What happened
On 10 February 1999, an ATR72-202, registration G-BWTM, was performing a scheduled public transport flight from London Gatwick Airport to Rotterdam. During the takeoff roll, just before the aircraft reached rotation speed, the crew heard a momentary 'shimmy sound'. As there were no other indications of technical difficulty, the flight continued to its destination.
Upon landing in Rotterdam, a post-flight inspection revealed that the right nosewheel and a portion of the right nosewheel axle's outboard section had been lost. The missing components, including the wheel, were later recovered at Gatwick Airport.
The investigation
Following the incident, the nose landing gear was replaced, and the damaged components were sent for detailed examination. Investigators found that while the left nosewheel assembly was intact, the right wheel showed significant damage to both half hubs and the outer bearing. The five bolts holding the hub halves together were loose, and the nuts on three of these bolts lacked sufficient thread protrusion.
Technical analysis of the axle revealed that the inner race of the bearing had spun against the axle, generating intense localized heat. This heat caused the axle to fail through a process known as liquid metal embrittlement, where the heated cadmium plating on the axle diffused into the steel's grain boundaries, weakening the metal.
Furthermore, the axle nut was found in a fully tightened position, with no locking bolt present in the assembly. The investigation determined that the locking bolt had not failed due to mechanical overload, but rather that the securing cotter pin was either missing or had failed, allowing the nut to unscrew.
Findings
- The primary cause of the axle failure was the absence of the axle nut locking bolt, which allowed the nut to rotate and eventually bottom out on the threads.
- The lack of a locking mechanism allowed the axle nut to tighten against the bearings, contributing to the eventual failure of the bearing inner race.
- The failure of the bearing inner race on the axle led to extreme frictional heating, resulting in liquid metal embrittlement of the axle material.
- The loss of the locking bolt likely occurred shortly before the incident, as the absence of the bolt would have been difficult to detect during routine daily tyre pressure checks.