What happened
On September 4, 2013, an Airbus AS350 B3 helicopter, registration HB-ZNB, was performing commercial passenger flights near Glarus, Switzerland. After transporting passengers to the Planurahütte landing platform, the pilot began the return flight to the Mollis base alone. During the descent phase, the pilot heard a loud bang and observed the "DOORS" warning light illuminating.
Despite the sudden event, the pilot maintained full control of the aircraft and experienced no vibrations. The pilot immediately reduced airspeed to prepare for a potential autorotation and successfully landed the aircraft at the Mollis base without further incident. Post-flight inspection revealed that the left cargo door had been torn from the aircraft and had collided with a main rotor blade. A portion of the interior lining was also lost due to airflow entering the compartment.
The investigation
Investigators examined the wreckage and the aircraft's maintenance history. The investigation confirmed that the damage to the main rotor blade was caused by the middle latch of the cargo door. While the front latch was found to be functional, the helicopter-side locking component showed deformations that suggested it might have been manually straightened in the past.
Technical stress analyses performed at 170 knots during descent showed that the deformations found on the hinges were consistent with the loads experienced if one of the latches was not properly engaged. The investigation also looked into the actions of the ground personnel at the Planurahütte, who had been recently instructed on safety procedures, but found no evidence that they had tampered with the cargo door locks.
Findings
- The primary cause of the incident was the detachment of the cargo door, which subsequently struck the main rotor blade.
- It could not be ruled out that the latches were not correctly secured prior to departure.
- The middle latch, which is designed to be secured with a key, had historically been left unlocked by the crew for convenience because the cargo compartment was rarely used.
- There were no evidence of pre-existing technical defects in the locking system that would have caused the failure.
Safety action
To prevent accidental opening during passenger handling or by third parties, the operator has swapped the positions of the front and middle latches. The lockable middle latch is now positioned at the front, and the crew now uses the provided key to ensure the door is securely fastened.