What happened
On May 10, 2011, at approximately 21:30, a police MD 900 helicopter was conducting a nighttime search for a missing person near Engelsbrand, Germany. The crew, consisting of a pilot, a flight technician, and a FLIR operator, had transitioned from daytime visual flight to night operations using night vision goggles (NVG).
While attempting to position the aircraft over a thermal target identified by the FLIR camera, the helicopter experienced a sudden loss of control. Witnesses on the ground observed the aircraft hovering low over a wooded ridge before it suddenly pitched nose-down and disappeared into the trees. Some witnesses reported hearing sounds similar to engine misfires just before the impact. The aircraft struck trees, causing the tail boom to separate and the main rotor blades to break, before the wreckage came to rest in the forest. All three persons on board sustained serious injuries.
The investigation
The BFU investigation examined the aircraft' and the crew's operational procedures. The investigation found that the aircraft's engines, main gearbox, and hydraulic systems were functioning correctly prior to the crash. Data from the engine's Data Collecting Units (DCU) revealed that the pilot reduced the collective pitch approximately eight seconds before the end of the recorded data, then rapidly increased it one second later.
The investigation also reviewed the crew's use of night vision goggles (NVG) and the lack of specific procedures in the police flight manual for conducting person searches at night. While the aircraft was equipped with a radar altimeter, the investigation noted discrepancies between the recorded altitude and the crew's perception of their height above the terrain.
Findings
The primary cause of the accident was the loss of control over the helicopter following an unnoticed loss of altitude and a subsequent backward movement. Several contributing factors were identified:
- The use of night vision goggles (NVG) which, while providing visibility, also imposed significant optical and spatial limitations.
- A loss of spatial awareness regarding altitude and airspeed during the transition to a hover.
- Distracted attention within the crew, as the FLIR operator and flight technician were focused on the search target rather than monitoring the aircraft's flight path.
- Inadequate operational procedures within the flight manual regarding the specific hazards of nighttime person searches and the transition from day to night flight.
- The pilot's attempt to correct a rightward yaw by reducing power, which inadvertently led to a fatal loss of altitude.