What happened
On December 30, 1987, two pilots departed from Sion Airport with the intention of practicing glacier landings in the Diablerets region. The first aircraft, a Piper PA-18-180 (HB-PCH), carried an instructor and a student pilot. It was followed closely by the second aircraft, a Piper PA-18-150 Super Cub registered as HB-OPF, carrying the accident pilot and a private pilot passenger. The weather conditions were favorable, with clear skies and light winds.
The first aircraft completed a reconnaissance flight and landed on the glacier before taking off again to join the circuit. The second aircraft, HB-OPF, also performed a reconnaissance pass. During its final approach for landing, the pilot attempted to establish radio contact with the other aircraft but received no response. As HB-OPF came in for its second landing, it disappeared from sight. Approximately five minutes later, at 11:10 local time, the instructor in HB-PCH discovered the wreckage of HB-OPF on the lower part of the glacier. The impact occurred around 11:05 near the base turn to final approach segment.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the flight record, aircraft maintenance logs, and the crash site. The Piper PA-18-150 HB-OPF had been maintained by its owner-pilot, who held a valid aircraft controller license. No mechanical failures or foreign object damage were found in the wreckage. The engine was intact but showed no signs of rotation at impact, indicating it was not providing power when the crash occurred.
Medical autopsies confirmed that both occupants died instantly from the impact forces, with no evidence of alcohol influence or pre-existing medical conditions. The aircraft's weight at the time of the accident was calculated to be 823 kg, which exceeded the maximum authorized takeoff weight of 794 kg by approximately 3.5%. The altimeters in the wreckage were set to standard pressure settings (1031/1032 hPa).
Findings
The investigation concluded that the accident was caused by a stall at low altitude resulting from exceeding the minimum sustaining speed. Several contributing factors created a critical situation:
- Overloading: The aircraft was 3.5% overweight, reducing its performance capabilities.
- High Altitude Performance: At the operating altitude of roughly 2,700 meters, the 150-horsepower engine suffered a power loss of approximately 30%, significantly degrading climb and maneuvering ability.
- Mountain Landing Technique: The pilot likely lost excessive altitude during the departure from the glacier due to downdrafts and failed to maintain sufficient airspeed while attempting to climb or turn toward the landing area. The combination of low speed, high weight, and reduced engine power made recovery impossible.
Safety action
The report highlights the dangers of operating underpowered aircraft at high densities altitudes when loaded near maximum limits. It underscores the necessity for pilots to accurately calculate performance margins and adhere strictly to mountain flying techniques that account for wind effects and terrain constraints.