Flight History
On 19 May 1986, two pilots, both holding mountain landing endorsements, departed Kägiswil at 1356 local time for glacier landing training at Kanderfirn. Pilot X performed four landings and Pilot Y performed eight. The return flight to Kägiswil began at 1537 with Pilot X as commander on the front seat. During the flight, the pilots decided to make an unscheduled landing at the Rosenegg mountain landing site. A reconnaissance flight over the sunlit landing site was satisfactory. During the final approach, diffuse light made height estimation difficult.
Accident
After landing, the aircraft was turned with engine power and assistance from Pilot Y. During takeoff, acceleration was notably slower than usual due to soft snow. Shortly after lift-off, the aircraft banked to the right, the right wingtip contacted the snow, and the aircraft overturned, coming to rest on its right side. Both pilots exited the aircraft uninjured.
Rescue
The pilots activated the emergency locator transmitter (ELT) and built an igloo. Their equipment and provisions would have allowed them to survive for about two days. The aircraft was reported overdue to the Swiss Air Rescue (REGA) at 1900. At approximately 1930, a SARSAT ELT signal was received. A search flight eventually located the accident site at about 2230, aided by clear night skies and moonlight. Guidance of the rescue helicopter was hampered by ELT signal reflections in the mountainous terrain; the pilots noted that a glow stick or flashlight would have been useful for signaling.
Findings
The aircraft, a Piper PA-18-150 (registration HB-OKV), was airworthy with valid documents. Weight and balance were within limits. No pre-existing defects were found. The mixture was set to rich, carburetor heat to cold, fuel selector to left tank, throttle full, and flaps retracted. Weather at the time included a thunderstorm line, strong local variations in cloud and visibility, wind from SE-E at 10-15 knots, temperature 3°C, dewpoint 0°C, and QNH 1023 hPa. The snow surface was soft and heavy, with the right ski track showing deeper sinking.
Cause
The accident was attributed to insufficient acceleration during takeoff on unexpectedly soft snow.