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Interim Recommendation issued to: Federal Aviation Administration (FAA)

Recommendation details
Output No: IR19990048
Date issued: 27 May 1999
Safety action status:
Background: Why this Interim Recommendation was developed

Output text

The Bureau of Air Safety Investigation recommends that the Federal Aviation Administration 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.

As a result of the investigation of this safety deficiency, the Bureau simultaneously issues the following interim recommendations:

IR990046

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.

IR990047

The Bureau of Air Safety Investigation recommends that Boeing Commercial Airplane Group review the effectiveness of Service Bulletin 737-32A1198 revision 2.

IR990049

The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority note the above safety deficiency and interim recommendations and initiate appropriate action to ensure the integrity of Australian Boeing 737 main landing gear trunnion pin assemblies.

IR990050

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.

Initial response
Date issued: 12 July 1999
Response from: Federal Aviation Administration (FAA)
Action status: Monitor
Response text:

The Office of Accident Investigation is in receipt of your recommendation concerning "B-737 MLIS Trunnion".

Your recommendation has been forwarded to the appropriate office for response, which is normally 90 days. Your recommendation has been identified as 99.231, and inquiries should reference this number.

You will be kept informed as to the progress and final resolution of your submission. If you have any questions, please contact this office.

Further correspondence
Date issued: 14 October 1999
Response from: Federal Aviation Administration (FAA)
Response status: Closed - Accepted
Response text:

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.

 
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Last update 01 April 2011