Accident Details
On December 6, 1993, at 1042 local time, a McDonnell Douglas 269C helicopter (registration HB-XSK) sustained substantial damage after tipping over at Lauchboden, near Muotathal in the canton of Schwyz, Switzerland. The helicopter was operated by R. Fuchs-Bamert of Schindellegi. The crew, consisting of an instructor and a student pilot, were uninjured.
Flight History
The flight departed from Schindellegi at 1000 local time for a training session focused on alpine operations. The weather in the lowlands featured dense fog up to approximately 800 m above sea level, with clear skies and calm winds above. The flight proceeded via the Pragel Pass to the Charetalp area, where the crew conducted several external landings and hover exercises.
The instructor then flew to nearby Lauchboden (elevation 2009 m) to demonstrate the hazards of landing on snow in shadow or diffuse light conditions. The approach was performed with both pilots on the controls and was uneventful. The ground was hard and covered with approximately 10 cm of snow; the area was in shadow and the outside temperature was around -2°C.
The student pilot initiated the subsequent takeoff. During the takeoff, engine rpm dropped below 3,200 rpm. The instructor took control and began a turn back to the landing site. Due to the reduced main rotor rpm, the instructor intended to let the helicopter slide on the snow. During this sliding maneuver, the forward part of the right landing skid penetrated a snow-filled depression. Unable to lift off due to insufficient power, the instructor pulled the cyclic backward. At that point, a main rotor blade struck the tail boom, causing the helicopter to tip onto its right side.
Both pilots exited the heavily damaged helicopter without injury.
Causes
The accident was attributed to:
- misinterpretation of the terrain during landing;
- a loss of rotor rpm during the takeoff phase.