Summary
On September 14, 1999, at approximately 0930 local time (UTC+2), a Piper PA34-220T Seneca III, registration HB-LRC, experienced a left main landing gear collapse during landing at St. Gallen-Altenrhein Airport. The aircraft, operated by Horizon Swiss Flight Academy Ltd., was being used for flight training. No injuries were reported.
Flight History
The pilot, accompanied by a flight instructor, performed a touch-and-go landing on runway 28. The aircraft touched down with all three wheels simultaneously in a flyable condition. After taking off again, the landing gear could not be retracted. The crew decided to return for landing. Due to a wind shift, an approach to runway 10 was made. Approximately 300 meters after touchdown, the left main landing gear collapsed, causing the aircraft to veer off the runway and come to rest in the grass after a 90-degree left turn.
Damage
The left main landing gear fractured. Subsequent damage occurred to the left wing, propeller, and engine. No third-party property damage was reported.
Landing Gear Inspection
Investigators from the Swiss Federal Institute for Materials Testing and Research (EMPA) examined both left (broken) and right (intact) main landing gear trunnion housings. The left unit exhibited a fatigue crack extending to a depth of 5 mm, followed by a ductile overload fracture. The right unit showed similar pre-damage in the form of fatigue cracks up to 3 mm deep at equivalent locations. Metallurgical analysis revealed that the fatigue cracks initiated due to unfavorable surface conditions: sharp grinding grooves and a heavily cold-worked, cracked surface layer, likely introduced during the manufacturing finishing process. No anomalies were found in the interior material.
Service Bulletins
Piper issued Service Bulletin No. 787B in 1985 for main landing gear trunnion housing inspection on certain serial numbers; HB-LRC was not initially affected. On November 7, 2000, Service Bulletin 787C was issued covering serial numbers 34-48001 to 34-48037, which included HB-LRC.
Operational Context
The aircraft was used primarily for flight training, involving a higher number of landings compared to private or commercial operations. The many, sometimes hard, landings typical of training operations likely contributed to the final overload fracture based on pre-existing fatigue cracks.
Probable Cause
The accident was caused by the fracture of the left landing gear leg due to a pre-existing defect. The pre-existing fatigue crack resulted from a structural defect on the polished surface (manufacturing process).