What happened
On August 16, 2018, a Pacific Aerospace 750XL, registration HB-TSA, was performing a series of private parachute jump flights at Luzern-Beromünster Airport (LSZO). During the takeoff roll for its sixth flight of the day, approximately 100 meters into the run, the right main landing gear detached from the aircraft. This caused the plane to tilt abruptly to the right, resulting in the right wing striking the runway. The departing gear leg, still attached by the brake line, struck and damaged the right flap before being left behind on the pavement.
Despite the structural damage, the pilot continued the takeoff roll and successfully lifted the aircraft into the air. Passengers noted a hole in the wing, and ground personnel radioed the pilot to report the lost wheel. After coordinating with air traffic control and receiving assistance from a Swiss Air Force aircraft that visually confirmed the missing gear, the pilot decided to land at Dübendorf (LSMD). To minimize fuel weight and fire risk, the pilot performed holding patterns to reduce fuel levels and shut down the engine just before touchdown. The aircraft landed on runway 29 at Dübendorf, coming to a stop near the centerline with no injuries to the 11 passengers or the pilot.
The investigation
Investigators examined the detached landing gear components and the remaining structure of the aircraft. They discovered that the upper attachment bolt of the torque link had been lost. This loss allowed the lower, movable part of the landing gear leg, including the wheel, to fall out of the upper, stationary part during the extension of the strut.
Upon inspecting the recovered bolt, investigators found that the threaded end had broken and exhibited significant signs of corrosion. Furthermore, the investigation revealed that the grease nipple for the joint in the torque link was missing. Evidence suggested the nipple had broken off some time ago and the remaining stub had been filed down rather than repaired.
Findings
- The primary cause of the incident was the fatigue failure of the torque link attachment bolt, which was driven by advanced corrosion.
- The corrosion was caused by a lack of lubrication, as the missing grease nipple prevented the joint from being properly maintained.
- The 300-hour inspection, performed approximately two months prior to the accident, failed to identify the corrosion on the bolt or the unserviceable state of the grease nipple.
- The rough surface of the grass runway at the departure airport contributed to the failure by causing the landing gear to extend abruptly and violently during takeoff rolls.