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NTSB updates investigation of fatal Otter accident

NTSB investigators reported October 24 that a missing lock ring may have allowed the horizontal stabilizer to swing free in flight before a de Havilland Canada DCH–3 Otter plunged into Mutiny Bay off Whidbey Island, Washington, on September 4. The accident killed the pilot and nine passengers.

The DHC–3 wreckage was found with the clamp nut at left not seated in the barrel at right, having apparently unthreaded prior to the September 4 accident. A lock ring designed to prevent this unthreading was not found, and the manufacturer is drafting inspection guidance for the rest of the fleet. NTSB image.
The DHC–3 wreckage was found with the clamp nut at left not seated in the barrel at right, having apparently unthreaded prior to the September 4 accident. A lock ring designed to prevent this unthreading was not found, and the manufacturer is drafting inspection guidance for the rest of the fleet. NTSB image.

Inspections of other aircraft of the same model previously prompted an FAA airworthiness directive to inspect other parts of the empennage of all DHC–3 Otter aircraft in service, noting "several recent reports of cracks in the left-hand elevator auxiliary spar." The NTSB investigative update published about three weeks after that AD focused on another part of the empennage: the horizontal stabilizer actuator that is connected to the pitch trim wheel. The actuator was found separated, with undamaged threads suggesting that threads connecting the horizontal stabilizer to the flight controls had rotated out of place, leaving half of the assembly still attached to the fuselage and the other half to the horizontal stabilizer. A lock ring installed to prevent this separation was not found.

"The actuator is the only means to hold the horizontal stabilizer in its position, and the lock ring keeps the assembly from unthreading," investigators wrote. "Unthreading of the clamp nut and the barrel during flight would result in a free-floating horizontal stabilizer, allowing it to rotate uncontrollably (trailing edge up or down) about its hinge, resulting in a possible loss of airplane control."

The NTSB does not yet know what happened to the missing lock ring, or whether it was installed when the airplane hit the water. Investigators determined that it is possible to install the lock ring improperly and not be able to visually confirm the correct assembly due to a range of factors including viewing angle and the presence of dirt or grease.

"The NTSB, in coordination with the Transportation Safety Board of Canada, has asked that the 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 released and provided to all operators of the DHC-3 airplane worldwide in a Service Letter."

Portions of the accident aircraft wreckage that have been recovered are highlighted in green, and the recovered portions of the flight controls are highlighted in orange. NTSB image.
Jim Moore

Jim Moore

Managing Editor-Digital Media
Digital Media Managing Editor Jim Moore joined AOPA in 2011 and is an instrument-rated private pilot, as well as a certificated remote pilot, who enjoys competition aerobatics and flying drones.
Topics: Accident

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