What happened
On 22 January 2000, a Boeing 757-2T7, registration G-MONE, was operating a scheduled flight from Salzburg to London Gatwick. During takeoff from Salzburg, the crew heard a loud noise from beneath the forward section of the aircraft, which was initially suspected to be a birdstrike.
Upon approach to Gatwick, the crew selected the landing gear to the down position, at which point a second loud bang and significant vibration occurred. The nose landing gear indication displayed an amber warning, signifying the gear was not locked. The crew subsequently performed a go-around. During the maneuver, observers on the ground and the crew of a taxiing aircraft noted that the nose gear appeared to be down but not securely locked. Following the execution of emergency procedures and a passenger briefing, the crew performed a precautionary landing with emergency services on standby. The gear remained in the down position, and the aircraft was able to taxi to the stand after ground locks were applied.
The investigation
An investigation into the mechanical failure revealed that the nose landing gear retract actuator eye-end had failed at the junction of the eye and its threaded section. Additionally, the lock actuator had failed, and several other components in the nose landing and gear area were bent.
Metallurgical analysis of the fracture surfaces determined that the lock actuator had failed due to ductile overload. The retract actuator, however, had succumbed to a fatigue failure. Examination of the component's surface showed that the machined face of the shoulder had been heavily marked by what appeared to be a saw or a file. These marks were mirrored on the locking collar, suggesting the damage was present during the component's last assembly.
Records indicated that the actuator had been worked on in 1988 as part of a fleet-wide Service Bulletin. The investigation found that the mechanical damage likely occurred during the disassembly process in the airline's hydraulic bay, possibly because the technician attempted to remove the locking collar using tools that damaged the underlying metal.
Findings
- The primary cause of the incident was the failure of the nose landing gear retract actuator eye-end.
- The failure originated from a fatigue crack that propagated from stress-raisers created by mechanical damage (saw or file marks) on the component.
- This damage was inflicted during the last disassembly of the actuator in 1988, likely due to inappropriate workshop practices used to free the locking collar.
- The failure of the lock actuator was a result of ductile overload, caused by the rapid, undamped descent of the nose gear once the up-lock released.
- The airline's quality control system at the time of the 1988 maintenance failed to identify the improper workshop procedures.