SEATTLE — The National Transportation Safety Board investigating the crash of a seaplane last month off Whidbey Island on Monday said its experts identified a broken actuator that moves the plane’s horizontal tail that could potentially be the cause of the fatal accident that killed 10 people.
The actuator is the only means to hold the plane’s horizontal tail — also called the stabilizer — in its position.
Its separation “during flight would result in a free-floating horizontal stabilizer, allowing it to rotate uncontrollably … about its hinge, resulting in a possible loss of airplane control,” the NTSB update said.
The plane, a de Havilland Canada DHC-3 Otter turboprop operated by Renton-based Friday Harbor Seaplanes, crashed into the Puget Sound just off Whidbey Island about half an hour into a flight from Friday Harbor to Renton on Sept. 4. The pilot and all nine passengers were killed.
The investigators said that when the wreckage was retrieved the upper portion of the broken actuator was found still attached to the horizontal stabilizer while the lower portion was “attached to its mount in the fuselage.”
They said the separation of this critical component occurred when a clamp nut that should have been fixed in place by a circular wire lock ring unthreaded and rotated.
The NTSB said that while the lock ring was not located in the wreckage, they found that three of five holes drilled in the clamp nut to accept the lock ring were damaged “such that they would not allow for the full insertion of the lock ring.”
“This suggests that it may be possible for a lock ring to be partially installed … not fully seated in a hole in the clamp nut,” the NTSB said. “Further, it might be difficult to visually determine if the lock ring is fully engaged in the clamp nut hole” depending on conditions such as lighting, viewing angle, and the presence of dirt or grease.
The most recent overhaul of the plane’s horizontal stabilizer actuator was completed on April 21.
“At this time, the NTSB does not know whether the lock ring was installed before the airplane impacted the water or why the lock ring was not present during the airplane examination,” the update said.
As a result of this finding, the NTSB, in coordination with the Transportation Safety Board of Canada, has asked that the manufacturer draft instructions for all operators of DHC-3 aircraft to inspect the actuator to ensure that the lock ring is in place and properly engaged to prevent unthreading of the clamp nut.
Dozens of witnesses saw the plane “nose dive” into Mutiny Bay and helped officials narrow down the crash site. Still, it took over a week and three types of sonar to locate what remained of the plane due to its depth and the current of the channel where aircraft hit the water before fracturing beneath the surface.
Crews using remotely operated vessels and cranes recovered the majority of the plane’s wreckage from the sea floor more than 150 feet below the surface in late September.
Six bodies have been recovered. Those include the body of 29-year-old Gabby Hanna, which was recovered by witnesses the day of the crash, and five others found during recovery efforts.
Family members of those who have not been recovered are privately contracting remote submersibles to continue searching, since NTSB and Navy crews have stopped recovery efforts.
-------
