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SERVICE LETTER
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
ATA: 1230-10
14 August 2017
SUBJECT:
DEICING/ANTI-ICING FLUID RESIDUES CAUSING FLIGHT
CONTROL RESTRICTIONS
MODEL:
707, 727, 737, 747, 757, 767, 777, and 787 Series
APPLICABILITY:
All 707, 727, 737, 747, 757, 767, 777, and 787 airplanes
REFERENCES:
a) Boeing Multi-Model Service Letter (SL) 737-SL-12-014, dated
11 January 2000
b) Society of Automotive Engineers Aerospace Recommended Practice
(SAE ARP) 4737, Revision H
c) Aircraft Maintenance Manual (AMM) Chapter 12-33-01
EXPORT COMPLIANCE STATEMENT:
Export of this technology is controlled under the United States Export Administration Regulations
(EAR) (15 CFR 730-774). An export license may be required before it is used for development
or production, or used by foreign persons from specific countries. The controller of this data has
the individual responsibility to abide by all export laws.
ECCN: 9E991
SUMMARY:
Dried residues from thickened deicing/anti-icing fluids can rehydrate and form into a gel-like
substance that can freeze during flight and cause restrictions to the flight control systems.
Therefore, attention to these residues should be part of a regularly scheduled inspection and
cleaning process. This service letter advises operators about deicing/anti-icing fluid residues and
provides instructions on where to inspect for, and how to clean, the residues from the airplane. It
also advises of revisions to the AMM chapters for deicing/anti-icing operations. Boeing
recommends periodic review of this information by operators to ensure that their procedures are
current and reflect the latest information and regulatory requirements.
BOEING PROPRIETARY
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
14 August 2017
Page 2 of 5
BACKGROUND:
Boeing has received several previous reports where operators have experienced restrictions in the
airplane flight controls due to rehydrated deicing/anti-icing fluid residues that froze during flight.
The reference a) SL previously advised operators about some of these events, and about the
potential for flight control problems due to fluid residues. The service letter also advised of a new
caution note that had been added to the reference b) SAE ARP 4737 “Methods Document” at that
time. The same note was also added to the reference c) AMM chapters for cold weather and
deicing/anti-icing operations at that time.
DISCUSSION:
During recent winter seasons there have been many reports of flight control restriction events on
various airplanes operating throughout the European Region. Most of these events have involved
regional and commuter size airplanes, while a few have been on small commercial jetliners, such
as the 737, MD-80 and MD-90 airplanes. These events have been caused by deicing/anti-icing
fluid residues that rehydrate into a gel-like substance and freeze during flight. The events have
occurred on airplanes that have both hydraulic and non-hydraulic powered flight control systems.
In several cases it was determined that the residues had been generated from fluids that were
applied during previous winter seasons.
The flight control restriction events are more common on small or medium-size short-haul
airplanes because, during severe winter weather, these airplanes may receive many fluid treatments
every day, increasing the possibility of deicing/anti-icing fluid residues accumulating. Larger
long-haul airplanes do not receive as many fluid treatments, and therefore have not incurred
problems due to fluid residues. However, service experience has shown that the residues can
accumulate over time, so it is possible for residue problems to occur on large airplanes.
A greater number of deicing/anti-icing fluid residue problems have occurred in Europe compared
to North America and Asia. Industry experts agree that one of the reasons for this is the difference
in deicing/anti-icing practices between the continents.
In Europe, a one-step deicing/anti-icing process is commonly used. This process involves the
application of de-icing/anti-icing fluid in a single application, using a heated mixture of Type II
fluid and water, usually in a ratio of 75/25. When this process happens repeatedly there can be a
significant buildup of residues in the aerodynamically quiet areas of the wings and stabilizers.
In North America, a two-step process is commonly used. This process involves deicing with
heated Type I fluid, or a heated mixture of Type I fluid and water, which is followed by an
application of Type IV anti-icing fluid. Experience and testing has shown that deicing with heated
Type I fluid will help clean away residues from previous anti-icing fluid treatments.
Export Controlled ECCN: 9E991
BOEING PROPRIETARY
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
14 August 2017
Page 3 of 5
Investigation has also shown that the volume of fluid and the application technique can be a
contributing factor in the formation of residues. It is important for operators and/or their service
providers to ensure that all deicing/anti-icing fluids are being stored and handled properly in
accordance with the fluid manufacturers’ recommendations. Improper storage and usage could
result in degraded fluid performance, or the use of greater volumes of fluid, which could contribute
to the formation of more residues. Also, spraying fluid from the rear of the wing or stabilizer,
rather than from the front which is the correct method, might result in more fluid entering the flight
control areas through the control surface vent gaps.
Research has shown that the interaction between airplane deicing/anti-icing fluids and runway
deicing fluids may contribute to the formation of residue gels. When these fluids combine, the
salts of the runway fluids will enhance the separation of the polymer thickening agents of the
airplane fluids, leading to a more rapid formation of the gel residue. The fluids can mix together
when the thrust reversers send fluids from the runway onto the wing during the landing roll. The
fluids can flow into the rear spar areas, through the control surface vent gaps, where airplane fluids
may remain from previous applications.
Due to the heightened awareness of the deicing/anti-icing fluid residues throughout the industry,
the revision H of the reference b) “Methods Document” includes revised wording in the caution
note in section 6.3.1.2. This note is included here. The italics show the words that are being added.
Caution: The application of Type II, III and IV fluid, especially when used in a one-step
process or in the first step of a two-step process, may cause residues to collect in
aerodynamically quiet areas, cavities and gaps. The application of hot water or heated Type
I fluid in the first step of a two-step process will minimize the formation of residues. Residues
may rehydrate and freeze under certain temperature, high humidity and/or rain conditions and
may block or impede critical flight control systems. If a Type II, III or IV fluid is used in a
one-step process or in the first step of a two-step process, then an appropriate inspection and
cleaning program shall be established, dependent on the operator’s experience and fleet type.
Whenever suitable, deice and anti-ice with only Type I.
BOEING ACTION:
The Boeing airplane maintenance manuals for cold weather and deicing/anti-icing operations have
been updated (except for 787 which is being investigated) to add more inspection and cleaning
information for residues. Additionally, Boeing participated in the industry working groups on this
subject. These include the “Residues Working Group”, which is a sub-group of the SAE G-12
Aircraft Ground Deicing “Fluids” subcommittee, and the European Regions Airline Association
“Winter Operations Workshop”. These working groups involve industry experts from airlines,
airports, service providers, airplane manufacturers, deicing/anti-icing fluid manufacturers, and
safety and regulatory agencies. The working groups recommended deicing/anti-icing fluid
modifications to reduce the possibility of residual fluid buildup. The recommendations were
Export Controlled ECCN: 9E991
BOEING PROPRIETARY
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
14 August 2017
Page 4 of 5
incorporated into an update of SAE Aerospace Material Specification (AMS) 1428
SUGGESTED OPERATOR ACTION:
We recommend that operators accomplish inspection and cleaning of deicing/anti-icing fluid
residues at appropriate intervals for all airplanes that are exposed to these fluids during winter
operations. Operators should be aware that failure to regularly remove these residues may degrade
the airplane performance by inhibiting proper operation of the flight control systems.
The frequency of the inspections should be based on each operator’s experience during winter
operations. Boeing recommends that all airplanes that have been exposed to de-icing/anti-icing
fluids should be inspected and cleaned prior to the winter season, and at the end of the winter
season. Boeing also recommends that during the winter season, each airplane should be inspected
and cleaned no less than once per month.
This proposed frequency is based upon information from operators that have experienced multiple
occurrences of flight control problems due to deicing/anti-icing fluid residues. Some of these
operators are performing inspections more frequently than once per month, and Boeing encourages
operators to inspect as frequently as practical until sufficient data has been collected to more
accurately define the inspection period.
The attachment to this service letter includes our recommended inspection and cleaning
procedures. However, operators may wish to refine these procedures based on their own service
experience. The following considerations should be part of your decision process:
•
•
•
Be aware of how frequently your airplanes are being deiced/anti-iced.
Be aware of what deicing/anti-icing fluids are being applied to your fleet. Is the fleet
undergoing a one-step or two-step process? Is the process the same at all airports?
Make sure that proper procedures are being followed by airline personnel or a third-party
service provider. Are the fluids being stored and handled properly, and are they being applied
properly?
Include the application of lubricants and corrosion inhibitors as necessary to the areas where
residue cleaning occurs.
WARRANTY INFORMATION:
Boeing warranty remedies are not available for the inspection and maintenance procedures given
in this service letter.
Export Controlled ECCN: 9E991
BOEING PROPRIETARY
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
14 August 2017
Page 5 of 5
RELATED INFORMATION:
The Air Accidents Investigation Branch (AAIB) in the UK issued four safety recommendations in
early 2006 regarding the subject of deicing/anti-icing fluid residues. In response to the AAIB
recommendations, the UK Civil Aviation Authority (CAA) issued Flight Operations Division
Communication (FODCOM) number 15/2006, which recommends specific procedures for the use
of deicing/anti-icing fluids, and for cleaning of fluid residues, on airplanes with non-powered
flying controls. Additionally, the European Aviation Safety Agency (EASA) has released Safety
Information Notice (SIN) 2006-09, issued 26 September 2006. This notice includes information
and recommendations regarding deicing/anti-icing fluid residues in section 8, “Special Operation
Considerations” and section 9, “Special Maintenance Considerations.” Please note that the SIN is
only an advisory document. EASA also released the “Advanced-Notice of Proposed Amendment”
(A-NPA) 2007-11 on 31 July 2007.
CMC / EICAS MESSAGE:
N/A
TG:nsn
Original:
Revision A:
Revision B:
Revision C:
Attachment:
Dated: 14 November 2006
Dated: 2 August 2007. To add information about interaction of runway fluids and
airplane fluids, update EASA regulatory information, and add a caution note for
residues inside the 737 elevators.
Dated: 28 July 2017. Revised to add 787 to the service letter and to update Boeing
Action section.
Revised to remove a typographical error that created confusion to some readers.
Added CMC / EICAS Message section.
Deicing/Anti-icing Fluid Residues - Inspection & Cleaning Recommendations
Export Controlled ECCN: 9E991
BOEING PROPRIETARY
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
ATTACHMENT To:
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
14 August 2017
Page 1 of 3
Deicing/Anti-icing Fluid Residues - Inspection & Cleaning Recommendations
Inspection
1. Gain access to the following areas where flight controls and other systems components are
located:
•
•
•
•
•
wing rear spar areas, including the actuating components for the
spoilers, ailerons, flaps and flaperons (if applicable), and the control
surface hinges and balance bays
wing leading edge devices, including the actuating components
the horizontal stabilizer rear spar, including the actuating
components for the elevators, elevator tabs (if applicable) and the
control surface hinges and balance bays
vertical stabilizer, including actuating components for the rudder,
and the control surface hinges
the APU Bay and the bilge area of the tailcone
2. Visually inspect for the presence of dry or rehydrated residues anywhere in these areas. The
residue may be very hard to see, especially if dry. Dry residue will normally be a thin film that
may be partially covered with dirt or grease. Rehydrated residue will be a gel-like substance of
more visible thickness.
Note: It may be necessary to use a flexible borescope to inspect inside the elevator panels on 737
airplanes. Fluids can get inside the elevators where the tab control rods extend through the elevator
forward spar. The fluids can migrate along the elevator inner rear spar and form residues that can
cause an unbalance condition of the elevator. The elevator may have to be removed from the
airplane to clean out the residues.
3. Spray the area with a fine mist of warm water to rehydrate any residue that may be present and
to make it easier to identify. In some cases rehydration may occur quickly, but the process often
may be slow, especially if residue has accumulated from multiple applications over a long period
of time. Wait at least 15 minutes to allow rehydration to take place.
4. If no rehydrated residue is visible, repeat this step at least three more times, if practical,
including the wait time of 15 minutes to allow rehydration to take place. This recommendation to
perform repetitive spraying and wait for rehydration to occur is based on the experience of several
operators during the previous two winter seasons.
Do not spray the flight control areas with water when the ambient temperature is below freezing
unless the airplane is in a heated hangar. Doing so may result in ice that impairs the flight controls.
BOEING PROPRIETARY
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
ATTACHMENT To:
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
14 August 2017
Page 2 of 3
Cleaning
Once identified, the residues should be removed by using warm water with rags and/or soft brushes
to hand clean the gel-like substances away. You may also use a low pressure stream of water or
compressed air to rinse away the residues. Make sure the water or compressed air stream does not
cause the residues to enter crevice areas that are not accessible.
Testing and experience have shown that the use of Type I deicing fluid, or a mixture of water and
Type I fluid is also a good cleaning agent for removal of residues. Test data indicates that use of
a detergent additive with water may actually reduce the cleaning effectiveness.
The cleaning process has the potential of removing grease from control system bearings, fittings
and cables, and removing corrosion inhibiters from various areas. Care should be taken to avoid
spraying cleaning fluids onto bearings, fittings, control cables and electrical connectors.
The cleaning process also has the potential to wash the residue into other areas, where it may
deposit and create a future problem. Runoff from the cleaning process should not be left to
contaminate other areas of the airplane, and these areas should be flushed until the operator is
confident that any de-icing/anti-icing fluid residue has completely left the airplane.
Make sure all the drain holes are open and clear. This includes the elevator balance panel area,
wing and stabilizer rear spar areas and any area where access panels were removed or replaced.
Do not spray the flight control areas with water when the ambient temperature is below freezing
unless the aircraft is in a heated hangar. Doing so may result in ice that impairs the flight controls.
Relubrication
If residue has been found and removed by cleaning, we recommend that all bearings, fittings and
control cables in the area that was cleaned should be relubricated in accordance with appropriate
AMM instructions as necessary.
Any areas where corrosion inhibiting compound (CIC) has been removed or depleted by the
residue cleaning process should be retreated per the appropriate standard overhaul practices
manual (SOPM) procedures.
Follow-up actions
Operators have reported that it can be very difficult to remove all residues with a single cleaning,
and that residues may slowly migrate out of crevices after it is removed from open areas by
cleaning. During the pre-winter cleaning, Boeing recommends re-inspection within three days of
any areas from which residue has been cleaned. If residue is found, additional cleaning is
recommended. The inspection and cleaning process should be continued on a frequent basis until
no additional residue is found. These recommendations are based upon the experience of several
Export Controlled ECCN: 9E991
BOEING PROPRIETARY
707-SL-12-016-C
727-SL-12-016-C
737-SL-12-019-C
747-SL-12-016-C
ATTACHMENT To:
757-SL-12-015-C
767-SL-12-019-C
777-SL-12-007-C
787-SL-12-007-C
14 August 2017
Page 3 of 3
operators during previous winter seasons. However, the recommendations are also dependent on
the specific schedules for each airplane and the amount of deicing/anti-icing treatments being
encountered. Many of these operators have developed their own maintenance programs to remove
existing deicing/anti-icing fluid residue based on their own data.
Export Controlled ECCN: 9E991
BOEING PROPRIETARY