What happened
On 28 April 2001, a Piper PA-25-235, registration G-AZPA, was performing a glider aero-tow at Talgarth Airfield in South Wales. As the aircraft reached approximately 50 mph, just before lift-off, the left wing suddenly dipped and momentarily stalled before striking the grass runway. Following this impact, the aircraft underwent a ground loop to the left, rotating roughly 160 degrees before coming to a stop within the airfield perimeter. The pilot promptly closed the throttle and shut down the engine. The glider pilot, noticing the instability of the tug, reacted by braking and turning the glider to the right to prevent a collision. There were no injuries reported, and the aircraft sustained relatively minor damage to the underside of its left wing.
The investigation
Investigators examined the left landing gear assembly to determine the cause of the wing drop. They identified a failure at the bolted joint where the landing gear truss connects to the spring damper unit. Specifically, the front lug on the truss fork and the connecting bolt had failed. While the physical components were not available for metallurgical testing because the lug had been previously repaired via welding and the bolt was lost, photographic evidence provided critical clues. The photographs showed age-related staining on the lug fracture, suggesting a pre-existing fatigue crack, and evidence of low-cycle fatigue on the bolt.
Findings
- The primary cause of the accident was the failure of the landing gear truss lug and bolt.
- A pre-existing fatigue fracture in the truss lug likely caused the bolt to transition from a double-shear load to a single-shear load, leading to its rapid failure.
- A secondary failure occurred when the wire strop, intended to prevent total gear collapse, also failed. This strop had been weakened by corrosion, likely caused by water trapped inside its sheath.
- The failure of the truss and the subsequent failure of the safety strop resulted in the two-stage collapse of the landing gear leg.