What happened
On July 19, 2001, a Reims Aviation FR 172H, registration HB-CVF, was conducting a private VFR flight from Samedan. The pilot, acting on a request to transport a passenger to Zurich, was operating under significant time pressure to return to Samedan before the airport closed at 19:00. After departing from runway 21, the pilot initiated a left turn toward La Punt and entered the narrow Albula Valley at a very low altitude.
Witnesses observed the aircraft flying extremely low, appearing to lose altitude as it progressed through the valley. In an attempt to navigate through a low point in the terrain near Plaun da las Ovas, the aircraft struck the ground and overturned in a mountain stream. The impact was nearly vertical, resulting in two fatalities, including the pilot and the passenger.
The investigation
SUST examined the aircraft's mechanical state and the pilot's operational decisions. Investigators found that the engine was producing power at the time of impact, but the fuel mixture was excessively rich. A reconstruction flight revealed that the electric auxiliary fuel pump had been left in the "low" position, which, when combined with a rich mixture, causes the engine to struggle. It was determined that the passenger's left knee likely moved the mixture control lever into the "full rich" position during the flight.
Furthermore, the investigation looked into the engine's performance at high density altitudes. The engine's maximum power is significantly reduced at the altitudes encountered during the flight (between 1,706 m and 2,280 m). The pilot's decision-making was also scrutinized, specifically regarding the lack of altitude margin for a turnaround in the narrow valley.
Findings
- The primary cause was the loss of control due to unsuitable mountain flying tactics.
- The pilot entered a narrow valley at an altitude too low to allow for a safe return turn.
- Significant time pressure to meet the airport's closing deadline contributed to rushed preparations and risky maneuvering.
- The pilot overestimated the available engine performance at high altitude.
- The electric auxiliary fuel pump was inadvertently left in the "low" position, contributing to an improper fuel-air mixture.