Accident Overview
On 20 September 1989 at 1235 local time (UTC+2), a Piper PA-18-150, registration HB-PBA, operated by a private owner on a commercial sightseeing flight, was involved in an accident during takeoff from the Jungfraujoch mountain landing site in Valais, Switzerland. The aircraft sustained heavy damage to the landing gear and fuselage.
Sequence of Events
The pilot, a Swiss national born in 1950 holding a commercial pilot license with mountain landing endorsement, had 2596 total flight hours, including 2400 hours on type. He landed solo at Jungfraujoch at 1135, having departed from Bern-Belp Airport. Between 1209 and 1221, he conducted a first sightseeing flight with a passenger, with uneventful takeoff and landing.
For a second flight, a female passenger took the rear seat. After briefly warming the engine, the pilot increased power, causing the aircraft to begin sliding slowly. Despite fully applying left rudder to reach the normal takeoff track, the pilot could not bring the aircraft into the fall line. The aircraft, already moving at considerable speed, could not be stopped. To avoid three tourists in the slide path, the pilot steered the aircraft onto a nearby track prepared by a snow groomer. Unable to leave this track due to snow accumulations on the sides, the pilot attempted to take off from the sloping track. After a few meters, he became airborne by extending flaps. However, the track leveled off after 180-200 m, and the left ski and left landing gear collided with partially frozen snow accumulations on the left side, causing the aircraft to impact the ground hard after 10-20 m.
Findings
- The pilot held a valid commercial pilot license with mountain landing endorsement and was authorized for the intended flight.
- The aircraft was airworthy and certified; no technical defects were found that could have contributed to the accident.
- The Piper PA-18 is a tailwheel aircraft with a fixed tail ski. The unit is steerable via the rudder through tension springs.
- Weather at the Sphinx Observatory (relevant to the accident): clear, visibility 15 km, wind from southeast at 20 kt with gusts to 24 kt, temperature 0°C, dew point -2°C, sun azimuth 165°, elevation 43°.
- At approximately 1330, the investigator observed that the ski tracks were still clearly visible. The edges of the tracks and the snow accumulations along the groomer-prepared track were partially frozen.
Assessment
When the tail ski enters an old, frozen track, it becomes very difficult for the pilot to change direction. If the aircraft slides on a sufficiently steep slope, the takeoff cannot be aborted. The pilot's decision to attempt takeoff on the nearby groomer track was correct, especially since no people were in the takeoff direction on that track.
The wind measured at the Sphinx Observatory came from the southeast at 20 kt, but it was deflected by the Mönch massif and the ridge between Mönch and Sphinx, blowing from the north at ground level on the northern part of the Jungfraufirn. Thus, the aircraft experienced a tailwind during takeoff in the track, worsening the situation given the low gradient. The collision of the left landing gear with snow accumulations on the left edge of the track was unavoidable.
Cause
The accident was due to the tail ski entering a wrong track during takeoff. A contributing factor was tailwind.