Incident Overview
On 4 September 2013, at approximately 11:05 local time, a Eurocopter AS350 B3 helicopter, registration HB-ZNB, operated by Heli-Linth AG, experienced a serious incident near Glarus, Switzerland. During descent, the left cargo door became detached and struck a main rotor blade. The pilot, alone on board, noticed a warning light and heard a bang, reduced speed, and continued to land at the base in Mollis without further issues at 11:10. The pilot was not injured; the aircraft sustained light damage.
Sequence of Events
Earlier that morning, the helicopter had conducted passenger flights from Mollis. After refueling, a different pilot took over with five passengers, landing at the Planurahütte mountain hut. The hut keeper opened the left sliding and front doors while rotors were turning, assisted passengers off, closed the doors, and signaled the pilot for departure. The pilot then flew solo back to base. During descent, the warning light and bang occurred. After landing, it was discovered that the left cargo door had been torn off and had collided with a main rotor blade. A thin interior panel was also sucked out. The cargo door had not been used that day; a pre-flight check that morning had found no discrepancies.
Investigation
The investigation was a summary report (no formal inquiry). Examination of the recovered forward half of the door and the hinges showed deformation consistent with overstress. The main rotor blade damage was caused by the middle snap latch. The front snap latch functioned properly. The closure angle of the helicopter-side component showed deformations possibly indicating previous manual straightening. Stress analysis at 170 knots descent matched the observed damage for scenarios where one or more latches were not properly closed.
Findings
No pre-existing technical defects were found in the latching system. It could not be excluded that the snap latches were not correctly closed before the flight. Photographs taken by a third party at the last takeoff from the hut showed the cargo door latches in the closed position. The middle latch, lockable with a key, had never been locked by the crew. The manufacturer stated this was the first known incident of this type since a 2009 modification; three previous similar incidents (2008, 2009, 2011) were documented.
Aftermath
Following the incident, the operator relocated the lockable latch to the front position and now always secures it with the key to prevent inadvertent opening.