What happened
On 29 December 1998, a Piper PA-25-235C, registration G-BSTH, was completing a glider towing operation at Portmoak Airfield in Tayside, Scotland. Following a successful aerotow launch, the aircraft was in the process of landing. Approximately five seconds after the wheels touched the ground, the left landing gear leg collapsed. This caused the left wing to drop and strike the grass. As the aircraft began to ground loop to the left, the pilot shut down the engine. The aircraft rotated roughly 180 degrees before coming to a halt. There were no injuries to the pilot, though the aircraft sustained damage to the landing gear and the left wing.
The investigation
The AAIB investigated the incident by reviewing the pilot's accident report and performing a detailed examination of the failed component. The investigation focused on the shock absorber assembly, which uses bungee shock cords wrapped around an oil-filled damper. This unit is held in place by bolts at both ends, with one bolt securing the ramrod to the fuselage and another securing the damper body to the landing gear via a welded attachment lug.
Upon inspection, investigators found that the attachment lug on the damper body had failed. Analysis of the fracture surfaces revealed two distinct areas: a smooth region and a more ragged region. The smooth area indicated a progressive fatigue cracking mechanism, while the ragged area indicated the final failure occurred due to an overload during the landing sequence. Microscopic analysis determined that the fatigue crack started at the surface of the bolt bore, initially progressing through high-cycle fatigue before transitioning to low-cycle fatigue as the crack grew.
Findings
- The primary cause of the gear collapse was the failure of the gear attachment lug on the damper unit.
- The fatigue crack originated at the surface of the bolt bore, exacerbated by a poor surface finish in that area.
- While the specific damper unit had been installed only 200 flying hours prior, the investigation noted that surface defects can induce stress concentrations that lead to fatigue.
- The aircraft's role as a glider tug, involving frequent take-offs and landings on rough grass surfaces, subjects the equipment to high cyclic loads.