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Factual Information


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.



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.



Operator fleet 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."


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.


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.


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.


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.


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.

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