What happened
On the night of October 18, 2014, a pilot and a passenger departed from the Frankfurt-Egelsbach airfield (EDFE) for a flight to Augsburg (EDMA). Shortly after takeoff, while flying east over a local industrial area, the occupants heard sudden, loud noises. The pilot, unable to identify the source of the noise, attempted to return to the airfield. During the maneuver, the pilot experienced a sudden loss of engine RPM.
In response to the power loss, the pilot initiated an autorotation and steered the Cabri G2 toward an open field. During the subsequent landing, the helicopter slid across the soft ground, causing the main rotor to strike the tail boom. The aircraft sustained heavy damage, including destruction of the Fenestron blades and the horizontal stabilizer. Both occupants escaped the aircraft without injury.
The investigation
The BFU investigation focused on the mechanical failure of the engine' and transmission components. Investigators examined the wreckage and found the drive belt, which transmits power from the Lycoming O-360-J2A engine to the main gearbox, had disintegrated into numerous strips and pieces. Some of these fragments had become wrapped around the tail rotor shaft.
Upon disassembly of the aircraft at the manufacturer's facility, investigators discovered that the left-hand drive belt guide was bent, which likely resulted in insufficient clearance between the guide and the belt. Furthermore, the area surrounding the drive belt, including the firewall and gearbox interface, was heavily contaminated with oil and grease. The recovered fragments of the belt were also found to be saturated with oil and grease, with the rubber ribs feeling soft and greasy to the touch.
Findings
- The primary cause of the engine power loss was the failure of the drive belt.
- The drive belt was contaminated with oil and grease, which likely contributed to its degradation.
- A bent belt guide reduced the necessary clearance, potentially contributing to the belt's failure.
- Previous maintenance records showed that the belt had been replaced earlier in 2014 due to slipping caused by similar oil and grease contamination.