A year after the deadly floatplane crash nearly Whidbey Island, the National Transportation Safety Board is focused on the design and maintenance of the part that could have failed and caused the crash, NTSB investigation documents released Friday show.

On September 4, 2022, a de Havilland Canada DHC-3 Otter turboprop operated by Renton-based Friday Harbor Seaplanes plunged into the waters of Puget Sound, killing all 10 people on board.

Less than two months later in October, the NTSB identified a potential mechanical cause of the crash. It concluded that the actuator that moves the horizontal tail and controls the pitch of the airplane that crashed probably came apart in flight.

The NTSB won’t formally confirm the probable cause of the crash until its final report comes out, but its actions and the documents released Friday that reveal new details point to that actuator as the chief suspect.

The final NTSB report with safety recommendations is expected before the end of the year, after which the Federal Aviation Administration will consider what actions are necessary to prevent a recurrence.

Local floatplane operator Kenmore Air, which has 10 Otters in its fleet and collaborated with the NTSB investigation, didn’t wait for the FAA.

Earlier this year, Kenmore designed, certified and installed on all its Otters a “belt-and-suspenders” fix that should prevent the actuator failure the NTSB suspects caused the crash.

Kenmore’s president David Gudgel said that most of the North American operators of DHC-3 Otters and DHC-2 Beavers meet once a year, along with FAA representatives, typically in Alaska to trade operational and safety insights.

When they met in Ketchikan in February, Kenmore presented the fix it had devised.

“This is just the way that we respond to any potential issue that arises in that plane,” Gudgel said. “We want to make sure that we have done everything we possibly can to ensure the safe operation.”

The investigation

The DHC-3 Otter 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, almost all updated with turbine engines.

The plane is a heavy, stable airplane, and it has a simple design that’s considered very safe. The Otters that ply the routes up and down Puget Sound fly only in calm weather with clear visibility.

The NTSB trove of documents released Friday represent the all-but-complete factual record of its investigation into the 2022 crash.

When the crash wreckage was retrieved, the lower barrel of the actuator linked by a cable to the pilot’s flight controls had separated from the top part connected to the horizontal tail.

The threads used to screw the two parts together were intact, suggesting that the parts had not sheared apart on impact. A wire lock ring used to secure the two parts together was missing.

The disconnection of this actuator would have left the pilot Jason Winters no way to control the plane, even though Chief Pilot Shane Carlson, who owns and runs Friday Harbor Seaplanes, told the NTSB investigators Winters was “the most capable pilot that I’ve ever employed.”

After the NTSB reached that conclusion last October, it recommended urgent and immediate inspections of the actuator on all Otters. The FAA then mandated the inspections.

Videos of lab tests conducted by the NTSB released Friday show that the lock ring securing the two parts of the actuator can disengage when the clamp nut at the top is tightened.

The NTSB also released maintenance records indicating the actuator was taken apart and reassembled with new bearings on April 21, 2022, more than five months before the crash.

Jim Lambert, the director of maintenance for Friday Harbor Seaplanes, told investigators he performed that work with another mechanic.

“He and I took it apart. He put the bearings on. It was a two-man deal,” Lambert told the NTSB. “We wanted it to go together perfect so we could finish out our annual inspection.”

Lambert recalled putting the lock ring in place and said he had never known it to pop off or disengage.

Asked if it could happen, he replied: “I suppose, if you really torqued it.”

FAA inspections before the accident

William Shinn, the FAA inspector assigned to oversee the operations of Friday Harbor Seaplanes, told NTSB investigators that he visited the business about once a year.

“They know the airplane very well,” Shinn told the NTSB. “They seem very professional and have a good safety record.”

He said he had last visited the company’s Renton maintenance base in June 2022, but didn’t see any of the airplanes, all of which were out flying. The only plane being worked on at the time was a third party’s plane, not an Otter. He inspected only the records.

Shinn told investigators that, “in hindsight,” requiring an inspection of the actuator that failed and the lock ring that secured it after maintenance would have been a good idea.

The FAA “can only encourage, because the operational rules that they are under, they are not required to have a second set of eyes look at it,” Shinn said.

Likewise, “in hindsight,” Shinn said Friday Harbor Seaplanes would have benefited from an FAA-approved aircraft inspection program — a formal routine of inspection procedures to include specific tasks necessary to mitigate risk.

“But the rules are not written that way, so it was really up to them,” Shinn told investigators.

The FAA would be supportive and would encourage it, but couldn’t require it, Shinn said.

The FAA said via email on Tuesday that the safety agency “is working closely with the NTSB on the accident investigation” and “will take appropriate action as necessary at the conclusion of the investigation.”

Kenmore Air acts quickly

Because Kenmore Air has a fleet of Otters, the NTSB worked closely with the operator as it initially investigated the accident.

The first action after the crash followed reports of cracks and corrosion found in a movable part at the back of the horizontal tail on some Otters still in service.

As it turned out, this is likely unrelated to the crash. Still, Kenmore created a new replacement part, doubling the thickness and adding flanges to make installation more flexible.

Soon afterward, Kenmore’s leadership learned from the NTSB of its conclusion that the horizontal tail actuator on the crashed plane had probably separated.

The company immediately grounded all Kenmore’s Otters until each of the actuators were inspected and declared safe.

Then Kenmore commissioned an engineer to develop a secondary lock feature in addition to the standard lock ring to secure in place the clamp nut at the top of the actuator.

Kenmore applied to certify the modification less than two months after the crash, and the FAA quickly approved it.

The solution provides a new clamp nut with a raised flange on the upper edge where safety wire is attached between the clamp nut and a bolt on the actuator barrel.

A smear of blue marker, a torque seal, crosses the join at the top of the actuator to provide a simple visual marker during inspections that will reveal if the clamp nut has rotated even a little from its tightened position within the barrel.

Kenmore’s Gudgel said the February meeting of Otter operators in Ketchikan “gave us an opportunity to present these developments and ideas and to make those available to the other fleet operators.”

The NTSB documents note just two previous accidents involving the horizontal tail actuator, though neither seems similar to what appears to have happened last year.

In two separate crashes in 1989 and 2001, the actuator barrel failed due to corrosion, which is a common problem for planes that land on saltwater.

Though the Otters have been flying since the 1950s, the NTSB turned up no previous instance of an intact, corrosion-free actuator separating. Gudgel said he’s not aware of any such incident.

Nevertheless, lawyers representing eight of the nine passengers killed in the 2022 crash have sued both Friday Harbor Seaplanes and Viking Air of Canada — which took over ownership of the Otter from de Havilland Canada — claiming the actuator was both badly designed and negligently maintained.

The complaint filed in King County Superior Court cites Kenmore’s fix as evidence of “feasible alternative designs of the actuator … that would have ensured that it not separate during flight.”

It states that the design of the actuator is “unreasonably dangerous and defective” and that Friday Harbor Seaplanes failed in its “duty to maintain and inspect” the aircraft.

The Otter that crashed was the only one Friday Harbor Seaplanes had. The company continues in business, flying people from its base at the southern tip of Lake Washington to the San Juan Islands, on its smaller DHC-2 Beaver floatplanes.

Kenmore continues to fly its fleet of Otters. In addition to its flights out of Lake Union and Lake Washington, in partnership with the Puyallup Tribe it has opened a new seaplane terminal on Ruston Way in Tacoma to fly scenic tours in the south Puget Sound region on an Otter.

Gudgel said employees have wondered if last year’s crash has had an impact on business, but “in the post-COVID era, trying to figure out what the new norm is has been a bit of a challenge.”

Seattle Times reporter Sarah Grace Taylor contributed to this report.