What happened
On July 25, 2005, a Pilatus PC-6, registration HB-FMQ, was conducting a training flight at Buochs Airport. The flight, involving a pilot undergoing type conversion and a flight instructor, was intended as a touch-and-go training session.
During the landing phase on runway 25L, the aircraft touched down and initially rolled straight for approximately 20 meters. However, the aircraft then began to drift sharply to the left. The crew attempted to correct the deviation using right rudder and the right brake, but the aircraft continued to veer off the runway. As the aircraft left the paved surface, it struck a perimeter fence. The impact damaged the propeller and engine cowling. No injuries were reported among the two occupants.
The investigation
Investigators examined the aircraft's mechanical systems following the excursion. While the brakes initially appeared functional during testing on-site, the investigation revealed that the tailwheel steering mechanism was compromised. Specifically, the locking plate, which is essential for maintaining directional stability during takeoff and landing, was missing because the bolts securing it had sheared off. This failure likely occurred prior to the flight.
Furthermore, the investigation looked into the lack of braking effectiveness. While the primary cause of the excursion was the tailwheel failure, the investigation also identified that the braking system was not performing as expected. Subsequent analysis of the aircraft's brake installation revealed that insufficient clearance and improper alignment between the pedals and the brake cylinders prevented the cylinders from returning to their full resting position, leading to intermittent brake failure.
Findings
- The primary cause of the accident was the aircraft veering left due to a defective tailwheel locking mechanism.
- The locking plate for the tailwheel had been lost after its mounting bolts sheared off.
- The crew was unable to counteract the leftward drift because the right brake was not providing the necessary deceleration.
- The aircraft's directional stability was significantly reduced because the tailwheel was in a free-swiveling state rather than a locked position.
- The pilot and instructor were unable to detect the missing locking plate during the pre-flight inspection.