What happened
On November 9, 2007, an Eurocopter AS 350 B1 helicopter crashed in a forested area near Hammerbrücke, Germany. The aircraft had been stationed at a forest clearing for a forest liming mission and had been parked overnight. Following a period of snowfall during the night, the pilot prepared for a weather reconnaissance flight before moving to a different location for the day's operations.
During the flight, the helicopter encountered difficult weather conditions, including low visibility due to clouds and wind gusts. The aircraft eventually struck trees in a descending forest area approximately 300 meters southwest of its departure point. The impact destroyed the landing gear and sheared the tail boom. The pilot sustained serious injuries.
The investigation
The BFU investigation focused on the mechanical state of the engine and the environmental conditions at the time of the accident. The aircraft was equipped with a sand filter (particle separator) on the engine air intake. Investigators examined the engine via borescope and conducted tests at the manufacturer's facility; no mechanical defects or issues with the fuel control pump were identified. The fuel itself was found to be free of water contamination.
Investigators also noted that the electrical fuel pumps were found in the "OFF" position, despite the flight manual requiring them to be active. However, the mechanical fuel pump was found to be fully functional. The wreckage pattern suggested that the main rotor was likely not under power and was rotating at a reduced RPM at the moment of impact with the trees, indicating a significant loss of engine power.
Findings
- The crash was caused by a highly probable engine disturbance.
- The specific cause of the engine disturbance could not be determined.
- While the electrical fuel pumps were switched off, the mechanical pump was capable of maintaining engine operation.
- Environmental conditions included freezing temperatures and high humidity, which could have led to icing.
- It was noted that the engine air intake sand filter could potentially be susceptible to engine failure if snow or ice accumulates on its underside, a phenomenon previously highlighted in an FAA safety bulletin.