What happened
On 16 August 2018, a Pacific Aerospace 750XL, registered HB-TSA, was performing its sixth flight of the day at Lucerne-Beromünster airfield, transporting eleven skydivers. During the takeoff roll, approximately 100 meters from the start, the right main landing gear detached from the aircraft. The loss of the gear caused the plane to tilt sharply to the right, resulting in the right wing striking the runway. The detached gear, still connected by a brake line, struck the right landing flap before coming to rest on the runway.
Despite the structural damage to the wing and flap, the pilot maintained the takeoff direction and continued the ascent. Passengers reported seeing a hole in the wing, and ground personnel confirmed via radio that the wheel had been lost. Following coordination with air traffic management and a visual inspection by a Swiss Air Force aircraft, the pilot proceeded to Dübendof airfield. To minimize fuel weight and fire risk, the pilot performed a low-fuel approach, even shutting down the engine just before touchdown. The aircraft came to a halt on the runway with no injuries to the occupants.
The investigation
Investigators examined the remains of the right main landing gear and discovered that the upper fastening bolt of the torque link had been lost. This bolt was found on the takeoff runway, where it exhibited significant corrosion and a fracture on the thread side. Analysis determined that the bolt had failed due to a fatigue fracture initiated by corrosion.
Further inspection of the torque link revealed that the lubricating nipple was missing. Evidence suggested the nipple had been broken off long ago and the remaining stub had been filed down rather than properly repaired. This lack of a nipple prevented the joint from being lubricated, which was a requirement for the aircraft's maintenance schedule.
Findings
- The primary cause of the incident was the failure of a fastening bolt on the right main landing gear due to fatigue.
- The fatigue was driven by advanced corrosion on the bolt, which resulted from a lack of lubrication in the joint.
- The lubrication failure was due to a broken and improperly filed lubricating nipple.
- A 300-hour inspection performed approximately two months prior to the accident failed to identify or rectify the corrosion or the unlubricated state of the joint.
- The rough surface of the grass runway at the departure airfield likely contributed to the mechanical stresses that accelerated the fatigue fracture.