What happened
On May 20, 1970, during the Swiss gliding championships at Amlikon, a Fournier RF 4 D motor glider, registration HB-2004, was performing a demonstration flight. The pilot began the flight at approximately 15:18 local time, executing several climbing turns and left-hand circles at an altitude of roughly 35 and 50 meters.
As part of the planned demonstration, the pilot reduced airspeed and shut down the engine with the intention of completing the flight via a dead-stick landing. To prepare for the landing, the pilot used a pull-cord to rotate the propeller into a horizontal position and extended the landing gear. During this phase, the aircraft lost sufficient airspeed, causing the left wing to drop and the aircraft to enter a spin.
Although the pilot managed to recover from the spin using opposite rudder and neutral elevator, the aircraft's altitude was critically low. During the recovery, the pilot attempted to restart the engine twice, which failed. The aircraft regained some speed just above the ground, narrowly clearing a power line, but the pilot was unable to arrest the descent. The Fournier RF 4 D struck the ground with a left-wing low attitude outside the airfield. The aircraft was destroyed, though the pilot escaped without injury.
The investigation
The investigation examined the aircraft's condition, the pilot's experience, and the environmental factors present at the time of the accident. The pilot was a highly experienced glider instructor with nearly 500 total flight hours. No pre-existing damage was found on the aircraft, and there were no indications of health issues affecting the pilot's performance.
Weather observations at the time indicated overcast skies, visibility of 10 km, and light wind turbulence. The investigation focused on the sequence of maneuvers involving the engine shutdown and the subsequent loss of airspeed.
Findings
- The primary cause of the accident was the pilot falling below the minimum flying speed during the demonstration.
- The aircraft entered a spin due to the combination of low airspeed and existing wind turbulence.
- The pilot's attempts to restart the engine at a very low altitude contributed to a loss of altitude that prevented a successful recovery of the flight state.