What happened
On July 5, 2015, a DB Aerocopter Ltd. AK1-3 helicopter departed from the Bitburg airfield for a short flight to an external landing site near Mülheim/Mosel. The flight involved a pilot and one passenger. After a 20-minute pause at the destination, the pilot attempted to return to Bitburg.
During the takeoff from the external site, the pilot initiated a vertical climb with a slight forward movement, flying parallel to the Moselle River. At an altitude of approximately 10 meters and roughly 20 meters from the riverbank, the engine experienced a loss of power. In an attempt to maintain rotor RPM, the pilot adjusted the collective pitch and the throttle, but the helicopter began to rotate counter-clockwise around its vertical axis and descend. Witnesses observed the aircraft performing several turns while sinking toward the water. The helicopter eventually settled on the water's surface, rolled to the right, and sank rapidly. Both occupants were able to exit the aircraft independently and swim to the shore without injury.
The investigation
The BFU examined the aircraft and the circumstances surrounding the power loss. The investigation included a technical inspection of the Subaru EJ-25 engine, the fuel system, and the cooling system. The aircraft, which was registered in France as an experimental model, had a total operating time of approximately 20 hours. Investigators also reviewed meteorological data, noting that while the official METAR reported temperatures around 33°C, the local temperature at the landing site was measured at approximately 39°C, resulting in a high density altitude of about 3,300 feet.
Findings
- The technical inspection of the engine, fuel supply, and cooling system revealed no evidence of mechanical failure, leaks, or malfunctions in the engine or flight controls.
- The investigation considered two possible triggers for the power loss: "overpitching" (excessive power demand leading to a loss of rotor RPM) or engine power loss caused by vapor lock (steam bubble formation) following the 20-minute pause in high temperatures.
- Neither of these potential causes could be definitively confirmed through the investigation.