Service Letter Published to Address Possible Issue with Pitch Control Assembly

The NTSB has issued an expected service letter regarding the De Havilland Canada DHC-3 after finding a possible cause in the course of investigation regarding the September Mutiny Bay Crash. 

The NTSB urgently recommends that operators 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. Investigation of the submerged wreckage and debris uncovered the lack of a critical locking part from the aircraft’s pitch control assembly that likely led 
to its downfall. 

“During the examination of the airplane wreckage,” the NTSB found, “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.”

Viking Air Limited, the holder of the DHC-3 certificate, published a service letter recommending that 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.”

The “fix”, as it were, is thankfully simple, and decidedly affordable as a simple inspection can ensure continued safety of flight. “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.”

FMI: www.ntsb.gov