What happened
On November 12, 1979, a Pilatus PC-7 Turbo-Trainer, registration HB-HCN, departed Buochs airfield for a factory test flight. The flight, operated by Pilatus Flugzeugwerke AG, was intended to verify the symmetrical emptying of wing tanks. The aircraft was fully fueled at the time of departure.
After climbing to 10,000 feet, the pilot communicated intentions to proceed toward Muotatal. At approximately 11:24, the pilot contacted Buochs tower to request landing permission. Shortly thereafter, a witness observed the aircraft in a vertical descent, spinning around its axis, before it disappeared behind the horizon. At 1s126, the aircraft struck a transport cable and subsequently collided with the slope of the Alps Gigi at an altitude of 1,700 meters. Both crew members, factory pilots, were killed in the impact. The aircraft was destroyed.
The investigation
Investigators examined the wreckage and recovered data from the aircraft's instruments, including the altimeter, clock, and compass. The investigation established that the aircraft's landing gear and flaps were retracted at the time of impact. Analysis of the propeller blades revealed traces of the transport cable, indicating the aircraft struck the cable at an angle of approximately 20 degrees. The cable, which was not marked but was recorded on aeronautical charts, was severed by the propeller.
To understand the aircraft's behavior, a test flight was conducted using the same model. The tests demonstrated that when the wing tanks are full, the aircraft can overspin during a spin maneuver by up to 1.5 rotations. The investigation also noted that the pilot's physical injuries were consistent with an attempt to forcefully recover the aircraft from the spin.
Findings
- The aircraft was fully fueled, which significantly increased the risk of an overspin during maneuvers.
- The pilot likely initiated a spin maneuver to rapidly lose altitude for the approach.
- The aircraft likely overspun beyond the intended direction, leading to a flight path toward the terrain.
- The aircraft struck a transport cable, which was positioned in its flight path, before hitting the mountainside.
- There were no evidence of mechanical failure, and the crew members were found to be medically fit for flight.