What happened
On September 14, 1999, a Piper PA34-220 T Seneca III, registration HB-LRC, was performing a touch-and-go maneuver at St. Gallen-Altenrhein Airport. Following the initial contact with the runway, the pilot found that the landing gear could no longer be retracted. The crew decided to land on runway 10 due to a change in wind direction. Approximately 300 meters after touchdown, the left landing gear collapsed. The aircraft veered off the runway and came to a halt in a grassy area. There were no injuries to the crew or passengers.
The investigation
Investigators examined the wreckage and the left trunnion assembly. The investigation, which included metallurgical analysis by the Swiss Federal Laboratories for Materials Science and Technology (EMPA), focused on the fracture surfaces of both the left and right landing gear components. The analysis revealed that the left trunnion had suffered a complete fracture. The damage to the left wing, propeller, and engine was determined to be a direct consequence of the gear collapse.
Findings
Technical analysis confirmed that the failure was caused by a fatigue crack originating from a pre-existing structural defect. The microscopic examination identified fatigue striations indicating a crack depth of 5mm before the final ductile failure occurred. The root cause of this fatigue was traced to surface anomalies created during the manufacturing finishing process. Specifically, sharp, notch-like grinding marks and an intensely cold-worked, cracked surface layer on the polished area initiated the cracks. While the internal material structure was sound, these surface imperfections provided the starting point for the fatigue. Additionally, the frequent, sometimes heavy landings typical of flight training operations likely contributed to the progression of the existing cracks toward the final failure.
Safety action
At the time of the accident, the aircraft was not covered by existing service bulletins regarding the inspection of the main landing gear trunnion housing. However, following the investigation, the manufacturer released Service Bulletin 787C, which expanded inspection requirements to include the serial number range of the involved aircraft.