aircraft Release Process
GARP
Refer to MAA02.
Government Furnished Equipment
GFE
Refer to MAA02.
Generic Helicopter Health and
Usage Monitoring Systems
GHHUMS
Generic Health and Usage
Monitoring System
Ground Maintenance System
GHUMS
SI Glossary Version 1.1
GMS
UK programme of individual helicopter monitoring systems for
Lynx, Sea King and Chinook (See HUMS qv).
A GMS is the equipment and software used to transform and
manage GSS-processed HUMS data and manually input
maintenance data into information pertinent to the management of
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Term
Abbreviation
Definition
GOX
aircraft maintenance at 1st through to 4th Line. A GMS may be
aircraft-specific, such as Typhoon’s Engineering Support
Subsystem (ESS), or corporate, such as Logistics Information
Technology Strategy (LITS) or Work Recording and Asset
Management (WRAM). (JAP 100A-01 Ch 7.2.5)
Refer to MAA02.
Ground Support Equipment
GSE
Phenomenon in helicopters when rotors turning on the ground
excites a natural frequency associated with roll about the
undercarriage legs and can lead to rapid, violent catastrophic
rolling onto side.
Refer to MAA02.
Ground Support System
GSS
Gaseous Oxygen
Ground Resonance
Gust
Hardened Aircraft Shelter
HAS
Hazard
A GSS is the equipment and software used to download, further
process, analyze, display and store health and usage monitoring
data from aircraft HUMS. The GSS may be used to reconfigure
airborne HUMS data collection parameters. (MAP-01 Ch 11.2)
(MAA02 Issue 5)
An atmospheric phenomenon in which a localised air mass has
vertical, or lateral velocity appreciably different to that of the
surrounding conditions thereby momentarily changing the
aerodynamic loads on the structure. The magnitude and intensity
of the gust increases as altitude decreases towards sea level,
therefore gusts may be particularly significant during the take-off
or landing phases.
Refer to MAA02.
Refer to MAA02.
High Cycle Fatigue
HCF
Hazard Division
HD
Hot-Spot Gauges
When the number of repeated loads applied to an item is more
than about 107 then the item is said to be subjected to high cycle
fatigue loading.
Refer to MAA02.
Description applied to strain gauge applied to a specific problem
area in an aircraft structure to measure strain directly. The stress
is inferred by direct relationship although the external loading on
the item of structure is not necessarily known accurately as the
installation is not calibrated in a load frame. (The strain gauge
itself will be calibrated though).
Headquarters Strike Command
HQSTC
Health and Usage Monitoring
Systems Support Cell
HSC
Refer to MAA02.
Health and Usage Centre
HUC
Refer to MAA02.
Health and Usage Monitoring
System
HUMS
Helicopter Under Slung Load
Equipment
Identification and Authentication
cycle
Issue Authority
HUSLE
The term Health and Usage Monitoring (HUM) encompasses a
variety of techniques including operational load monitoring,
vibration analysis, visual inspections, oil and wear debris analysis.
The data obtained through such monitoring is used to preserve
and enhance the airworthiness of the platform. Health and Usage
Monitoring Systems (HUMS) have been developed to support
condition-based maintenance by the acquisition of health and
usage data from on–board sensors.
Refer to MAA02.
I&A cycle
Refer to MAA02.
IA
Refer to MAA02.
Individual Aircraft Tracking
IAT
International Committee on
Aeronautical Fatigue
ICAF
Inspector of Explosives
IE
A term applied to a process that leads to the output from a system
that collects aircraft-by-aircraft cumulative lifing data, which is
used to manage retirement, modification or examination as
appropriate to the aircraft type and fatigue design philosophy.
Bi-annual conference and symposium covering all aspects of
aeronautical fatigue. Member Nations present a review of
aeronautical fatigue work in each country during the first 2 days,
following which there is a 3 day Symposium.
Refer to MAA02.
Interactive Electronic Technical
Publication
Integrated Ground Support Facility
IETP
Refer to MAA02.
IGSF
Tyhpoon term concerned with GSS (qv).
Integrated Logistic Support
ILS
Refer to MAA02.
Incident
Refer to MAA02.
Inspection Threshold
The point at which inspection should be carried out expressed in
terms of units of usage (Flying Hours, FI, years etc.) The initial
threshold is based on the smallest detectable defect in the
structure concerned. (MAA02 Issue 5)
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Term
Abbreviation
Integrated Operational Support
IOS
Definition
Internal Quality Audit
IQA
Refer to MAA02.
Inspect and Repair As Necessary
IRAN
Refer to MAA02.
Information System
IS
Independent Safety Advisor
ISA
Independent Structural
Airworthiness Advisor
ISAA
Independent Technical Evaluator
ITE
A person or team independent of the design activity who carries
out process audits of the Designer’s work for adherence to the
Safety Plan, reviews the safety documentation including the
safety assessment , and carries out independent safety audit
where commissioned by the PTL. (MAA02 Issue 5)
A competent individual, independent of the Designer, who carries
out an independent technical evaluation of aircraft structures.
They must have a first degree in an engineering discipline and
prior experience of 5 years minimum in aircraft structures design,
structures safety assessment or structures maintenance, relevant
to aircraft types for which advice will be given.
A person or team independent of the Design organization (DO)
who carries out an independent technical evaluation and analysis
of the data evidence supporting the contractor’s Safety
Assessment including a qualitative assessment of aircraft
handling. (MAA02 Issue 5)
Refer to MAA02.
In-Use equipment
IU
Joint Aviation Authority
JAA
Refer to MAA02.
Joint Airworthiness Committee
JAC
A sub committee of the DASB (qv). Refer to MAA02.
Joint Air Publication
JAP
Refer to MAA02.
Joint Aviation Requirement
JAR
Refer to MAA02.
Joint Force Harrier
JFH
Joint Helicopter Command
JHC
Refer to MAA02.
Joint Services Specification Guide
JSSG
LAD
US forces combined guide for weapons systems issued following
acquisition reform initiatives to emphasise performance
requirement and to replace single service documents. For aircraft
structures the guide is 2006.
Factors applied to the Endurance Limit obtained in a laboratory on
the rotating beam specimen to generate the Endurance Limit for a
mechanical element. These account for Surface finish, size
effects, reliability, temperature, stress concentrations and other
miscellaneous effects. K – Stress Intensity Factor: This is a
measure of the severity of the stress at a fatigue crack tip. As the
crack grows, K increases until it reaches a critical value (Kc or
Klc) at which fast fracture will occur. Kt -Stress Concentration
Factor: The factor is defined as the ratio of stress at a fatigue
critical feature to nominal stress based on minimum. This term is
used to simplify the selection of appropriate S-n curves in classic
Safe Life analysis. The factor is defined as the ratio of stress at a
fatigue critical feature to nominal stress based on minimum area.
factors for most geometries of aircraft joints were catalogued by
RAE in the 1960s and are now available as ESDU (qv) data.
Refer to MAA02.
Life Cycle Costing
LCC
(Also known as Edge Distance) Distance between a row of
fasteners and the free edge of the material.
Important loading actions on most aircraft types differentiated
between whether the aerodynamic loading on the lifting surfaces
is fully dissipated (Full stop) or not (Roller or touch-and-go).
Refer to MAA02.
Life Extension Programme
LEP
K Factors
Light Aid Detachment
Land
Landing, Full Stop, Roller
Logistic Information System
LIS
Measures taken to extend the period of operation of a platform, as
expressed in flying hours, calendar time or more directly in fatigue
usage beyond the original cleared life.
Specific case of aeroelastic (qv) oscillation in which a steady state
is reached neither divergent nor damped.
Refer to MAA02.
Logistic Information Technology
Strategy
Limit Load
LITS
Refer to MAA02.
LL
The maximum load that the aircraft should experience in service.
Limit Cycle Oscillation
Load
Load Path
SI Glossary Version 1.1
Often used indiscriminately by engineers to mean: the
acceleration of an aircraft; the external force exerted on an aircraft
structure; or the resultant stresses induced in a loaded structure.
Method of visualising the build up of stresses throughout a
structure by considering the notional “flow” of stress from the
point(s) of application of the load into more remote supporting
structure.
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Term
Abbreviation
Load Spectrum/G-Spectrum, Stress Spectrum,
Usage Spectrum
Loading Action
Local Strain
Low-Cycle Fatigue
Liquid Oxygen
LOX
Definition
A statistical expression of the number of times the load (normally
expressed in acceleration multiples of g) on an aircraft structure
exceeds specified levels. Data is often presented with a
logarithmic x-axis indicating exceedances per 10 to the power x
flying hours.
The source and nature of the forces introduced into the structure
at a particular point in the aircraft’s operation. eg during landing.
A method developed in the US for use in predicting the life to
crack initiation by assessing the strains around a stress
concentration using techniques such as Neuber's rule as an
alternative to considering stresses.
Low cycle Fatigue, stress reversals up to about 103 cycles, needs
to be considered for the design of other components when the
possibility of some very high loads during the lifetime of the
component is high.
Refer to MAA02.
Low-Profile Platforms
LPP
Refer to MAA02.
Line Replaceable Unit
LRU
Refer to MAA02.
Logistic Support Analysis
LSA
Refer to MAA02.
Logistic Support Analysis Record
LSAR
Refer to MAA02.
License to Act
LTA
Materials & Structures Group
M&SG
Military Aircraft Release
MA Release
Obsolete: Refer to MAA02.
Military Aviation Authority
MAA
Refer to MAA02.
MAA Regulatory Policy
MAA01
MAA Master Glossary
MAA02
Military Air Systems Certification
Process
Military Air Environment
MACP
MAE
Refer to MAA02.
Major Airframe Fatigue Test
MAFT
See full Scale Fatigue Test.
Manual of Maintenance and
Airworthiness Processes
MAP
Military Aircraft Structures
Airworthiness Advisory group.
MASAAG
Obsolete: See MIG
Master Armament Safety Switch
MASS
Refer to MAA02.
Main Airframe Static Test
MAST
A programme conducted in the pre-production phase of a new
design in which simulated flight and ground loads are applied to
confirm that the aircraft will not collapse before the Ultimate Load
(qv) is reached. The overall clearance may be achieved in stages,
with each stage clearing specific packages of flight trials. Usually,
the test will be taken above ultimate loads in a selected critical
load case to explore the potential strength.
Mobile Aircraft Support Unit
MASU
Materials Assurance and
Technology Section
Military Certificate of Airworthiness
MATS
Miniature Detonating Cord
MDC
Manual Data Recording Exercise
MDRE
Multi-Element Damage
MED
MC of A
Mid Sortie Mass
Materials Integrity Group
Military Aircraft
Miner's Rule/Law
50
MIG
Obsolete Short-lived (1996) term intended to replace CAR (qv)
but quickly changed to MA Release to prevent confusion with
civilian terminology.
Refer to MAA02.
Simple method of collecting flight condition data by installing a
trained observer in a helicopter who, for a sample of sorties,
categorises type and records the duration of flight condition.
MED is the simultaneous presence of fatigue cracks in similar
adjacent structural elements, such as frames and stringers.
Term used in a fatigue meter formula to identify the assumed
mass of the aircraft for all damaging manoeuvres. Strictly not the
mass at mid-sortie by time but a percentage of the mass including
stores, fuel etc, at the take off and landing. The percentage used
may vary depending on the configuration the aircraft is flown in.
An organisation within Navy Command that is responsible for
scientific and technical advice on materials and monitoring of
equipment in the air environment.
An aircraft in respect of which there is in force a certificate issued
by the SofS (for Defence) that the aircraft is to be treated as a
military aircraft for the purpose of the Air Navigation Order (qv).
Fundamental axiom of fatigue damage that an item cycled
through a variable amplitude stress programme will fail when the
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Term
Abbreviation
Definition
sum of the ratios of number of load cycles at a given stress
amplitude to the number of cycle required to fail the item at that
constant amplitude is equal to unity. The Miner's Rule can be said
to be "Calibrated" to an actual failure at a known number of load
cycles of variable amplitude on a fatigue test.
Master Maintenance Schedule
MMS
Main Operating Base
MOB
Modification
Monitored Structure
Military Registered Civil Owned
Aircraft
MRCOA
Machine-readable device
MRD
Main Rotor Head
MRH
Master Record Index
MRI
MAA Regulatory Publications
MRP
Multi-Site Damage
MSD
Maintenance Steering Group
MSG
Not-At-Risk
NAR
Non-Destructive Examination
NDE
Non-Destructive Testing
NDT
Neuber's Rule
A term used in the assessment of structure in its vulnerability and
susceptibility to AD/ED.
The beginning of the physical changes in the material properties
to allow a crack to develop when subjected to a load.
NVST
Nz
Vertical acceleration.
Officer Commanding Engineering
and Supply Wing
OC ESW
Officer Commanding Handling
Squadron
Operational Data Recording
OC HS
ODR
On-the-Job-Training
OJT
Operational Loads Measurement
OLM
SI Glossary Version 1.1
The simultaneous development of fatigue damage in large
numbers of adjacent features qv such as fastener qv holes that
would result in the coalescence of fatigue cracking and major
structural failure if not checked. Cause of the Aloha B-737 cabin
rupture in 1980´s. Also see WFD.
NIU
Nucleation
No Volts Safety Test
Obsolete. Refer to MAA02.
A method of predicting fatigue life for notched members if the
notch root strain history and the smooth specimen strain-life data
or fatigue properties are known. One of the advantages of this
method is that it accounts for changes in local mean residual
stresses.
See Artificial Neural Network.
Neural Network
Not-In-Use equipment
Defined as a change, authorised by the customer, to the Baseline
Build Standard (BBS) of an item of the weapon system, including
hardware, software and support material. It is considered a
modification if it affects the fit, form or function of the weapon
system.
Considered monitored if there is a fleet wide load monitoring
procedure in place which records the fatigue consumption on an
individual A/C or component basis. Traditionally, fatigue
consumption for monitored structure is determined using Fatigue
Meter Formulae (FMFs), which relate A/C normal acceleration in
service to the results of MAFT.
Term for aircraft previously known as: COM - Military type aircraft
not in employment of the crown, often used for flight testing of
military aircraft prior to expor COM- Aircraft usually of civil design
and qualification, employed on contractorised tasks (eg flying
training). Airworthiness of the aircraft is maintained to MOD
regulations by a MOD appointed In-Service PT with CAA
oversight. (RA1121.) (MAA02 Issue 5)
Refer to MAA02.
UK validation programme for monitoring in service strains on
helicopter components with strain gauges, together with detailed
flight parameters such as accelerations, roll rates, turn rates, side
slip.
Refer to MAA02.
(Sometimes referred to, incorrectly, as Operational Loads
Monitoring). An OLM programme is designed to capture the
actual in service fatigue usage of an aircraft for comparison with
the assumptions made in design and qualification, particularly the
major fatigue tests of components and the airframe. It normally
covers both monitored structure, where the aim is to validate the
normal fleetwide fatigue monitoring process, and unmonitored
structure which is normally cleared simply in terms of FH,
landings etc. Strain gauges bonded to the surfaces of metallic
components give electrical outputs, which are recorded and
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Term
Out-of-Service Date
Abbreviation
OSD
Overload
Definition
converted into strain. In addition other parameters are recorded.
On some programmes, the airframe/component may be calibrated
in a loading rig in order to determine actual loads,(including
bending moments and torque).
The point at which an aircraft or system is planned to leave
service.
Situation in which a structure is subjected to a single high load
that exceeds its limit strength. If proof strength is exceeded the
structure may be permanently deformed and if the higher ultimate
strength is exceeded there is a high risk of structural failure. The
actual strength (proof, ultimate) is not often known precisely, it is
the CLEARED STRENGTH that should be known. The airframe is
cleared to its req’d strength, not its actual or potential strength.
So, it will not necessarily permanently deform above proof load or
fail above ultimate load.
Refer to MAA02.
Publication Authority
PA
Preliminary Design Review
PDR
Post Design Services
PDS
Project Engineer
PE
A review held with the customer during development. Any
permitted exceptions shall be detailed as design deviations.
Covers mods, development, repair, fault assessments and
investigations. (MAA02 Issue 5)
Refer to MAA02.
Protected Earth Monitoring
System
Potential Explosive Site
PEMS
Refer to MAA02.
Penetrant Flaw Detection
PFD
Production Flight Test Schedule
PFTS
Refer to MAA02.
Proof Installation
PI
Refer to MAA02.
Personal Identification Number
PIN
Point In The Sky
PITS
Permanent Joint Headquarters
PJHQ
Portable Maintenance Data
System
Plastic Media Stripping
PMDS
PMS
(NDE) Probability of Detection
POD
Point On the Ground
POG
PES
Pressurisations
Probabilistic SI approach
Production Permit
Proof Load Test
Principal Structural Element
PSE
Project Team
PT
52
Refer to MAA02.
A representative flight condition, expressed in terms of
configuration (stores etc), mass, fuel load, mach number, altitude
which design engineers use to define a particular load (flight
manoeuvres, gusts or other loading actions qv) for either static
strength or fatigue analysis.
Refer to MAA02.
Typhoon term for the data transfer memory module used to get
data from aircraft to the GSS (qv).
Method of removing surface finish by directing a controlled stream
of plastic beads at the surface to wear finish away.
Faults present in materials cannot always be detected using Non
Destructive Testing. A statistical approach can be used to
describe the likelihood, with a specified confidence level of finding
a specified flaw within a specified structural configuration.
A representative condition, expressed in terms of configuration
(stores etc), mass, fuel load, prior to take-off and post landing
which design engineers use to define a particular load (ground
manoeuvring or other loading actions qv) for either static strength
or fatigue analysis.
The number of loading cycles caused by pressurising the cabin or
cockpit above the external pressure either when flying at altitude
or during maintenance on the ground. A combat aircraft operating
a HI-LO-HI sortie may experience more than one pressurisation
per sortie.
An approach which aspires to consider the full range of probable
strengths of a structure with the full range probable loads applied
to it to determine the probability of the structure surviving (or
failing). This is emerging technology. Cf Deterministic.
Requested when it is obvious before manufacture that the product
will not be to the actual spec. For example, when producing old
products an alternative component may have to be used if it has
become obsolescent. On occasions an alternative raw material
may be used.
A Proof Load test may be carried out on the MAST to
demonstrate the Proof Strength req’t, ie no permanent
deformation).
A basic, functional feature within an airframe structure: for
example, wing carry-through structure or undercarriage back-up
structure. Each PSE contains a number of sub-elements or SSIs
that are located in the same region and subjected to similar stress
fields and loading spectra.
Replacement term for Integrated Project Team (IPT) which,
between 2000 & 2008, was the body responsible for managing a
project from Concept to Disposal. (MAA02 Issue 5)
Structural Integrity Handbook Issue 1.1
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Term
Abbreviation
Definition
Project Team Leader
PTL
Refer to MAA02.
Quality Assurance
QA
Refer to MAA02.
Quality Audit
QA
Refer to MAA02.
Quality Assurance Representative
QAR
Refer to MAA02.
Qualified Aircrew Instructor
QFI
Refer to MAA02.
Qualified Helicopter Instructor
QHI
Refer to MAA02.
Quality Management System
QMS
Refer to MAA02.
Quality Occurrence Report
QOR
Refer to MAA02.
Qualification Programme Plan
QPP
BAE SYSTEMS term related particularly to the fatigue life
qualification work on Tornado (PDT qv 1283 etc).
Refer to MAA02.
Quality System Coordinator
QSC
Quality System Owner
QSO
Refer to MAA02.
Qualification
Verification that a system meets its design specification.
Qualification Life
Amount of safe life demonstrated by test, calculation or other
means against a specified design loading requirement.
Regulatory Article
RA
Radiation Hazard
RADHAZ
Rainflow Stress Cycle Counting
Repair assessment Programme
RAP
Residual Current Device
RCD
Reliability Centred Maintenance
RCM
Repair and Design Liaison Office
RDLO
Request for Engineering
Information/Instruction
Residual (Static) Strength Test
REI
Residual Strength
Reserve Factor
RF
Refer to MAA02.
Rainflow stress cycle counting is the most common and practical
form of stress cycle counting. Rainflow counting is used to
measure the likely impact of the most damaging stress cycles in a
sequence that combines load reversals in a manner that defines
cycles by closed hysteresis loops. The rainflow counting
technique uses a plot with strain time history plotted vertically
downward on a time axis. The history is plotted so that the largest
strain magnitude occurs as the first and last peaks or valleys, with
the lines connecting strain peaks ending up looking like a series
of ‘pagoda roofs’. The fatigue damage is then counted using a
series of conventions, by counting the ‘rainflow’ of drops falling
from the roof of the pagoda.
A programme of identifying and then assessing the repairs
carried out on an aircraft structure, from both a static strength and
damage tolerance perspective, to ensure continued structural
airworthiness.
Refer to MAA02.
The systematic approach for identifying preventive maintenance
tasks for an equipment or item in accordance with a specified set
of procedures and for establishing intervals between maintenance
tasks [with minimum expenditure of resources] (DEF STAN 0040).
Note: Outside the MAE, RCM is commonly known as
Maintenance Steering Group (MSG) logic. (MAA02 Issue 5)
Refer to MAA02.
Now Repair Instructions and Aerospace Equipment Design
Requests (MAP-01 Ch 9.13.2)
At the cessation of a successful Full Scale Fatigue Test (qv), an
aircraft may be subjected to a design static loading case, usually
wing-up, nose down bending, to confirm that the structure will still
sustain a percentage (for example 80%) of the ultimate load (qv).
The static strength of a structure (or component) after a specified
amount of usage or degradation.
The ratio of the failure stress of a component to the stress at
ultimate load (Normally 1.5 x Limit Load). Ideally, this factor is 1.0
for minimum weight but other factors such as AD/ED (qv) may be
more important.
Refer to MAA02.
Royal Fleet Auxiliary
RFA
Regulatory Instruction
RI
Regulatory Notice
RN
Regional Non-Destructive Testing
Team
Resident Project Officer
RNDTT
Refer to MAA02.
RPO
Obsolete. Refer to MAA02.
Repair and Salvage Unit
RSU
Routine Technical Instruction
RTI
Refer to MAA02.
Release to Service
RTS
The release document that authorizes Service flying on behalf of
the Service Chief of Staff. The RTS refers to the Safety
Assessment documentation for the aircraft or equipment,
including the limitations and aircraft description, and defines the
as-flown standard of the aircraft. (MAA02 Issue 5)
SI Glossary Version 1.1
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Term
Abbreviation
Definition
Remote Viewing Aid Equipment
RVA
Refer to MAA02.
Rotary Wing
RW
Support Authority
SA
Senior Aircraft Engineer Officer
SAEO
Safe Life
Service Amendment Leaflet
SAL
Station Administrative
Management Aid
Sampling
SAMA
Structural Airworthiness
SAw
Scatter
Structural Configuration Control
SCC
Stress Corrosion Cracking
SCC
Obsolete RAF term. Refer to MAA02.
The period during which the risk of unexpected structural collapse
of an aircraft's structure, determined by statistical prediction, is
deemed to be acceptably small. The life depends on both the
flying hour rate of the aircraft and the severity of its usage. In the
UK, the 2 parameters are combined, for convenience, into the
single non-dimensional parameter Fatigue Index (qv). Thus, the
Safe Life is expressed as a specific number of Fatigue Index units
or 'FI'. Other operators, notably the USN, periodically update the
expected Flying Hours to life expiry using the most recent
structural usage data. Tornado is designed to safe life principles,
that is, fatigue tests using a specified design loading spectrum,
and appropriate scatter factors to produce sufficient confidence
not to require dedicated structural inspections to maintain safety.
Safe life limitations can also be expressed in terms Flying Hours
and/or Landings.
Refer to MAA02.
Normally taken to mean structural sampling, a programme to
validate that design assumptions and sustaining measures
applied in service use are adequate by examining parts of
structure that were assumed not to need routine examination, on
a sample of fleet.
Refer to MAA02.
Statistical term describing the spread of data points. Often used in
context of fatigue or strength testing or fatigue lives.
The maintenance of effective control of the approved
configuration of materiel (applying to aircraft structure).
This is cracking in a metallic material originating at a site
subjected to continual tensile stress, which is susceptible to
corrosion and which is subjected to the agents of corrosion. The
tensile stress may result from externally applied loads or may be
residual, arising from the metal working processes during
manufacture. Not to be confused with corrosion fatigue (qv).
Supply Control Centre System
SCCS
Senior Civilian Engineer
SCE
Refer to MAA02.
Structural Control Point
SCP
Service Deviation
SD
The particular SSI or feature within a PSE that represents the
lead fatigue feature and for which the structural examination
requirement would be most frequent. By examining the SCP only,
the condition of all sub-elements within the PSE can be inferred.
(MAA02 Issue 5)
Obsolete. Refer to MAA02.
Strategic Defence and Security
Review
Station Engineering Management
Aid
SDSR
Structural Examination
Programme
SEP
SEMA
Service Provider (or Support
Provider)
Simulated Flight Hours
SFH
Special Flying Instruction
SFI
Serious Fault Signal
SFS
Safety, Health, Environmental and
Fire
Structural Health Monitoring
SHEF
Obsolete. SEMA was an early information technology system
designed to manage selected engineering functions for Phantom
and Tornado aircraft.
Set of measures including directed inspections (sustaining) and
opportunity inspections and Teardowns (validating) that
underwrite continued SI for in service aircraft.
The company/group delivering a contracted service or capability
to the MOD.
Refer to MAA02.
SHM
Shot Peening
Servicing Instruction
SI
Process during manufacture of metallic parts in which residual
tensile stresses at the surface are relieved by blasting the item
with lead shot or other media thereby reducing the susceptibility
to surface fatigue cracking.
Refer to MAA02.
Significant Item
SI
Refer to MAA02.
Structural Integrity
SI
The ability of an aircraft structure to retain its strength, function
and shape within acceptable limits, without failure when subjected
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Term
Abbreviation
SI Plan
Special Instruction (Technical)
SI(T)
Strategic/Source Intelligence
Fusion Testbed
Structural Integrity Strategy
Document
SIFT
Structural Integrity Working Group
SIWG
SISD
SLAP/SLEP
SLE
Service Modification
SM
Schedule Maintenance Agency
SMA
Definition
to the loads imposed throughout the aircraft’s service life by
operation within the limitations of Release To Service (RTS) and
to the usage described in the Statement of Operating Intent (SOI)
or the Statement of Operating Intent and Usage (SOIU). Refer to
MAA02 Issue 5.
Plan forming part of the TLMP in which the key structural
management events are marked to allow for coordination and
budgeting.
An instruction, issued by the PT, that gives instructions to
undertake a work package to repair or prevent a potential fault.
There are 4 types of SI(T): STI, SI, UTI and RTI. (MAA02 Issue 5)
The document that communicates the Structural Integrity strategy
and methodology and records historic structural integrity
actions/events employed. The document covers the strategic SI
actions and contains the SI plan and the explanation behind the
actions contained within it.
Meeting between all SI stakeholders including PT, Designer, Air
staff and Specialist Structural adviser to address SI issues and
obtain TAA endorsement that the approach to SI assurance on a
specific aircraft type is being satisfactorily undertaken. Usually
held twice a year and chaired by an LoAA holder at OF4/B2 level.
Service Life Assessment Programme/Structural Life Extension
Programs (US) by Boeing and McDonald Douglas.
Structural Life Extension programme for the Tornado.
Refer to MAA02.
Subject Matter Expert
SME
Refer to MAA02.
Safety Management System
SMS
Refer to MAA02.
S-N Diagram (or Curve)
To determine the fatigue strength of materials under the action of
fatigue loads, specimens are subjected to repeated or varying
forces of specified magnitudes while the cycles or stress reversals
are counted to destruction. Due to the statistical nature of fatigue,
a number of tests are completed. The results are plotted on an SN Diagram. The ordinate of the S-N diagram is called the Fatigue
strength; a statement of this strength must always be
accompanied by a statement of the number of cycles N to which it
corresponds.
That structure whose failure would cause direct loss of the aircraft
or whose failure, if undetected, would result in loss of the aircraft.
(US Joint Services Specification Guide 2006, though with “air
platform” substituted by “aircraft” in UK)
Used at the project inception stage to formally describe and
communicate the aircraft’s expected usage to the Designer.
The aircraft Topic 15S, the SOIU is a narrative and graphical
description of the expected and/or actual usage of an aircraft
type. Specific types of sorties are summarised in the SPCs (qv)
and the purpose of the SOIU is to provide Designers, aircrew and
engineers with information on which to base fatigue management
decisions. Part of the aircraft document set used to underpin
airworthiness as set out in the platform safety case.
Refers to modifications applicable to a limited range of ac tail
numbers. Not unique to SI work.
UK industry term used to define the specific actions expected of
the company in order to fulfill a task. Normally drafted by the
company who will undertake the work for approval by the tasker.
Not specific to SI work.
A code number used within the MOD to identify a specific type of
sortie. Among many uses, SPCs have been used to develop the
fatigue formulae for aircraft with fatigue meters, the SPC
continues to provide a useful summary of generic sortie types and
are defined in the SOIU (qv).
A fabrication technique used in the manufacture of the Eurofighter
Typhoon Titanium foreplane.
Safety of Flight Structure
SofFS
Statement of Operating Intent
SOI
Statement of Operating Intent and
Usage
SOIU
Special Order Only
SOO
Statement of Work
SOW
Sortie Profile Code
SPC
Superplastic Formed/Diffusion
Bonded
Support Policy Statement
SPFDB
Service Engineered Aircraft Radio
Installation Modification
SRIM
Obsolete. Refer to MAA02.
Structural Significant Item
SSI
Any detail, element or assembly, which contributes significantly to
carrying flight, ground, pressure or control loads and whose
failure could affect the structural integrity necessary for the
continued safe and controlled flight of the aircraft. (MAA02 Issue
5, see also AP 100C-22, MSG-3 and MASAAG 103B)
SI Glossary Version 1.1
SPS
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Term
Abbreviation
Definition
Structural Sampling Programme
SSP
See Sampling.
System Security Procedures
SSP
Refer to MAA02.
Standard Serviceability Test
SST
Refer to MAA02.
Static Overload
See Overload.
Static Strength
The static load at which a structure fails.
Static Test
See MAST.
Strike Command
STC
Special Trial Fit
STF
Special Technical Instruction
STI
Stochastic
Static Type Record
A time dependent variable often associated with statistics.
STR
Strain
Strain Gauge
Strength Envelope
Stress Rupture Fracture.
Structural Configuration Control
Database
Structural Sampling (Scheduled,
Opportunity)
Structure
Special-to-Type Test Equipment
STTE
Support Authorities
Properly known as the Type Record (qv) but sometimes “Static”
used to differentiate from Fatigue Type Record (qv).
Engineering term describing the ratio of the change in length of a
loaded component to it’s original, unloaded length.
A device based a wheatstone bridge that has an electrical current
running through it, the device is bonded to the structure and as
the structure deforms due to the application of load the electrical
resistance of the gauge changes. The changes in resistance are
then recorded, from which the deformation and strain can
determined and subsequently the stress in the structure.
Description of the limitations on an aircraft’s static strength and
usually portrayed graphically by normal acceleration, altitude and
velocity boundaries.
Fracture of a material due to prolonged exposure at a high
temperature.
A database on which the structural configuration, in terms of
repairs, corrosion, damage, modifications etc of individual aircraft
can be recorded.
See Sampling.
Aircraft structure consists of all load-carrying members including
wings, fuselage (including some transparencies), empennage,
engine mountings, landing gear, flight control surfaces and related
points of attachment, control rods, propellers and propeller hubs if
applicable and, for helicopters: rotor blades, rotor heads and
associated transmission systems. The actuating portion of items
such as landing gear, flight controls and doors must be subject to
System Integrity Management regulation (RA 5721) as well as
Structural Integrity Management regulation. (MAA02 Issue 5)
Refer to MAA02.
Obsolete. The original multi-disciplinary group (MDG),
incorporating air, engineering, supply, finance, contracts and Post
Design Services staffs which provided all aspects of support
management for a particular ac, weapons system or range of
equipment. Nominated engineer staff, the Engineering Authority
(EA), was responsible for exercising engineering judgement wrt
airworthiness. (MAA02 Issue 5)
See Service Provider.
Support Provider
Severe Wind And Moisture Prone.
SWAMP
Refer to MAA02.
Service Working Party
SWP
Refer to MAA02.
Security Operating Procedures
SyOPs
System
Type Airworthiness Authority
TAA
Typed Air Station
TAS
Teardown
Technical Log
56
Refer to MAA02.
A combination of physical components, procedures and human
resources organised to achieve a function. The system organised
to achieve the function of SI is the aircraft Structure. (MAA02
Issue 5)
The Type Airworthiness Authority is the individual, often an
aircraft PTL, who on behalf of the Secretary of State for Defence,
oversees the airworthiness of specified air system types. As the
TAA the PTL responsibilities are as laid down and agreed in their
Letter of Airworthiness Authority from their respective Director.
(MAA02 Issue 5)
Refer to MAA02.
Usually termed a Structural Teardown, this is a destructive
dismemberment of a Structure during which key joints are
carefully disassembled and key features are inspected for
previously unknown degradation from fatigue, corrosion,
delamination, disbonding.
Civilian equivalent of Form 700 incorporating F724/725 data and
fault rectification information similar to MOD F707A.
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Term
Abbreviation
Definition
Temporary Earth Electrode
TEE
Refer to MAA02.
Technical Enabling Services
TES
Obsolete. Refer to MAA02.
Test Factor
Sometimes called the Life Factor or Safety Factor. A factor
applied to the results of a test to allow for uncertainty and to
introduce an acceptable level of confidence when the factored life
is imposed on in-service structures. Various figures are used in
practice depending on national practice, the uncertainty of loads
applied in service and/or whether the structure’s usage is
monitored or unmonitored in service.
The life (to failure or end of test) of a component or whole aircraft
as measured in a representative fatigue test where no Safety
Factors are considered.
The 4 threat groups to SI are: Overload; Accidental and
Environmental Damage; Fatigue, fretting and wear; and
Maintenance/Supply Error.
Test Life
Threats to SI
Technical Information
TI
Trial Installation
TI
Technical Information sponsor
TI sponsor
Refer to MAA02.
Top Level Budget
TLB
Refer to MAA02.
Test and Measuring Equipment
TME
Trade Qualification Annotation
TQA
Turn Round
TR
Tail Rotor Blade
TRB
Type Record
Refer to MAA02.
Unmanned Air Vehicle Air Vehicle
UAV
UK mandatory summary of the structural design of an aircraft with
the loading assumptions and Reserve Factors for all Structure.
Definition of which structure to include has been interpreted
differently by different manufacturers over the years but intention
is that SSIs qv are included as a minimum.
Obsolete. Refer to MAA02.
Uninstalled Engine Test Facility
UETF
Refer to MAA02.
Unsatisfactory Feature Report
UFR
Refer to MAA02.
Urgent Technical Instruction
UTI
Safety factor between the Limit Load and the Ultimate Load.
Conventionally, Ultimate was 1.5 time Limit Load but this factor
has been lowered to 1.4 for some parts of the Eurofighter
Typhoon structure because of the constraints to pilot input
imposed by flight control software. It can be and is used wherever
it can be shown that a system will control a load eg A pressure
relief valve maximum value could be used with 1.4.
Load on the structure at Limit Load x Ultimate Factor and at which
structural failure would normally occur with loss of SI.
A material property giving the stress at which the material will fail.
Usually qualified by terms “tensile”, “compressive” or ”buckling” to
indicate mode of failure.
Structure for which there is no fleet wide load monitoring
procedure in place for UK military aircraft. Therefore, generally,
structure which is affected by loading actions other than normal
acceleration is considered as unmonitored structure. Unmonitored
structure is qualified by a mixture of MAFT, separate fatigue tests
and also by calculation.
Comparative measure of the fatigue life consumed by component
in service with that for a component on a fatigue test. Needed
when flight data indicates that the stresses measured in the test
specimen are unrepresentative of those in Service for the
equivalent flight condition.
Refer to MAA02.
Very Light Aircraft
VLA
Ultimate Factor
Ultimate Load
Ultimate Strength
Unmonitored Structure
Usage Factor
Vne
Weibull Distribution
Widespread Fatigue Damage
SI Glossary Version 1.1
WFD
Never exceed velocity – specified because at greater speeds the
aerodynamic loading would exceed limit loads in parts of the
structure, unstable aeroelastic phenomena would develop or flight
control would become ineffective.
Statistical distribution defined by 4 parameters, sometimes used
instead of Log Normal distribution to describe fatigue life against
probability of failure for a large population of test samples. Weibull
distribution is asymmetric and for fatigue lives will assume zero
probability value at time zero whereas Log Normal distribution has
a non zero value at time zero.
WFD in a structure is characterized by the simultaneous presence
of cracks at multiple structural details that are of sufficient size
and density whereby the structure will no longer meet its damage
tolerance requirement.
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Term
Abbreviation
Work Recording and Asset
Management
WRAM
58
Definition
Structural Integrity Handbook Issue 1.1