Swiss Investigation of Piper PA-18-180M Landing Gear Failure at Montricher (HB-PDL)

Incident Overview and Aircraft Details On 8 April 2021, at 14:16 local time, a Piper PA-18-180M, registration HB-PDL, operated privately, experienced a serious incident during landing at the aerodrome of Montricher (LSTR) in Switzerland. The aircraft was being used for glider towing operations. The pilot, a Swiss national born in 1941, held a Private Pilot Licence (Aeroplane) with a glider towing privilege issued by the Federal Office of Civil Aviation (OFAC). At the time of the incident, the pilot had a total of 2,110 flight hours, including 240 hours on the type. The aircraft, manufactured in 1978, had accumulated more than 6,695 flight hours and 19,105 landings. ## Sequence of Events During the tenth landing of the day, in calm wind conditions on grass runway 21, the aircraft collapsed onto its main landing gear after making contact with the ground. The pilot managed to keep the aircraft aligned with the runway and brought it to a stop on the left side, with the wingtip touching the ground. The pilot sustained minor injuries, while no passengers or other persons were involved. The aircraft sustained minor damage to the right main landing gear structure. ## Investigation Findings The Swiss Transportation Safety Investigation Board (STSB) conducted a summary investigation under Article 46 of the relevant ordinance. The investigation found that the failure of the right main landing gear attachment point exhibited characteristics of structural fatigue resulting from repeated and accumulated stresses during a very high number of landings. The specific attachment point was obscured by two washers, making it difficult to detect a crack during the pre-flight inspection. The last maintenance intervention on the landing gear was performed on 16 March 2018, involving the replacement of the four upper axes of the triangles. ## Probable Cause and Safety Lessons The probable cause of the incident was the rupture of one of the attachment points of the tubular structure of the main landing gear to the fuselage, which caused the aircraft to collapse and come to rest on its left wing. The high number of landings was identified as a causal factor. The investigation recommended that particular attention be paid to these attachment points during periodic inspections to detect weakening of the landing gear before failure.