History of the flight
The Piper PA-25-235 (registration G-AZPA) was conducting its seventh glider aero-tow of the day at Talgarth Airfield, South Wales. Just prior to liftoff at about 50 mph, the left wing suddenly dropped, halted momentarily, and then contacted the grass runway. The aircraft ground-looped to the left through approximately 160° before coming to rest within the airfield boundary. The pilot closed the throttle and stopped the engine using the magneto switches. The glider pilot observed the tug's behaviour, applied the brake, and turned to the right to avoid a collision. No injuries were sustained, and damage was limited to the underside of the tug's left wing.
Examination of the left landing gear
Subsequent examination revealed that a failure had occurred in the left landing gear truss at the attachment point to the bottom of the spring damper unit. This bolted joint featured a fork end on the truss that attached to a lug on the damper, with the bolt loaded in double shear. The front lug on the truss fork and the bolt had failed. The lug and one half of the bolt were found on the grass runway but were not available for metallurgical examination; the lug had been welded back onto the truss during repair, and the bolt portion was mislaid. Photographs of the failed lug and bolt indicated age-related staining on part of the lug fracture, suggestive of a pre-existing fatigue fracture in the lug, and a low cycle fatigue failure of the bolt. The lug failure would have placed the bolt in single shear, inducing rapid low cycle fatigue failure.
Safety strop failure
The aircraft was fitted with a modification consisting of a wire strop on each main landing gear, looping around the truss and internal wing structure. This strop was designed to prevent complete leg collapse in the event of failure of the bungee cord within the spring/damper unit. Following the truss fork failure, the strop was loaded and failed immediately, resulting in a two-stage leg collapse. Subsequent investigation showed that the strop cable had been weakened by corrosion, most likely due to water accumulation within the cable's sheath. During repair, the sheath was replaced with a clear plastic tube to enable visual monitoring of the cable's condition.