What happened
On 13 May 1999, a BAe ATP, registration G-MANM, was performing a scheduled public transport flight to Jersey Airport in the Channel Islands. The aircraft completed a routine touchdown using 29 degrees of flap without initial incident. However, as the aircraft decelerelated to approximately 60 knots, a severe vibration developed during the landing roll. The pilot managed to complete the roll and taxied the aircraft clear of the runway, where the engines were shut down. All 47 passengers and 5 crew members disembarked safely via bus, and there were no injuries reported.
The investigation
Upon inspection of the aircraft on the taxiway, investigators discovered that the right main landing gear wheels were misaligned by roughly 20 degrees to the right. The apex pin from the right main landing gear torque links was missing. The pin and a portion of a sheared locking bolt were recovered from the runway, though the retaining nut and the other end of the bolt could not be located.
Engineering analysis revealed that the threads on the apex pin were severely worn and distorted. This distortion indicated that the retaining nut had been stripped from the threads, which subsequently caused the locking bolt to shear. Further investigation into the aircraft's maintenance history showed the gear had completed 10,389 landings since its last overhaul. While the joint had been adjusted in November 1998, the investigation focused on the inherent design and maintenance of the torque link joint.
Findings
- The primary cause of the incident was the separation of the apex pin due to severe thread wear on the pin and nut.
- The design of the torque link joint allows the nut to remain free to rattle on the pin threads when no axial load is applied, a condition that increases as the joint wears.
- The threads of the pin and nut were found to have insufficient residual strength to withstand the loads generated by wheel shimmy.
- The manufacturer's service letter had not explicitly warned about the likelihood of thread wear caused by the lack of pre-load in a high-vibration area.
- A fleet-wide inspection following the incident identified four other aircraft with unacceptably worn pin and nut combinations.