What happened
On December 31, 1976, a Pilatus Porter PC-6/B1-H2, registration HB-FFY, was operating a parachuting mission out of Sion Airport. The aircraft, operated by Air-Glaciers S.A., was tasked with dropping eight skydivers at an altitude of 3,000 meters.
Immediately following takeoff from runway 08, the aircraft entered an extremely steep climb. During this maneuver, the pilot attempted to retract the landing flaps, which were initially set at 28 degrees. As the flaps were being retracted, the pilot likely continued to apply back pressure on the elevator, causing the airspeed to decay. The aircraft began to lose altitude at approximately 30 meters above the ground. Despite efforts to recover the aircraft by pulling back on the controls, the plane remained in a high pitch attitude and struck the ground roughly 20 meters east of the runway threshold. The aircraft slid 40 meters along the ground before colliding with a runway arrestor bed winch. The impact resulted in the destruction of the aircraft and one injury among the passengers.
The investigation
Investigators examined the wreckage and flight controls, finding no evidence of pre-existing mechanical failure. The engine was found to have been operating at full power at the moment of impact. The investigation also reviewed the aircraft's weight and balance, noting that the total weight was approximately 5,223 lbs, which exceeded the maximum authorized weight of 4,850 lbs by about 373 lbs. Additionally, the aircraft was carrying eight passengers, one more than the maximum permitted capacity.
Crucially, the investigation looked into the status of the stall warning system. It was discovered that the stall warning had been intentionally deactivated. This practice was part of the pilot's training transition, as an instructor had recommended disabling the warning to prevent it from alarming passengers during turbulent mountain landings.