The National Transportation Safety Board (NTSB) reports that the failure of a single component of a critical flight control is to blame for the deadly crash of a floatplane near Seattle on September 4, 2022. The 1967 de Havilland DHC-3 Turbine Otter went down in Mutiny Bay near Freeland, Washington, killing all 10 on board.
The aircraft was owned by Northwest Seaplanes and the flight operated by West Isle Air, a Part 135 operation. The aircraft was en route between Friday Harbor Seaplane Base (W33) and Renton Municipal Airport (KRNT) at the time of the accident. It was the pilot’s second flight of the day, and the accident occurred approximately 18 minutes into it.
According to the 59-page NTSB report, the ADS-B data indicates the aircraft was at an altitude between 600 and 1,000 feet msl, and the groundspeed was between 115 and 135 knots.
The aircraft was over Mutiny Bay approximately 34 miles northwest of Seattle at an altitude of about 1,000 feet when, according to ADS-B, the groundspeed decreased to 111 knots and the aircraft pitched up 8 degrees. However, according to witnesses and surveillance video, the aircraft appeared to be in level flight then suddenly pitched down, plunging into the water nose first. Investigators estimated the rate of descent to be in excess of 9,500 feet per minute. The impact was so violent that the aircraft was heavily fragmented, and the pilot and all nine passengers were killed.
The woman’s body was recovered from the water by citizens who witnessed the event. The bulk of the fuselage sank into 200 feet of water. Several government agencies, including the Coast Guard, NTSB, and U.S. Navy, spent several days searching for the wreckage. Because of strong currents and murky water, the recovery of the wreckage and the remains of the occupants took the better part of a month. Crews were able to recover approximately 85 percent of the aircraft pulled from the ocean floor.
The postaccident investigation revealed the actuator to the elevator, which controls the pitch of the airplane, had become disconnected from a control linkage. According to the NTSB, this “would have made it impossible for the pilot to control the airplane’s pitch.” Based on the evidence presented, the agency concluded that the flight control failure happened before the crash, not as a result of it.
Close inspection of the wreckage revealed the lower barrel of the actuator that attaches the cable for the elevator had separated. The threads that screw the two parts together were found intact. However, a single wire lock ring used to secure the two parts together was missing.
The NTSB found that if a lock ring is not present to secure the actuator barrel and the clamp nut together, they can become separated, and the actuator would not be able to control the position of the horizontal stabilizer, resulting in a loss of airplane pitch control.
“The probable cause of this accident was the in-flight unthreading of the clamp nut from the horizontal stabilizer trim actuator barrel due to a missing lock ring, which resulted in the horizontal stabilizer moving to an extreme trailing-edge-down position rendering the airplane’s pitch uncontrollable,” the NTSB report stated.
Emergency Action
Based on this finding, on October 26, 2022, the NTSB issued an urgent recommendation to the FAA and Transport Canada to require all operators of DHC-3 airplanes to conduct an immediate inspection of the aft flight control system.
The NTSB noted that when the airplane’s design was certificated by the FAA in 1952, there was no requirement for a secondary locking device to secure flight control linkages. However, in 1996, regulations were amended to require newly designed aircraft to have a secondary locking device “if the loss [of the first device] would preclude the continued safe flight and landing.”
There was no requirement for retrofitting existing airplanes with a similar safety feature, and as such the accident airplane had only a single locking device.
Recommendations
As a result of the investigation, the NTSB recommended the FAA and Transport Canada require operators of DHC-3 airplanes to install a secondary retention feature to prevent a single point of failure in the flight control system. Additional recommendations were made to both agencies as well as to the current type certificate holder, Viking Air.
“The Mutiny Bay accident is an incredibly painful reminder that a single point of failure can lead to catastrophe in our skies,” said NTSB Chair Jennifer Homendy. “To adequately protect safety, we must build in the necessary redundancies across the entire aviation system. We’re calling on the Federal Aviation Administration and [its] Canadian counterparts to eliminate the safety vulnerability identified by NTSB investigators, so this kind of tragedy never happens again.”
The NTSB’s 59-page final report can be found at NTSB.gov.
