What happened
On August 14, 2016, a pilot and one passenger were conducting a scenic flight over the Valais Alps in an HK36 Super Dimona, registration HB-2360. The flight originated from Sion under visual flight rules. At approximately 10:20 local time, while flying over the Rocher de Naye, the pilot intentionally shut down the engine to transition the aircraft into glider mode, setting the propeller to the feathered position.
As the flight continued toward Bex, the pilot attempted to restart the engine using the starter and transition the propeller to a low pitch to allow for windmilling. Despite multiple attempts, the engine failed to restart and the propeller failed to windmill. Due to insufficient altitude to reach a suitable airfield in glider mode, the pilot performed an emergency landing in a field near Chessel (VD) at 1s0:33. The occupants sustained one slight injury, though the aircraft remained undamaged.
The investigation
Following the incident, assistance arrived at the landing site and successfully restarted the engine by following the manufacturer's prescribed procedures. Because the cause of the in-flight failure remained unknown, the aircraft was disassembled and transported by road to Sion for further inspection. Upon reassembly, investigators could not identify a definitive technical fault that prevented the engine from starting. The only anomaly detected was low pressure in the nitrogen accumulator used for propeller pitch adjustment. While this accumulator is essential for transitioning the propeller from feathered to low pitch while the engine is stopped, the engine should theoretically still be capable of starting via the electric starter even in the feathered position.
Findings
- The low nitrogen accumulator pressure prevented the propeller from being adjusted to a low pitch while the engine was stationary.
- It is probable that the engine restart procedure applied by the pilot was inadequate for the specific conditions encountered.
- Although the low nitrogen pressure hindered the propeller pitch change, it should not have prevented the engine from starting via the starter motor.