History of the flight
At the appropriate time during the approach sequence, the
co-pilot of the Boeing 737, who was the handling pilot for the
sector, called for the landing gear to be extended. When the pilot
in command placed the landing gear lever to the "down" position, a
loud thump was heard and the "gear safe" green light for the right
main gear illuminated immediately. This was followed by the
illumination of the left main and nose landing gear lights,
consistent with a normal extension sequence. The aircraft rolled
approximately 4 degrees to the right while the gear was extending.
This was counteracted by a left roll control input.
As the crew had received indications that the landing gear was
safely locked down, they continued the approach and completed a
normal landing.
Ramp maintenance staff briefly inspected the aircraft but did
not find any immediate cause for the reported thump. The aircraft
was then placed on jacks for a retraction test. When the landing
gear lever was selected to the "up" position, the right main
landing gear moved inboard approximately 15 cm before a grinding
noise was heard. The test was immediately suspended and the landing
gear was extended.
When access panels were removed, it was found that the actuator
beam arm inboard lugs and beam hanger had fractured. The rear wing
spar, landing gear beam, aileron bus cable, pulley bracket, aileron
and spoiler cables and hydraulic lines had been damaged extensively
following the fracture of the lugs and hanger.
Actuator beam arm examination
Inspection of the actuator beam arm revealed that the fracture
of both lugs was due to stress corrosion cracking.
Service information
The manufacturer addressed main landing gear (MLG) corrosion and
cracking problems with the issue of Service Bulletin (SB)
737-32A1224 in July 1989 and with production changes to the beam
arm assembly. Revision 2 of this SB was issued in April 1991 and
Part A of this SB was mandated by the issue of Federal Aviation
Administration (FAA) Airworthiness Directive (AD) 91-05-16, and by
the issue of Australian Civil Aviation Safety Authority AD/B737/57.
The AD action required in-situ inspection of the beam arm at 600
flight-cycle intervals. Alternatively, modification or replacement
of the beam arm with a new production arm could be accomplished as
a terminating action.
The modified beam arm assemblies incorporated the following
changes to improve the corrosion resistance of the actuator beam
arm and actuator beam attach bolt:
- improved bushings,
- an increased bushing interference fit,
- an improved actuator beam bolt,
- more extensive cadmium plating, and
- improved lubrication of components.
SB 737-32A1224 Rev 3, issued in October 1992, recommended
replacing the actuator beam arm components with production
components rather than part modification.
There have been two reports of lug failures on beam arm
assemblies that had been modified by AD 91-05-16. These failures
occurred overseas in March 1997 and October 1998 and were similar
in nature to this incident.
Maintenance history
The operator advised that in July 1991, the right beam arm
fitted to the aircraft involved in the March 1999 incident was
reworked for incorporation of SB 737-32A1224 Rev 2 at the
operator's maintenance facility. In June 1993, the right MLG was
overhauled at an overseas maintenance facility. In June 1993, the
beam arm was installed with the MLG onto the aircraft. At the time
of failure, the beam arm had been in service for 33,681 flight
hours, 25,025 flight cycles and 10.25 years. The period since last
overhaul was 17,879 flight hours, 12,296 flight cycles and 5.75
years.
ANALYSIS
When the aircraft was on the line it showed no visible damage,
however, the operator elected to carry out a more comprehensive
inspection. Fracture of the lugs causes the main landing gear to
fall back to the extended position and is therefore fail-safe in
that respect. However, an actuator beam arm fracture allows the
actuator, actuator beam, and support link to travel outboard,
contacting and possibly damaging the main landing gear beam and the
wing rear spar. It can also result in contact with nearby flight
control cables (aileron and spoiler) and hydraulic lines. This
contact has the potential of inadvertent flight control inputs that
could reduce the controllability of the aircraft.
The aileron bus cable in this incident was found to be almost
completely severed. This damage potentially could have resulted in
serious control problems.
The aircraft right roll experienced during gear extension was
considered to have been the result of the right landing gear
extending more rapidly than the left landing gear, and not as a
result of flight control inputs caused by the actuator beam arm
failure.
The location of the fractures in both inboard lugs of the right
actuator beam arm coincided with the positions for possible cracks
identified in SB 737-32A1224. The fractures in the lugs were a
result of stress corrosion cracking. Previous failures analysed by
the manufacturer were also attributed to stress corrosion
cracking.
Stress corrosion cracking of high-strength steel components of
aircraft main landing gears occurs when the components are
subjected to a sustained tensile stress and are exposed to an
environment that allows stress corrosion cracking to initiate. The
components are susceptible to stress corrosion cracking when
exposed to the normal operating environment of landing gear
(moisture, salt laden moisture). Stress corrosion cracking in the
actuator beam arm lugs occurred as a result of the movement of the
bushes installed in the lugs and the penetration of moisture into
the gap created between the bushes and lugs.
The ATSB Technical Analysis Report 19/01 further analysed
failures of B737 main landing gear pin/lug joints and is available
on the ATSB website or on request from the ATSB.
SAFETY ACTION
inspection
As a result of this incident, the operator conducted visual and
ultrasonic inspections of the actuator beam arms of its fleet of
Boeing 737 aircraft. A cracked actuator beam arm was found during
inspection of another aircraft.
As a result of this occurrence, the ATSB (which includes the
former Bureau of Air Safety Investigation) issued interim
recommendations (IRs) to the manufacturer, regulators and operators
on 19 March 1999 and identified the following safety
deficiency:
"Damage to flight control cables and primary aircraft
structure resulting from the failure of main landing gear (MLG)
actuator beam arm lugs of B737 aircraft has the potential to
seriously affect the safety of flight."
IR19990019
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.
The following response (in part) was received from the Boeing
Commercial Airplane Group on 6 April 1999:
"We have released Boeing All- Operator Message
M-7200-99-02383 dated 26 March 1999. In summary we plan to release
an alert service bulletin (737-32A1314) by the end of April, 1999.
This bulletin will call for the resumption of the ultrasonic
inspections of all Main Landing Gear Beam Arms every 600 flight
cycles upon the accumulation of 10,000 cycles or 4 years (whichever
comes first). This will be considered interim action until we have
analyzed and implemented the course of terminating action to
preclude the onset of corrosion noted in the parts."
Boeing released alert service bulletin 737-32A1314 on 15 April
1999 requiring all 737-100, -200, -300, -400, -500 airplane
operators to do a visual and ultrasonic inspection of the clevis on
the actuator beam arm. If cracks or corrosion are found, the
actuator beam arm is to be replaced. Boeing recommended this
inspection be done before the actuator beam arm has 10,000 cycles
or four years of service. Boeing recommended the inspection be
repeated every 600 flight cycles or 90 days (whichever occurs
first).
Boeing advised that they were evaluating design improvements
that will further inhibit the initiation of corrosion of these high
strength steel parts. Boeing expected to complete the evaluation
and begin implementation of the new hardware by the fourth quarter
of 2002.
Response classification: CLOSED-ACCEPTED.
IR19990020
The Bureau of Air Safety Investigation recommends that Boeing
Commercial Airplane Group review the effectiveness of Service
Bulletin 737-32A1224.
The following response (in part) was received from the Boeing
Commercial Airplane Group on 6 April 1999:
"Prior to the original release of the bulletin in 1989,
operators were reporting medium - to - heavy corrosion of the beam
arms (in addition to several reports of fractures). Following the
second post - bulletin fracture report (dated 6 October 98 in the
table above), we conducted a survey of operators and asked for an
assessment of the condition of beam arms in service. In general,
most operators who responded (representing just over 1300
airplanes) reported very little (if any) corrosion. The basic
response is that, although the bulletin appears to have been
effective in reducing the reported corrosion, it has not eliminated
the potential for corrosion that leads to the possibility of
cracking of the beam arm lugs. Therefore, we are undertaking the
actions as noted in the response to recommendation IR990019
above."
Response classification: CLOSED-ACCEPTED.
IR19990021
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 actuator beam arm assemblies.
The following response was received from the US Federal Aviation
Administration on 24 May 1999:
"The US Federal Aviation Administration issued an
Airworthiness Directive (AD 99-10-12) as Amendment 39-11165 to all
B737 operators, effective 27 May 99:SUMMARY: This amendment supersedes all existing airworthiness
directive (AD), applicable to certain Boeing Model 737-100, -200,
-300, -400, and -500 series airplanes, that currently requires
repetitive inspections to detect cracking, plating degradation, and
corrosion of the main landing gear (MLG) actuator beam arms and
actuator beam attach bolts; and rework or replacement, if
necessary. The existing AD also provides for optional terminating
action for the repetitive inspections. This amendment removes the
requirement to inspect the actuator beam attach bolts, expands the
applicability of the existing AD to include additional airplanes,
and removes the optional terminating action. This amendment is
prompted by reports of cracked MLG actuator beam arms. The actions
specified in this AD are intended to detect and correct corrosion
and cracking of the MLG actuator beam arm, which could result in
damage to the control cables for the aileron and spoiler and
consequent reduced controllability of the airplane."
Response classification: CLOSED-ACCEPTED.
IR19990022
The Bureau of Air Safety Investigation recommends that the Civil
Aviation Safety Authority initiate appropriate action to ensure
that Australian operators of Boeing 737 aircraft immediately
inspect all main landing gear actuator beam arm assemblies for
evidence of cracking.
The following response was received from the Civil Aviation
Safety Authority on 08 June 1999:
"I refer to your interim recommendation IR990022 in
regard to the Actuator Beam Arm failure on B737 VH-CZL on 12th Mar
1999. The incident resulted in substantial secondary damage to the
wing structure and flight control cables, and it was this damage,
rather than the undercarriage failure, which was of major concern
(the undercarriage is designed to fail safe).CASA has investigated the incident, and found that this failure
was previously covered by inspections mandated by AD/B737/57. Those
inspections ceased after a modified actuator beam arm was
installed, as nominated by the AD as closing action. VH-CZL had a
modified actuator arm beam, and inspections for cracking were
therefore not required.The significance of the secondary damage suffered by VH-CZL was
such that CASA decided to reintroduce initial and repetitive
inspections by reissue of AD/B737/57 Amendment 1. This amended AD
was issued on 29 March 1999 and requires visual and ultrasonic
inspections even if the actuator arm had been replaced. Boeing was
advised of the action and requested to advise us when suitable
terminating action is developed.The US FAA was also advised of our action. The FAA has
subsequently issued AD 99-10-12 effective 27 May 99 to perform
almost identical work as required by the CASA AD. The FAA AD was
issued as a final form without industry consultation.The action initiated is considered adequate to correct the
unsafe condition revealed by the incident involving VH-CZL. This
action will be reviewed when further information is received from
Boeing or the FAA.Receipt of IR990022 enabled urgent corrective action to be
initiated by CASA, and subsequently by Boeing and the FAA.
Expeditious issue of IR990022 by BASI is therefore much
appreciated."
Response classification: CLOSED-ACCEPTED.
IR19990023
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 actuator beam arm assemblies.
The following response was received from Ansett Australia on 8
April 1999:
"As you are aware the company conducted initial visual
checks and then NDT inspections of the affected area of the B737
landing gear. All the inspections were completed some weeks ago. We
found another failed part in VH-CZU and those components have been
sent to BASI for analysis.The company has received no further information from the
manufacturer at this time."
The following response was received from Qantas Airways Limited
on 12 April 1999:
"Qantas was notified by Ansett of the VH-CZL occurrence
through Safety and Engineering Departments contacts. Qantas
Engineering conducted initial visual inspections on the entire B737
fleet on 12 and 13 March 1999.In addition, an instruction was issued to carry out both
ultrasonic inspection in accordance with SB 737-32A1224 Revision 2,
and also a bush rotation check, which is to check for bolt binding
within the bush, causing bush movement.The initial inspections targeted beam arms with greater than
10,000 hours and no defects were discovered by the end of March
1999. All Qantas units are post SB737-32A1224 status and have
either been modified in-house or are Boeing as-new
replacements."
Response classification: CLOSED-ACCEPTED.