What happened
On June 26, 1963, a Pilatus Turbo-Porter PC-6A, registration HB-FAD, was conducting a low-altitude aerial filming mission over the Crans golf course. The aircraft, operated by the Wallis section of the Ae.C.S., was carrying the pilot and three passengers. After completing four low-level passes, the pilot attempted to initiate a climbing left turn at approximately 80 mph.
During this maneuver, the pilot attempted to increase turbine power by adjusting the propeller pitch via the cockpit lever. However, the propeller pitch mechanism failed to increase the pitch. While the pilot found that the pitch could still be decreased by pulling the lever, the inability to increase power disrupted the aircraft's performance. The pilot subsequently experienced difficulty maintaining level flight, and the aircraft entered a continuous descent.
To manage the descent, the pilot deployed the landing flaps and moved the propeller into the braking position shortly before impact. The aircraft performed an emergency landing on a bushy slope near the hamlet of Luc, northeast of the Ayent area. While the aircraft sustained heavy damage, all four occupants escaped without injury.
The investigation
Investigators examined the mechanical state of the Pilatus Turbo-Porter PC-6A and the functionality of its Astazou II-E engine and Ratier-Figeac propeller. Post-accident inspections of the propeller pitch indicator showed a reading of approximately +30°, while the actual blade pitch was found to be around +45°. An electrical test of the propeller pitch motor showed it was functioning correctly, and no visible external defects were found in the adjustment mechanism.
The investigation also looked into the aircraft's flight control surfaces. The stabilizer was found in a full nose-heavy position. While the pilot reported a temporary loss of elevator and stabilizer effectiveness, investigators found no technical evidence of a control system failure and suggested this may have been due to pilot error under high-pressure conditions.
Findings
- The primary cause of the incident was a mechanical jam of a specific design element within the propeller pitch control mechanism, likely involving the guide piece inside the hub.
- The pilot's attempts to troubleshoot the malfunction by repeatedly manipulating the propeller lever exacerbated the situation. By pulling the lever to check for movement, the pilot inadvertently reduced the propeller pitch to its minimum setting of +2.5°, which significantly degraded the aircraft's glide performance and increased the sink rate.
- The aircraft's descent was further complicated by the pilot's management of the flight controls, which led to a highly nose-heavy configuration during the emergency descent.