What happened
On 12 September 2000, an American General AG-5B, registration G-NODY, was involved in a landing gear failure at Coventry Airport. The incident occurred during a training flight. Following a successful landing by a student pilot, the instructor took control of the aircraft to taxi toward the southern taxiway for a scheduled crew change. While executing a left turn, the left-hand main landing gear wheel detached from the gear leg, causing the aircraft to come to an immediate stop. There were no injuries to the two crew members or the two passengers on board.
The investigation
The investigation was based on the accident report submitted by the pilot and a metallurgical analysis conducted by the Civil Aviation and Aviation Authority. Investigators focused on the failure of the left wheel's axle. Upon inspecting the operator's wider fleet of GA-5B aircraft, several other axles were found to exhibit corrosion in the same area where the failure occurred.
Metallurgical examination of the failed axle revealed that material between the upper bolt hole counterbores, which secure the brake assembly, had been lost to exfoliation-type corrosion. This loss of material created sharp peaks that served as initiation points for small cracks. These cracks progressed through the axle body via a fatigue mechanism. A subsequent high-load event caused two cracks to propagate from the remaining material peaks, which then merged and eventually led to a final fracture after the load-carrying cross-section had been significantly reduced.
Findings
- The primary cause of the failure was corrosion-induced fatigue of the left axle.
- The axle's surface exhibited a coarse-grained recrystallized structure, likely resulting from improper process controls during forging or heat treatment.
- This surface structure, combined with grainflow patterns around the forging flash line, made the component highly susceptible to corrosion.
- Exfoliation-type corrosion had significantly weakened the structural integrity of the axle, leaving insufficient material to withstand operational loads.