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Interim Recommendation issued to: Boeing Co

Recommendation details
Output No: IR19990047
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 Boeing Commercial Airplane Group review the effectiveness of Service Bulletin 737-32A1198 revision 2.

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.

IR990048

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.

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: 16 February 2001
Response from: Boeing Co
Action status: Closed - Accepted
Response text:

Subject: 737 Main Landing Gear - Trunnion Pin Corrosion

References: (a) e-mail from [name supplied] (ATSB) to [name supplied] (Boeing), same subject, dated February 5, 2001

(b) Quantas 737, "MLG actuator trunnion pin failure", 4/4/99, aircraft VH-TAK, BASI Ref. No. 0001455

(c) Ansett 737, "MLG actuator beam arm fracture", 3/12/99, aircraft VH-CZL, BASI Ref. No. 9901073

(d) Maintenance Tip, 737-MT-32-009 R1, "The use of BMS 3-27 (Mastinox 6856K) corrosion preventive compound for landing gear corrosion protection during disassembly and reinstallation of gear components during maintenance operations", dated 1/18/01

This letter is in response to your reference (a) e-mail requesting an update to progress on the issue of corrosion problems with the 737 Main Landing Gear (MLG), specifically trunnion pins since 1999. This request is based on the incident and subsequent Australia Bureau of Air Safety Investigation (BASI) interim recommendations made relative to reference (b).

Before providing the requested status, I would like to review the failures associated with the reference (b) and (c) events. The failures associated with the reference (b) and (c) events happened in the same area of the MLG installation but have significantly different failure effects. Both failures will prevent retraction of the associated MLG when gear up is selected, or will result in free fall of the MLG when gear down is selected. However, this is where the similarity ends.

The MLG actuator beam arm failure of reference (c), can result in secondary damage to structure and flight control cables when gear up is selected, or when fracture of the beam arm occurs.

However, the MLG actuator trunnion pin failure of reference (b), does not create any secondary damage during fracture, or when gear up is selected. Consequently, Boeing agrees that the beam arm failure is a safety issue, but does not agree that the trunnion pin failure is a safety issue.

Reference (a) states: "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." Based on the discussion above, Boeing does not agree that failure of the 737 MLG actuator trunnion pin will result in damage to aircraft structure and, as a result, does not believe this failure mode is a safety issue.

Relative to the reference (b) event, 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 recently released the reference (d) Maintenance Tip to highlight the need to properly restore corrosion prevention compounds when components are removed and replaced during maintenance.

Furthermore, Boeing investigation of other in-service occurrences of trunnion pin lug fractures concluded that some of these fractures were due to high preload on the clevis due to bolt clampup. 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.

Recommendation IR990046 was made relative to reference (b) and states that Boeing should alert 737 operators to this safety deficiency and that Boeing should review the effectiveness of Service Bulletin 737-32A1198. Given our evaluation of the consequences of the trunnion pin failure, we believe the reference (d) maintenance tip accomplishes the action to notify operators to the need for proper maintenance to avoid corrosion in these parts. Additionally, we believe that the aforementioned service bulletin is properly categorized and does not need to be upgraded to an "Alert" level bulletin. Please note that the correct number for the Service Bulletin is 737-32-1198.

Recommendation IR990047 was made relative to reference (b) and states that Boeing was currently reviewing the entire 737 MLG for corrosion problems. We have now completed this review and we do not anticipate releasing further service bulletins or similar type documents relative to the trunnion pin failure. However, we are continuing to review the MLG actuator beam arm fracture to determine what action is required for the in-service fleet.

Presumably, the on-going activities associated with the MLG actuator beam arm fracture are covered by IR990019 and IR990020. We therefore request that IR19990046 and IR19990047 of reference (b) be closed based on the data provided in this letter. If you have any questions, please do not hesitate to contact us.

 
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Last update 01 April 2011