SEATTLE — The federal agency investigating the September seaplane crash off Whidbey Island said Monday that its experts have identified a potential cause of the deadly accident and called for similar planes to be grounded until they are inspected.
“We’re concerned that another plane could crash as a result of something similar,” said Jennifer Homendy, chair of the National Transportation Safety Board, the agency investigating the Sept. 4 crash that killed 10 people. “Right now, the focus has to be (on) what could contribute to a further tragedy in the future.”
However, the NTSB cannot order the grounding of the de Havilland Canada DHC-3 Otter turboprops. That authority resides with the Federal Aviation Administration, which has not yet issued a directive.
When investigators examined the wreckage of the crashed plane, they found that a critical component that moves the plane’s horizontal tail had come apart. In a lab examination last week, the NTSB determined the part had come apart prior to impact.
Its failure could lead to loss of control and send a plane into a nose dive, which is what dozens of witnesses reported seeing.
The detail in the NTSB update strongly suggests a possible maintenance oversight.
David Gudgel, chief operating officer of local operator Kenmore Air, said his company was made aware of the vulnerability last week and immediately grounded its fleet of 10 Otters. All inspections were then quickly completed without a break in service, he said.
Homendy said the agency issued its investigation update to get the word out to all companies flying the Otter to “make sure that they are inspected immediately before they operate these planes again.”
The DHC-3 Otter that crashed was operated by Renton-based Friday Harbor Seaplanes. This aircraft is an aging workhorse of the Puget Sound air transportation system that routinely flies tourists and commuters to the San Juan islands and Canada.
It was first built in the 1950s and a total of 466 were produced through 1967. Today, only 65 Otters remain flying in the U.S. and 160 worldwide.
The NTSB is working with Viking Air of Canada, which took control of the Otter’s maintenance and certification requirements, on a service bulletin that will go out to all operators of the airplane with detailed inspection instructions.
Doug Brazy, the NTSB investigator in charge of the Whidbey crash, said he expects that letter to be approved and sent out “sometime this week” and that it will likely have urgent wording recommending that the inspections be completed before further flight.
In a statement, the FAA said the agency is in close communication with Transport Canada, which certified the Otter, and Viking Air.
“The FAA will take appropriate action based on the manufacturer’s service bulletin and any associated actions from the Canadian authorities,” the statement said.
Kenmore Air, with all its Otters having already passed inspection, on Monday afternoon was offering tickets for a scenic tour of the San Juan Islands from its Lake Washington base aboard an Otter.
A critical component comes apart
The component that NTSB investigators found had come apart is an actuator that swivels the horizontal tail — also called the stabilizer — to control the pitch of the airplane.
A cable from the cockpit wraps around the barrel of the actuator, providing the mechanical connection between the stabilizer and the pilot.
This component coming apart “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 seaplane crashed into Puget Sound just off Whidbey Island about half an hour into a flight from Friday Harbor to Renton. The pilot and all nine passengers were killed.
The investigators said that when the wreckage was retrieved, the upper portion of the actuator was found still attached to the horizontal stabilizer while beside it, but disconnected, the lower portion was “attached to its mount in the fuselage.”
They said the separation of this component occurred when a clamp nut that should have been fixed in place by a circular wire lock ring unthreaded and rotated.
NTSB’s Homendy said the separation of the actuator almost certainly didn’t happen as a result of the impact. If that were the case, the threads on the clamp nut and inside the barrel of the actuator would have been stripped, she said.
The fact that the threads were intact suggests instead that it separated because the clamp nut and lock ring were not securely fastened.
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.
Homendy said one possibility is that the clamp nut “was not screwed in at all.” Or, she said, it could also have been partially screwed in but not fully secured with the lock ring.
The most recent overhaul of the plane’s horizontal stabilizer actuator was completed April 21. Homendy said that during this major maintenance event, mechanics would have taken the actuator assembly apart and put it back together.
“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.
Brazy, the investigator in charge, said he hasn’t yet seen the maintenance records for that specific task.
He said the NTSB priority is to get the word out quickly about this vulnerability and that afterwards they will comb through the maintenance records to find out what happened.
Hommendy said the agency has not officially determined the probable cause of the crash and is still looking at other components of the plane, maintenance records and meteorology. Initially, the NTSB estimated the full investigation could take up to two years to complete.
In coordination with Transport Canada, the NTSB has asked Viking to draft instructions for all DHC-3 operators to inspect the actuator to ensure that the lock ring is in place and properly engaged to prevent unthreading of the clamp nut.
Witnesses who saw the plane nose dive into Mutiny Bay helped officials identify 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 the 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.