What happened
On September 16, 1959, a Piper Super Cub HB-OPT, operated by the Swiss Air Rescue (Schweizerische Rettungsflugwacht), departed from Speck airfield to conduct a search mission near Andeer. The mission was initiated to locate a hiker who had gone missing in the mountains near Lai Lung.
During the search, the aircraft flew through several phases, covering areas including the slopes south of Lai Lung and the hills west of the Hinterrhein. At approximately 15:15, while flying from a northerly direction, the aircraft entered the terrain depression of Alp D/Durnan at an altitude of no more than 50 meters. This was significantly below the required minimum flight altitude of 150 meters. The aircraft struck the rising ground in a nearly horizontal attitude with high engine power but insufficient forward speed and altitude. The impact destroyed the aircraft. The pilot sustained minor injuries, while the passenger, an operations leader, suffered severe injuries, including a brain contusion.
The investigation
The investigation examined the flight conditions, the pilot's background, and the aircraft's mechanical state. The aircraft was found to be in good condition, though it was noted that the flight was overweight by approximately 30 kg due to the inclusion of heavy mountaineering gear. The investigation also reviewed the pilot's history, noting that while he had no prior accidents, he had been previously warned by airport authorities for unsafe and undisciplined flying habits.
Investigators also looked into the fuel system, noting a discrepancy regarding the fuel selector valve, which was found in the right-tank position, though the pilot provided conflicting accounts of its setting. The weather at the time featured light to moderate clouds and visibility limited by haze, with potential downdrafts of up to 5 m/sec caused by winds from the south.
Findings
- The primary cause of the accident was that the pilot entered a mountain valley at an altitude below the prescribed minimums without sufficient altitude or airspeed reserves.
- The aircraft encountered unfavorable wind conditions, specifically downdrafts, which contributed to the impact.
- The pilot's flying style was characterized by a lack of sufficient safety margins.
- The aircraft was operating above its maximum allowable takeoff weight due to the weight of the search equipment.