The Office of Accident Investigation convened a Safety
Recommendation Review Board to review the enclosed responses to
your safety recommendation. As a result, the Review Board
classified your recommendation as follows:
99.231 - "Closed-Acceptable Action"
Memorandum
U.S. Department of Transportation
Federal Aviation Administration
Subject: INFORMATION: FAA Safety Recommendation Date: SEP 20
1999
99.231 - B737 MLG Trunnion Pin
This is in response to Federal Aviation Administration (FAA) Safety
Recommendation 99.23 1, regarding the trunnion pin on the Boeing 73
7 main landing gear. The attached memorandum dated September 17,
1999, from the Seattle Aircraft Certification Office (SACO)
provides a discussion on the issue raised by the Australian Bureau
of Air Safety Investigation. We concur with position taken by the
SACO and request that the safety recommendation be closed.
We thank the Australian Bureau of Air Safety Investigation for
bringing this issue to our attention.
Memorandum
U.S. Department of Transportation
Federal Aviation Administration
Sbject: INFORMATION: FAA Safety Recommendation Date: Sep 17
1999
9.231 - B737 MLG Trunnion Pin; ANM-1 14 Memo dated 7/12/99
This is in response to Federal Aviation Administration (FAA) Safety
Recommendation 99.23 1, regarding the trunnion pin on the Boeing
737 main landing gear.
Background:
On April 4, 1999, a Qantas Boeing 737-300 airplane experienced a
gear extension problem on the main landing gear during its approach
but completed a normal landing. Examination of the right main
landing gear revealed fracture of the trunnion pin actuator attach
lugs. This resulted in disconnection of the right actuator from the
landing gear assembly and caused free fall of the right gear during
extension to the down and locked position. There was minor damage
to the actuator and surrounding structure, but no damage to the
control cables or hydraulic lines. Metallurgical analysis of the
failed lugs revealed that the fracture of the lugs was attributed
to stress corrosion cracking. Inspection of the trunnion pin and
the actuator rod end bolt is addressed in Boeing Service Bulletins
(S/B) 737-32-1198, Revision 2 and 737-32A1224, Revision 2. The
right trunnion pin was reworked per the latter service bulletin in
March 1993 to remove corrosion in the pin thread. There have been
three previous reports of fractures of the trunnion pin lugs at the
actuator attachment and none of these resulted in collapse of the
gear.
The Australian Bureau of Air Safety Investigation (BASI) has
determined that the fracture of the trunnion pin actuator attach
lugs prevents retraction of the gear, and may result in damage to
the surrounding structure. The BASI states that the inspection
defined in the service bulletins is not mandated. They also state
that the service bulletins do not direct inspection to the trunnion
pin actuator attach lugs. Therefore, the BASI recommends that the
Federal Aviation Administration address this safety deficiency and
initiate the appropriate action to ensure the integrity of the main
landing gear trunnion pin.
ANM-100S Response:
The Federal Aviation Administration (FAA) does not concur that non
retraction of the gear is a safety deficiency. A safety issue will
arise if after extension, the gear is not in the down and locked
position, and the gear collapses upon landing. If the actuator is
detached, the gear will still be able to extend down by free fall.
Lock mechanisms in the gear assembly, such as the downlock spring
bungees, will ensure that the gear is in the down and locked
position. As in this case and the previous three cases, the
affected gear did not collapse; thus there is assurance that the
down lock features were effective.
We have evaluated the BASI's concern that the lug failures and
subsequent actuator disconnection may result in damage to the
surrounding structure. Disconnection of the actuator may cause
damage to the surrounding area but this affected structure is
secondary structure, and therefore not a safety issue. The BASI
also relates the trunnion pin lug failures to the previously
reported actuator beam arm incident. Fracture of the beam arm led
to the disconnection of the reaction to the actuator loads, which
caused the gear to translate outside of its envelope and resulted
in damage to the primary wing structure and control cables. As a
result, the FAA issued Airworthiness Directive 99-10-12 to address
this safety concern. In the case of-the disconnection of the
actuator, the actuator beam arm was still intact. Therefore, an
intact beam arm should prevent any excessive outboard translation
of the gear that may cause damage to primary structure and control
cables.
Regarding BASI's determination that the Boeing Service Bulletin
737-32-1198, Revision 2, does not provide direct inspection of the
trunnion pin lugs, we will contact the manufacturer on this matter.
Considering that there have been four cases of lug failures, we
will recommend that the manufacturer consider adding instructions
to inspect for corrosion and/or cracks in the lugs to the subject
service bulletin.
In conclusion, the FAA does not concur that a safety deficiency
exists and hence has determined that an airworthiness directive to
mandate the inspections in accordance with the appropriate service
bulletin is not warranted.
We thank the Australian BASI for bringing this issue to our
attention. We recommend that this safety recommendation be
closed.
