Accident Overview
On 26 July 2003, at approximately 15:08 local time, a Robinson R44 Raven helicopter, registration HB-ZCU, was involved in an accident near Selzach, in the municipality of Grenchen, Solothurn, Switzerland, about 25 km north of Bern. The helicopter was on a private VFR flight with a pilot and two passengers on board.
Flight History
The flight originated from Sitterdorf at around 11:50. After a stop in Bern for refueling and a subsequent landing at Kernenried BE to exchange passengers, the helicopter departed Kernenried at approximately 14:45. The route took the pilot over several locations before contacting Grenchen control tower to request transit through the control zone towards Biel.
Engine Failure
While cruising at about 4500 ft AMSL and 80 kt, the pilot experienced low-frequency vibrations. The engine rpm gauge fluctuated, and the clutch warning light briefly illuminated. Despite reducing power and speed, the situation worsened. The helicopter began to oscillate abruptly around the vertical axis. As rotor rpm decreased, the low rpm warning light and horn activated, prompting the pilot to perform an autorotation.
Autorotation and Landing
During the autorotation, the pilot informed the tower of the emergency landing. In the final phase, the tail rotor protection skid contacted the ground. The helicopter landed with significant sink rate but negligible forward speed. At that moment, a main rotor blade struck the tail boom. After coming to rest, the pilot shut down the engine and applied the rotor brake. The occupants exited the helicopter uninjured. The helicopter sustained damage, and minor crop damage occurred from oil spillage, which was contained by police.
Cause
The investigation determined that the accident occurred because a main rotor blade struck the helicopter's tail boom at the end of an autorotation. The autorotation was necessitated by a loss of engine power during cruise flight. The engine failure resulted from a blocked exhaust valve in one cylinder and a broken pushrod.