What happened
On June 4, 2012, a Piper Aircraft PA18-150, registration F-BKBE, was conducting a mountain flight training mission from Luchon to the Peyresourde altiport. The flight was supervised by an instructor, with a student pilot acting as the pilot in command. During the landing sequence for runway 09, the instructor noted a northwest wind between 5 and 10 knots.
Immediately following touchdown, a significant shimmy developed in the tailwheel. The instructor attempted to take control and apply nose-down elevator to relieve pressure on the tailwheel, but these maneuvers failed to stabilize the aircraft. As the aircraft continued its roll, the trajectory abruptly shifted 9/0 degrees to the left. The aircraft exited the runway and subsequently flipped onto its back. The two occupants were uninjured and managed to escape the cabin by breaking the side windows.
The investigation
The investigation focused on the mechanical integrity of the tailwheel assembly. Investigators examined the retaining bracket for the leaf spring shock absorber, which is secured to the fuselage via two bolt-and-nut assemblies. The left-hand assembly was found to be damaged, creating abnormal lateral and vertical play.
Technical analysis revealed that the left-hand bolt did not meet manufacturer specifications regarding both diameter and length. Furthermore, the bolt utilized a metric standard, while the accompanying nut was of an American standard with a different thread pitch. The hardness of the nut was also inferior to that of the bolt. This mismatch caused the components to fail under operational stress.
Findings
- The primary cause of the accident was an uncontrollable tailwheel shimmy during the landing roll.
- The shimmy was facilitated by excessive play in the bolt-and-nut assembly, which altered the damping characteristics and allowed the leaf spring stack to shift laterally.
- The intensity of the oscillation led to the misalignment and eventual locking of the tailwheel at a 90-degree angle, rendering rudder inputs ineffective.
- The improper hardware was installed in March 2012 by the flight club, which lacked the necessary certification and equipment for such maintenance.
- The use of incompatible metric and imperial hardware created the mechanical instability that triggered the event.