What happened
On May 3, 1966, a Cessna 175, registration HB-CRM, was performing a local flight from Sion Airport with three passengers. During the initial takeoff roll on the asphalt runway, the pilot's adjustable seat suddenly slid backward by approximately 35 cm. This unexpected movement caused the pilot to lose control of the aircraft.
With the engine still at full throttle, the aircraft veered left and began to skid. The nose gear struck the pavement with enough force to create deep gouges in the asphalt. The aircraft then exited the paved runway, traveling across a grass strip and onto a taxiway. As the aircraft continued to drift, the left wingtip scraped the ground, causing the plane to crab sideways. The intense friction on the taxiway caused the nose gear assembly to snap. The resulting pitch forward caused the propeller to strike the ground heavily, which eventually stalled the engine. The aircraft finally came to a stop approximately 10 meters from a parked Venom fighter jet, which sustained minor damage. All four occupants were uninjured.
The investigation
Investigators examined the aircraft's seat mechanism and found that the seat's left front leg had become dislodged from its rail. This occurred because the original seat stops, which were secured by steel bolts, had been replaced by simple pins during a recent 100-hour inspection. These pins were found to be deformed, which increased the seat's travel range.
It was determined that when the seat was pushed forward against the new, inadequate stops, the left leg could inadvertently slip out of the rail. In this dislodged position, the locking mechanism was no longer secure. If the pilot applied pressure to the top of the seat back, the lock could be triggered, allowing the seat to slide backward. The investigation also noted that the pilot, while understandably startled, failed to reach the throttle to reduce power or shut down the engine during the excursion.