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.
