Boeing 737-376, VH-TAK

Trunnion pin examination

Inspection of the trunnion pin revealed that the fracture of the lugs was a result of stress corrosion cracking. This had initiated at the surface of the lug bore, where extensive corrosion pitting was present. The hard chromium plating on the actuator rod end attach bolt had been lost from the bolt surface in areas that were in contact with the bushes.

Stress corrosion cracking of high strength steel components of aircraft MLG occurs when the items are exposed to moisture. Stress corrosion cracking in the trunnion pin lugs occurred because of movement of the bushes installed in the lugs and penetration of moisture into the gap created between the bushes and lugs.

Failure of the trunnion pin occurred during extension of the landing gear; however, the same failure could have occurred during landing gear retraction. Fracture of the MLG trunnion pin attach lugs causes the MLG to fall back to the extended position and is therefore fail-safe. A MLG trunnion pin fracture prevents retraction of the landing gear since it results in disconnection of the MLG actuator. Damage to the surrounding aircraft structure and landing gear assembly may result from a subsequent attempt to recycle the landing gear.

Trunnion pin service and maintenance

The manufacturer issued a Service Bulletin (Boeing SB 737-32-1198 revision 2) in January 1993 for inspection, preventive modification, rework and replacement to improve the corrosion resistance of the MLG trunnion pin and actuator rod end bolt. The manufacturer recommended inspection of the trunnion pin at the next convenient maintenance check with modifications or rework at the next major landing gear overhaul.

The accomplishment instructions for the initial inspection of the MLG trunnion pin vary slightly between SB 737-32-1198 and SB 737-32A1224. Neither bulletin specifically directs inspection to the actuator attach lugs.

The operator completed a rework and overhaul of the right MLG fitted to the aircraft in March 1993. As part of this rework, the operator performed a magnetic-particle inspection of the trunnion pin under SB 737-32A1224 revision 2 (now incorporated in SB 737-32-1198 revision 2), with nil defects found. Some thread corrosion damage was repaired by blending before reinstallation on the aircraft.

The MLG trunnion pin had accumulated 34,316 flight hours, 23,159 flight cycles and 12.8 years in service. The period since last overhaul was 19,316 flight hours, 12,211 flight cycles and 6.1 years. The time remaining on the trunnion pin before overhaul was 3,084 flight hours with 51841 cycles remaining life based on the manufacturer's life limits.

The manufacturer advised that it had received three previous reports of trunnion pin fractures at the actuator attach lugs, none of which resulted in MLG collapse. These trunnion pins had previously been inspected and reworked as necessary under either SB 737-32A1224 revision 2 or SB 737-32-1198 revision 2.

The manufacturer's service bulletins did not appear to be effective in preventing recurrence of corrosion and cracking in these parts. The bulletins did not direct inspection for cracking to the MLG trunnion pin attach lugs unless the trunnion pin was subject to a rework.

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