What happened
On April 8, 2021, a Piper PA-18-180M, registration HB-PDL, was performing glider towing operations at Montricher airfield. During the tenth landing of the day, conducted under calm wind conditions on the grass runway 21, the aircraft's main landing gear collapsed immediately after touchdown. While the pilot managed to keep the aircraft aligned with the runway centerline, the aircraft eventually came to a stop on its left side after the wingtip made contact with the ground. The pilot sustained slight injuries during the incident.
The investigation
Investigators examined the structural integrity of the aircraft's classic landing gear, which utilizes bungee cord shock absorbers. The investigation focused on the steel tubular structure that supports the main wheels and attaches to the fuselage. It was discovered that a connection point on the right side of the tubular frame had fractured, causing the forward section of the lateral tube to detach from its attachment point.
Technical records revealed that the aircraft, manufactured in 1978, had accumulated over 6,695 flight hours and a significant total of 19,105 landings. The last recorded maintenance on the landing gear components occurred in March 2018. During the pre-flight inspection, the crack was not detectable because the specific attachment point was obscured by two washers.
Findings
- The primary cause of the incident was the structural fatigue failure of a main landing gear attachment point.
- The failure was driven by the extremely high number of accumulated landings performed by the aircraft over its operational lifespan.
- The fracture occurred at a location that is difficult to inspect visually due to the presence of hardware covering the attachment point.