What happened
On 28 October 2008, a Boeing 73/400, registration PK-GZI, was operating a scheduled passenger flight from Yogyakarta to Denpasar. The aircraft, carrying 139 people, landed safely at Ngurah Rai Airport at 14:12 UTC. However, during the taxi phase toward the apron, the number-one main wheel axle failed, causing the axle to break and detach from the landing gear strut. There were no injuries to the 132 passengers or 7 crew members on board.
The investigation
Investigators examined the failed component, which had been part of a landing gear assembly overhauled in the United States and installed on the aircraft only four days prior to the incident. The landing gear had completed 7 cycles since its overhaul and 15,345 cycles since new.
Metallurgical analysis conducted in Indonesia and the United States revealed that the failure originated from a corrosion pit located within a brake assembly attachment hole. This pit acted as the starting point for a fatigue crack that propagated approximately 8 cm through the axle wall before a sudden, final fracture occurred. Evidence indicated that the crack was already present before the landing gear underwent its most recent overhaul.
Findings
- The primary cause of the failure was an undetected fatigue crack originating from a corroded hole in the brake attachment flange.
- The crack propagated through the axle wall until it reached a critical length, leading to the structural break.
- The failure occurred only seven flight cycles after the landing gear assembly had been reinstalled following an overhaul.
Safety action
Following the investigation, the NTSC issued recommendations to aviation authorities, including the FAA and EASA, to review inspection and overhaul requirements for Boeing 737 main landing gear axles. In response, Boeing announced plans to revise the Aircraft Maintenance Manual (AMM) to emphasize visual inspections for corrosion and coating damage at the brake flange location. Additionally, the overhaul facility involved implemented enhanced inspection techniques, including the use of fluorescent penetrant inspection.