What happened
On 20 September 2001, a Piper PA-34-2/00-2 Seneca, registration G-ELBC, was engaged in a training flight at Stapleford Airfield, Essex. The flight involved a student pilot under instruction, performing circuit work on the airfield's 1,120-metre grass runway. The weather conditions were favorable, with good visibility and a light crosswind.
Following two successful landings, the aircraft began its takeoff roll on runway 04L. During this sequence, the right main landing gear suddenly collapsed. The crew immediately aborted the takeoff, but the aircraft slid off the right side of the runway. The aircraft sustained damage to the right wing, the right propeller, and the right engine, which was shock loaded. The right main landing gear became detached from the airframe. There were no injuries to the two occupants, and no fire occurred during the incident.
The investigation
Technical examinations of the aircraft and its components were conducted by the flying club's maintenance organization and investigators. The inspection revealed that the right main landing gear leg had structurally detached from the aircraft, with both the forward and aft attachment fittings having fractured.
Investigators focused on the trunnion assembly, which connects the landing gear leg to the wing structure via two spigots housed in aluminum alloy castings. The forward fitting was found to have a horizontal fracture through its center, which released the pintle bearing. The aft fitting showed a broken portion of the boss and distorted bolt flanges.
Analysis of the mounting surfaces revealed ovalization of the bolt holes and significant fretting on the surfaces in contact with the wing spars. Specifically, the forward fitting showed extensive pitting. While some level of fretting is expected in such components, the severity of the damage in this instance suggested that the attachment bolts may have been undertorqued at some point in the aircraft's history.
Findings
- The right main landing gear failed due to an overload.
- The structural failure of the fittings caused the trunnion to pivot downward and rearward about the aft pintle.
- Evidence of severe fretting and pitting suggested the possibility that the fitting attachment bolts had been maintained below the required torque levels.
- There was no evidence to suggest that pre-existing fatigue cracking or the observed fretting damage was the direct cause of the immediate failure.