The National Transportation Safety Board today issued an urgent safety recommendation to the Federal Aviation Administration (FAA) and Transport Canada, acting upon the NTSB’s findings into the ongoing investigation of the Sept. 4, 2022, crash of a De Havilland Canada DHC-3 in Mutiny Bay, Washington.
The NTSB urgently recommends the FAA and Transport Canada require all operators of De Havilland Canada DHC-3 airplanes to conduct an immediate one-time inspection of the horizontal stabilizer actuator lock ring in accordance with the instructions in the Viking Air Limited service letter and report their findings to the FAA and Transport Canada, respectively.
“Immediate action needs to be taken to inspect the actuator of DHC-3 airplanes, of which 40% operate in the United States, to prevent a similar tragedy from happening,” said NTSB Chair Jennifer Homendy. “NTSB is issuing this urgent recommendation as a result of a significant finding made by NTSB investigators.”
Viking Air Limited, the current certificate holder for the DHC-3 airplane, yesterday published a service letter recommending DHC-3 airplane operators “visually confirm that the stabilizer actuator lock ring is present, correctly seated in the groove in the upper housing…and the lock ring tang is engaged in the clamp nut.” Viking Air Limited stated that this action was to be performed upon receiving this service letter, “regardless of when the most recent maintenance was completed.”
During the examination of the airplane wreckage, the NTSB found that the clamp nut that attaches the top eye end and bearing assembly of the horizontal stabilizer actuator to the actuator barrel had unscrewed from the barrel. The examination also found that the circular wire lock ring, which was designed to prevent the clamp nut from unscrewing, was not present. If the actuator barrel and the clamp nut are not secured together and become separated, the actuator would not be able to control the position of the horizontal stabilizer, resulting in a reduction or loss of pitch control.