What happened
On 29 January 1999, an AS365N2 helicopter, registration G-BTEU, was performing a public transport flight from the 'Audrey X' offshore satellite. Shortly after takeoff, the crew noticed a significant restriction in the aircraft's flight controls during the climb. As the aircraft reached 500 feet, the pilots attempted to assess the situation, finding that both the cyclic and collective controls were severely limited, making the aircraft difficult to maneuver.
The crew initiated an emergency diversion to the 'Loggs' platform. During this transit, the control restrictions worsened. In the final stages of the approach, both pilots were required to use the controls simultaneously to maintain sufficient command to land the aircraft safely. There were no injuries to the two crew members on board.
The investigation
Following the landing, the aircraft was shut down and inspected. Investigators discovered that the gas strut for the starboard rear door had become detached at its forward end. This detached strut had pierced the trim base of the 9-o'clock frame, which also serves as part of the heater diffuser for the rear cabin.
The investigation established that the strut had become trapped by the displaced inner wall of the diffuser box. This placement caused the strut to make contact with the roll control rod, which runs in close proximity to the trim. The resulting contact pressure increased friction within the roll control channel, thereby impairing control harmonisation.
Findings
- The primary cause of the control difficulty was the mechanical interference between the door gas strut and the roll control rod.
- The gas strut is secured via a ball fitting and a clip; however, the clip had been removed to facilitate the door jettison function for this specific aircraft's mission.
- A review of the MOR database revealed that an identical incident had occurred in September 1997.
Safety action
Following the 1997 incident, the operator implemented an interim modification involving a protective plate designed to prevent the strut from penetrating the trim panel. To address the underlying issue of the strut disconnecting, a second modification was planned for completion by late March 1999. This new design aimed to prevent the inadvertent disconnection of the door restraint strut while still allowing the door to be jettisoned in an emergency.