National Transportation Safety Board investigators have identified a possible safety issue with a flight control of the De Havilland Canada DHC-3 which crashed in Mutiny Bay, Washington on September 4, 2022. The pilot and nine passengers were fatally wounded.
Investigators found that the horizontal stabilizer actuator, which is part of the aircraft’s pitch trim control system and is also referred to as a trim ram, had separated into two parts. The actuator was found separated where the clamp nut threads into the barrel section. Examination of the threads inside the cylinder and the threads of the clamp nut revealed that the two components separated while unscrewing instead of being separated under tension. A circular wire lock ring is used to prevent the barrel and collet nut from unscrewing. The locking ring was not yet located in the wreckage. This possible safety issue could lead to an eventual loss of control of the aircraft.
At this time, the NTSB does not know if the lock ring was installed before the aircraft hit the water or why the lock ring was not present when examining the plane. the plane. The NTSB, in coordination with the Transportation Safety Board of Canada, has instructed the manufacturer to write instructions for an inspection of the actuator to ensure that the locking ring is in place and properly engaged to prevent the unscrewing the clamping nut. These instructions will be disseminated and provided to all DHC-3 aircraft operators worldwide in a service letter.
The investigation into this accident will continue with examinations of the two elevators in the NTSB Materials Laboratory; interviews with principal FAA operations and maintenance inspectors assigned to the operator; a review of maintenance records, including historical records of accidents, incidents and service difficulty reports and interviews with maintenance personnel. An assessment of lock ring failure modes and lock ring installation instructions will be completed in addition to a study of aircraft performance.
The wreckage recovery operation was completed on September 30 with approximately 85% of the aircraft recovered from the seabed. A field examination of the wreckage took place during the week of October 3, with follow-up activities at the NTSB Materials Laboratory in Washington, DC. The horizontal stabilizer actuator and both elevators were shipped to the NTSB Materials Lab for further examination.
All aspects of the accident are being investigated while the NTSB determines probable cause, with the goal of issuing safety recommendations to prevent similar accidents. The information in the update is preliminary and subject to change as the investigation continues.