Accident Sequence
The accident occurred on a maintenance test flight after the No. 1 engine had been replaced. During the return leg of the flight, the crew experienced a 'severe bump' followed by a 'loud bang'. Subsequently, the helicopter began to rotate to the right. The pilot executed a landing in a field, but during the landing the helicopter rolled over onto its left side.
Post-Accident Findings
Examination of the helicopter after the accident revealed that the No. 1 engine cowling had separated from the aircraft in flight. This separation caused damage to the main rotor blades, leading to an imbalance. The imbalance subsequently resulted in the loss of the tail rotor gearbox.
Inspection of the Hartwell cowling latches showed both latches in the closed position. However, the aft latch was undamaged, whereas the forward latch exhibited damage.
Manufacturer Response
The manufacturer is aware of similar problems with the cowling latches and has issued multiple service bulletins addressing the issue. The most recent service bulletin, SB-MBB-BK 117-20-109, had not been incorporated on the accident helicopter prior to the flight.