NTSB issues urgent safety recommendation regarding DHC-3 Otter aircraft

Avatar for Skies MagazineBy Skies Magazine | October 31, 2022

Estimated reading time 6 minutes, 39 seconds.

The U.S. National Transportation Safety Board (NTSB) issued an “urgent safety recommendation” to both the FAA and Transport Canada on Oct. 27, following a recent finding in its ongoing investigation of the crash of a DHC-3 Otter (N725TH) in Mutiny Bay, Washington.

The accident aircraft, operated by Renton, Washington-based Northwest Seaplanes under Part 135, was destroyed on Sept. 4, 2022, when it impacted Mutiny Bay and sank, killing the pilot and all nine passengers. It was a 1967 model, powered by the GE H80 engine.

N725TH, the accident aircraft. FlightAware Photo

The aircraft — which was destined for Renton Municipal Airport from Friday Harbor Seaplane Base — flew for about 18 minutes at altitudes between 600 and 700 feet mean sea level (msl), and at speeds between 115 and 125 knots. In the final minutes of the flight, security footage showed that the aircraft was in level flight, then climbed to a maximum altitude of roughly 1,000 feet msl, and suddenly pitched down “in a nose-low, near-vertical descent until water impact,” the investigation report states.

During its examination of the airplane wreckage, “the NTSB found that the clamp nut — which 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,” and there was no evidence of stripped threads.

It’s not yet clear whether the lock ring was present before the aircraft impacted the water. However, the investigation is ongoing, and the NTSB intends to review maintenance records and lock ring installation instructions, among other things.

The finding was identified as a serious safety concern, as the separation of the actuator barrel and the clamp nut would result in the actuator not being able to control the position of the horizontal stabilizer, resulting in a reduction or loss of pitch control.

As such, the NTSB is urging the FAA and Transport Canada to require all operators of De Havilland Canada DHC-3 Otter aircraft “to conduct an immediate one-time inspection of the horizontal stabilizer actuator lock ring.”

​​Clamp nut and barrel separation on accident airplane. NTSB Photo ​

NTSB chair, Jennifer Homendy, emphasized that “immediate action needs to be taken to inspect the actuator of DHC-3 airplanes . . . to prevent a similar tragedy from happening.”

Prior to the NTSB’s urgent recommendation, Viking Air Limited — the current certificate holder for the Otter — published a service letter stating that, regardless of when the most recent maintenance was completed, all DHC-3 operators should “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.”

The NTSB acknowledged that if DHC-3 operators follow Viking Air’s service letter, the safety issue should be adequately addressed. However, the service letter “is only advisory, and the potential for a catastrophic loss of control warrants immediate and mandatory action,” the NTSB said. “The FAA and Transport Canada have the authority to mandate such action.”

Clamp nut from accident airplane. NTSB Photo

Richmond, British Columbia-based Harbour Air is one of many operators in Canada flying the DHC-3 Otter, with 22 of the type in its fleet. The company has already completed the one-time inspection recommended by the NTSB and Viking Air; VP of sales and marketing, Meredith Moll, confirmed that nothing was found, and Harbour Air’s DHC-3s are back in the air.

Outside of Canada, 40 percent of the DHC-3 fleet operates in the U.S.

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