A newly released National Transportation Safety Board update notes that investigators have identified a possible “safety concern” with the horizontal stabilizer of the float-equipped De Havilland Canada DHC-3 Turbine Otter that plunged nose down into Mutiny Bay, Washington, on September 4. The pilot and nine passengers died in the crash.
Investigators found the elevator actuator trim jack had separated into two pieces, with separation occurring where the clamp nut threads into the barrel section. Threads inside the barrel and those on the clamp nut revealed that the two components separated by unthreading as opposed to being pulled apart in tension. A circular wire lock ring used to prevent the barrel and clamp nut from unthreading was not located in the wreckage. About 85 percent of the aircraft has been recovered from the sea floor.
This issue could “result in a possible loss of airplane control,” the NTSB said. The Otter involved in the fatal accident was said to have taken a sudden nearly straight dive into the water from about 1,000 feet msl. At this time, the NTSB does not know whether the lock ring was installed before the airplane hit the water or why the lock ring was not present when the airplane wreckage was examined.
All aspects of the crash remain under investigation. Meanwhile, the NTSB, in coordination with the Transportation Safety Board of Canada, has asked that the airframe manufacturer draft instructions for an inspection of the actuator to ensure that the lock ring is in place and properly engaged to prevent unthreading of the clamp nut. Those instructions will be provided to all operators of DHC-3s worldwide in a service letter.
“The FAA is in close communication with Transport Canada, which originally certified the DHC-3,” said an agency spokesperson. “The Canadian manufacturer, Viking Air Limited, is working on a service bulletin. The FAA will take appropriate action based on the manufacturer’s service bulletin and any associated actions from the Canadian authorities.”