What happened
On January 16, 2019, a Robinson R44 Raven I, registration HB-ZDW, departed from Luzern Beromünster (LSZO) for a private sightseeing flight. The flight was intended to include a stop at Kemmeribodenbad, carrying the pilot and three passengers. After completing standard pre-flight inspections and refueling with approximately 95 liters of AVGAS, the pilot initiated the takeoff from runway 33.
During the initial climb, once the aircraft reached an altitude of approximately 2,800 ft MSL and a speed of 70 kt, the pilot experienced a sudden, violent leftward yaw. Simultaneously, the engine RPM dropped to 0%, triggering the "LOW RPM Rotor" warning signal. The pilot immediately lowered the collective pitch lever to initiate an autorotation and executed a wide left turn to return to the airfield.
As the pilot attempted to flare for landing on the glider runway, the helicopter made contact with the ground approximately 100 meters past the runway threshold. Due to the forward momentum and the soft nature of the terrain, the left landing skid sank into the soft grass. This caused the aircraft to pivot 90 degrees and eventually tip onto its right side. The aircraft sustained heavy damage, though all occupants were able to exit the helicopter uninjured.
The investigation
Investigators examined the aircraft's engine, fuel system, and electrical components to identify the source of the power loss. The fuel system was found to be functioning correctly, and laboratory analysis of the fuel showed no signs of water or significant contamination, though some deviations from standard AVGAS 100LL specifications were noted due to the presence of unleaded automotive gasoline, which is an approved alternative for this aircraft.
The Lycoming engine was subjected to a test run and performed within all manufacturer-defined operational limits. Additionally, the ignition switch and associated wiring were inspected; while the switch showed high contact wear due to its age, it was found to be fully functional. No mechanical or technical defects were identified that could have caused the failure.
Findings
- The engine failure occurred during the climb phase, necessitating an immediate transition to autorotation.
- The pilot successfully stabilized rotor RPM and managed the return to the airfield.
- The landing gear sank into soft ground, causing the aircraft to tip over.
- There was no evidence of carburetor icing, as the carburetor heat was fully engaged during the climb.
- The cause of the initial engine failure could not be determined through the technical investigation.