As a result of this occurrence the Bureau of Air Safety
Investigation, now the ATSB, issued interim recommendations (IRs)
to the manufacturer, regulators and operators on 27 May 1999 and
identified the following safety deficiency:
Cracking and failure of main landing gear (MLG)
trunnion pin actuator attach lugs of Boeing 737 aircraft prevents
retraction of the MLG and may result in damage to the aircraft
structure. While the manufacturer's service information regarding
MLG corrosion requires a general one-time inspection of the
trunnion pin, the accomplishment instructions do not specifically
direct inspection to the attach lugs for cracking. The inspection
of the trunnion pin is not mandated by regulatory
authorities.
IR19990046
The Bureau of Air Safety Investigation recommends that Boeing
Commercial Airplane Group alert Boeing 737 operators to this safety
deficiency and implement an appropriate inspection program.
Boeing response
Boeing responded that it did not agree that failure of the 737 MLG
actuator trunnion pin would result in damage to aircraft structure
and, as a result, did not consider this failure mode was a safety
issue. A metallurgical examination of the failed trunnion pin by
Boeing determined that improper restoration of finishes during
trunnion pin overhaul was the likely cause of corrosion that
resulted in the trunnion pin failure. Boeing released Maintenance
Tip, 737-MT-32-009 R1, in January 2001, to highlight the need to
properly restore corrosion prevention compounds when landing gear
components are removed and replaced during maintenance.
Boeing advised that investigation of other in-service occurrences
of trunnion pin lug fractures concluded that some of these
fractures were due to a high preload on the clevis due to bolt
clamp up. As a result Boeing revised the Airplane Maintenance
Manual (AMM) 32-32-11 in 1997 to reduce the bolt nut torque when
installing the actuator rod end bolt at the clevis in question.
This was to reduce the clamp up loads and accompanying stresses
that may contribute to the initiation of lug stress corrosion
cracking.
Boeing advised that given their evaluation of the consequences of
the trunnion pin failure, they considered that the maintenance tip
accomplished the action to notify operators to the need for proper
maintenance to avoid corrosion in these parts.
Response classification: CLOSED-ACCEPTED
IR19990047
The Bureau of Air Safety Investigation recommends that Boeing
Commercial Airplane Group review the effectiveness of Service
Bulletin 737-32-1198 revision 2.
Boeing response Boeing responded that
it had completed a review of the entire 737 MLG for corrosion
problems and did not anticipate releasing further service bulletins
or similar type documents relative to the trunnion pin failure.
Additionally, Boeing considered that the aforementioned service
bulletin was properly categorised and did not need to be upgraded
to an "Alert" level bulletin.
Response classification: CLOSED-ACCEPTED
IR19990048
The Bureau of Air Safety Investigation recommends that the
Federal Aviation Administration (FAA) note the safety deficiency
and recommendations and take appropriate action as considered
necessary to ensure the integrity of Boeing 737 main landing gear
trunnion pin assemblies.
FAA response
The FAA responded that an inability to retract the landing gear was
not considered a safety deficiency since the landing gear was in
the down and locked position. The FAA considered that an
airworthiness directive to mandate the inspections under the
service bulletin was not warranted. The FAA after considering there
had been four cases of lug failures, advised that they would
recommend that the manufacturer consider adding instructions to
inspect for corrosion and cracks in the lugs to the subject service
bulletin.
Response classification: CLOSED-ACCEPTED
IR19990049
The Bureau of Air Safety Investigation recommends that the Civil
Aviation Safety Authority (CASA) note the safety deficiency and
interim recommendations and initiate appropriate action as
considered necessary to ensure the integrity of Australian Boeing
737 main landing gear trunnion pin assemblies.
CASA response
CASA responded that Boeing was currently reviewing the entire
Boeing 737 landing gear assembly for corrosion problems and was
expected to address this, and other faults, via service bulletin or
similar document. CASA advised it would continue to monitor the
manufacturers response to this problem.
CASA also advised that pending issue of such data by the
manufacturer, both Australian operators had developed inspections
to check the trunnion pin lugs for corrosion. CASA believed that
this action was appropriate under its existing regulatory
framework, and the inspections carried out were suitable to prevent
further problems. CASA considered that issue of an Airworthiness
Directive was unnecessary because of the operators' actions and the
impending issue of improved manufacturer's data.
Note: Qantas and Ansett were the only Australian Boeing 737
operators at the time the recommendation was issued
Response classification: CLOSED-ACCEPTED
IR19990050
The Bureau of Air Safety Investigation recommends that
Australian operators of Boeing 737 note the above safety deficiency
and interim recommendations and take appropriate action as
considered necessary to ensure the integrity of Boeing 737 main
landing gear trunnion pin assemblies.
The Australian operators of Boeing 737 aircraft
conducted a visual inspection of the MLG trunnion pins of their
fleet of B737 aircraft shortly after this event. Qantas issued an
Engineering Instruction (EI 737-032-0105) on 06 May 1999 to perform
ultrasonic inspections for cracks in trunnion pins that had
approximately 10,000 cycles since new or overhaul. Ansett raised an
Alert Engineering Release (B73-32-10-19) on 08 April 1999 to
perform a lug ultrasonic inspection on Boeing 737 MLG trunnion pins
at 10,000 flight cycles or 4 years since new or overhauled. Repeat
inspection was to be at 600 cycle intervals.
Note: Qantas and Ansett were the only Australian Boeing 737
operators at the time the recommendation was issued.
Response classification: CLOSED-ACCEPTED