What happened
On September 9, 1970, a Pilatus PC-7 (registration HB-HON) was conducting a VFR radar flight near Unterehrendingen, Switzerland, to test a fire-control device. The flight was being operated by Contraves AG. During the mission, the pilot was managing fuel levels across several tanks, including wing tip tanks and main wing tanks. At an altitude of approximately 80 meters, the engine power suddenly dropped. Despite the pilot's attempts to restart the turbine and manipulate the fuel selector, the engine failed to recover.
Unable to maintain flight, the pilot performed an emergency landing on a field with the landing gear retracted. The aircraft struck the ground on its tail and fuselage, sliding 85 meters before coming to a halt in a field. While the aircraft sustained significant damage, including a punctured wing tip tank and a separated propeller, the pilot and the passenger escaped the wreckage without injuries.
The investigation
Investigators examined the aircraft's fuel system, which had been modified from its original piston-engine configuration to a turboprop setup. The investigation included a technical teardown of the engine and fuel components at the manufacturer's facility in Stans. The team inspected the fuel pumps, filters, and valves, as well as the integrity of the various tanks.
Findings revealed that the fuel selector lever had been deformed during the impact, which could have caused it to settle in an intermediate position. Additionally, the investigation noted that the checklist being used by the pilot was outdated and did not reflect the specific fuel switching procedures required for the modified tank configuration.