MILITARY AIRWORTHINESS
ACCEPTABLE LEVEL of SAFETY
Bob Wojcik
Air and Naval Technology
General Dynamics Canada
Ottawa, Ontario, Canada
Email bob.wojcik@gdcanada.com
26-29 September 2005
1
“If you are looking for perfect safety, you will
do well to sit on a fence and watch the birds;
but if you really wish to learn, you must mount
a machine and become acquainted with its
tricks by actual trial.”
-Wilbur Wright, 18 September 1901
2
“There are two critical points in every
aerial flight – its beginning and its end.”
-Alexander Graham Bell, 1906
3
Outline
 Background
 Safety Definitions
 What is an Acceptable Level of Safety?
 How Safe is Safe?
 Acceptable Level of Safety – Civil
Aircraft
 Acceptable Level of Safety – Military
Aircraft
 Conclusion/Recommendation
4
Background
 Military authorities have always been
interested in aviation safety
 Many military authorities are
introducing formal Airworthiness
Programs
 International Military Aviation Authority
Conference 22-23 June 2004
Common theme – need for military
airworthiness regulatory authority
Many programs are modeled on civil
aviation safety programs
 Lack of military airworthiness standards
has led to reliance on civil airworthiness
standards
5
Safety Definitions
 Concise Oxford Dictionary - Being safe,
freedom from danger
 MIL-STD 882 – Freedom from those
conditions that can cause death, injury,
occupational illness, damage to or loss
of equipment or property, or damage to
the environment
 FAA System Safety Handbook –
Freedom from all forms of harm.
 British Standard 4778 – The freedom
from unacceptable risks of personal
harm
6
What is an Acceptable Level of Safety?
 A relative concept based on freedom
from danger or risk
 Involves consideration of:
Severity of the effect
Certainty of the occurrence
Reversibility of the effect
Knowledge or familiarity of the risks
Voluntary acceptance of the risk
Compensation for the risk
Advantages of the activity
Risks and advantages of the alternatives
7
Safety Targets - Determination
 Consider all consequences including
both risks and benefits
 Acceptance by both individuals and
societies in general
 Precedent of other regulatory
organizations
 What is reasonable and practical
An acceptable level of safety could be defined as
the point when the benefits outweigh the risks
from either an individual or a society
perspective.
8
Regulatory Authorities
 Airworthiness Regulatory Authorities
conduct risk-benefit tradeoffs and
decide what level would be acceptable
considering:
Requirements
Impact on industry
Technology available
Input from stakeholders including public
interest organizations
Action by other regulatory agencies
9
What Is Safety?
 From a technical perspective Safety is a
design attribute which is part of the
overall development process.
 Safety properties:
Safety has no absolutes
Safety is non deterministic
Accident rates are generally very small
10
How Safe Is Safe?
Probability of Deaths
per year
1 in 100
Activity
five hours of solo rock
climbing every weekend
1 in 5,000
work in the UK coal mining
industry
1 in 50,000
taking the contraceptive pill
1 in 500,000
passenger in a scheduled
airline
1 in 1 million
electrocution in the home
1 in 10 million
Lightning in the UK
Source: UK MOD “What is safety”
11
How Safe Is Safe? -Transport
Mode of
Transport
Accident rate
per 100,000
hours
0.7
Fatality Rate
3.7
0.4
Rail Travel2
0.06
0.02
Marine2
1.9
0.53
0.08
0.01
Civil aircraft –
airline1
Civil aircraft commuter1
Motor Vehicles2
0.1
Source:
1 – Transport Canada 5 year average (1993 – 1997)
2 – DND/DGAEPM Airworthiness Risk Assessment Report (1996)
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Aircraft Accident Cause Factors
 Technical Causes
Airframe structural failure
Landing gear failure
Fire
Engine failure
System failure
 Operational Causes
Weather
Controlled Flight Into Terrain
Undershoot
Overshoot
13
Prevention – Technical Causes
 Structural Failure (including landing
gear) – safe life, fail safe, damage
tolerance
 Fire – fire prevention and control
technology
 Engine Failure - safe life, fail safe,
damage tolerance, health monitoring
 System Failure – fail safe, system safety
assessment process
14
Prevention – System Failures
 Largest technical cause of aircraft
accidents
 Prevention of accidents due to system
failures is one of the primary concerns
of civil airworthiness regulatory
authorities (FARs 23.1309, 25.1309 &
29.1309)
 Severity Categories
Catastrophic
Hazardous
Major
Minor
No Effect
15
Acceptable Level of Safety – Civil
 The accident rate for large civil
transport aircraft has been steadily
declining since the early 60’s
 Generally an accident rate of 1 per
million flight hours has been considered
acceptable for large civil passenger
transport aircraft
 Therefore the probability of a serious
accident should be not greater than one
per million flight hours (1 x 10-6)
16
Acceptable Level of Safety – Civil
 System failures account for 10% of
accidents (probability of occurrence of
1 x 10-7)
 100 potential failure conditions that
could have a Catastrophic effect
 Target average probability of
occurrence established as 1 x 10-9 for
each failure condition with a
Catastrophic effect
 General principle - inverse relationship
should exist between a failure condition
probability of occurrence and severity
17
Acceptable Level of Safety – Civil
Note: Civil Transport Category
Individual System
10-9
10-8
Acceptable
10-7
10-6
10-5
Unacceptable
10-4
10-3
10-2
10-1
1
Catastrophic
Negligible
Severity
18
Acceptable Level of Safety – Military
 Most military airworthiness authorities
have not published military
airworthiness design standards
 Reliance on civil regulatory material for
military type certification and design
change certification programs
Airworthiness design standards (FARs,
CARs, JARs, etc)
Associated advisory material (FAA Advisory
Circulars, RTCA DO-178B, RTCA DO-254,
SAE ARP4754, SAE ARP4761, etc)
19
Acceptable Level of Safety – Military
 Civil processes provides an excellent
basis for military aircraft programs
 Civil target levels may be problematic for
military aircraft, equipment or missions
 Military/Civil Gaps
Handling qualities
Weapons and stores
Self defence suites
Wartime operations
Military role/mission/task - operational
necessity
Operational and usage environment
Rapid advances of military technology
20
Handling Qualities
 Civil aircraft handling quality requirements do
not adequately address military tactical
role/mission/task requirements in the intended
operating environment
21
Weapons and Stores
 Civil airworthiness standards have no
equivalent to military weapons and
stores
22
Self Defence Suites
 Military aircraft operate in a hostile
environment requiring the use of chaff,
flares and other self defence technology
23
Wartime Operations
 Military wartime operations include
extremely hazardous missions under
conditions of operational necessity
24
Military Roles/Missions/Tasks
 Many military roles/missions/tasks are
unique and have no civil equivalent
25
Environment and Usage
 Military aircraft often operate in a
harsh environment which is more
severe than equivalent civil aircraft
types
26
Military Technology
 Military performance requirements
demand rapid advances in technology
which may often be implemented
before they are mature
27
Acceptable Level of Safety – Military
 Application of civil standards must be
done with judgment, care and
forethought
 Difficult to separate military mission
and airworthiness requirements
 Traditionally military equipment
qualified to performance requirements
rather than certified to minimum
essential safety requirements
 No equivalent civil standards exist for
military unique equipment
28
Acceptable Level of Safety – Military
 Civil airworthiness design standards are
generally based on a specific aircraft
category intended for use within a
defined operational environment
 A higher accident rate should be
considered acceptable for military
aircraft
 Factor of 10 is often used in comparing
a military aircraft type with an
equivalent civil aircraft type
29
Acceptable Level of Safety – Military
10-9
Military
Transports
10-7
10-5
Note: Transport Category
Individual System
Acceptable
Civil
Transports
Unacceptable
10-3
10-1
Catastrophic
Negligible
Severity
30
Acceptable Level of Safety – Military
10-9
Note: Military Aircraft Types
Individual System
Military
Transports
Acceptable
Military
Helicopters
10-7
Military
Jets
10-5
10-3
10-1
Unacceptable
Catastrophic
Negligible
Severity
31
Acceptable Level of Safety – Military
 More flexibility required for military
aviation than just defining level of safety
as acceptable / unacceptable
 UK MOD - As Low As Reasonably Possible
(ALARP)
 MIL-STD 882 - Risk Index (defined as a
function of severity and probability of
occurrence)
 DND/CF TAM Risk definitions
Extremely High Risk - Normally unacceptable
High Risk - May be acceptable
Medium Risk - Should be acceptable
Low Risk - Acceptable
32
Acceptable Level of Safety – Military
10-8
Low Risk
(acceptable)
Medium Risk
10-6
(should be acceptable)
High Risk
10-4
10-2
(may be acceptable)
Extremely High Risk
(normally unacceptable)
1
Catastrophic
Severity
Negligible
33
Conclusion
 Acceptable Level of Safety is generally
based on an acceptable accident rate
 The associated probability of
occurrence for military aircraft types
should be higher than the equivalent
civil aircraft type
 Acceptable Level of Safety for military
aircraft types may be based on a risk
assessment process
34
Recommendation
 Need a forum for military aviation
authorities to discuss airworthiness for
military aircraft types
 Defence industries need to present the
problems associated with the
application of civil standards on military
aircraft programs
 Closer cooperation/liaison between
civil and military airworthiness
authorities
35
“If we die, we want people to accept it. We are
in a risky business, and we hope that if anything
happens to us it will not delay the program. The
conquest of space is worth the risk of life.”
-Astronaut Virgil I. Grissom, 27 January 1967
Paraphrased: If we die, we want people to
accept it. We are in a risky business, and we
hope that if anything happens to us it will
not delay the program. The need for military
aviation is worth the risk of life.
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