Issuing final report for Whidbey crash that killed Spokane’s Sandy Williams, NTSB calls for change to popular plane

The deadly 2022 crash of a DHC-3 Otter floatplane into Puget Sound that killed all 10 people aboard was caused by a broken link between the pilot’s controls and the plane’s tail, the National Transportation Safety Board concluded.

The federal safety agency’s final report into the accident, released Thursday, cites “a single point of failure” and recommends a retrofit fix for all Otter aircraft.

NTSB Chair Jennifer Homendy in a statement called upon “the Federal Aviation Administration and their Canadian counterparts to eliminate the safety vulnerability identified by NTSB investigators, so this kind of tragedy never happens again.”

The plane that crashed on Sept. 4 last year was operated by Friday Harbor Seaplanes on a routine flight from the San Juan Islands to the plane’s Renton base at the southern tip of Lake Washington.

Among the 10 dead were Spokane’s Sandy Williams, 60, a civil rights activist and the founder of the Carl Maxey Center and the Black Lens Newspaper, and Williams’ partner, 66-year-old retired school teacher Patricia Hicks.

Witnesses reported, and surveillance video confirmed, the airplane was in level flight before it climbed slightly and then abruptly pitched down, the NTSB said.

The dive into the water was an “unrecoverable, near-vertical descent,” the NTSB said. The airplane sank in about 200 feet of water.

The NTSB report confirmed a finding made by investigators just weeks after the crash that the metal actuator that moves the plane’s horizontal tail had come apart in flight.

That meant the pilot would have had no way to control the pitch of the airplane, which dived into the waters of Mutiny Bay off Whidbey Island.

Homendy said the coming apart of the tail actuator components “is an incredibly painful reminder that a single point of failure can lead to catastrophe in our skies.”

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, a clamp nut connected to the horizontal tail.

The threads used to screw the two parts together were intact, indicating that the two parts had not ripped apart on impact. Instead, the clamp nut had unscrewed from the barrel.

A single wire lock ring used to secure the two parts together was missing and never found.

The NTSB conducted lab tests on an exemplar in-service lock ring and concluded that deformation of the lock ring due to either wear and tear over time or defective manufacturing could lead to the ring not being properly seated in the groove so that it could potentially pop off and “prevent the lock ring from retaining the clamp nut as intended by design.”

The report states that “the probable cause of this accident was the in-flight unthreading of the clamp nut from the horizontal (tail) trim actuator barrel due to a missing lock ring, which resulted in the horizontal (tail) moving to an extreme trailing-edge-down position rendering the airplane’s pitch uncontrollable.”

The NTSB recommends scheduled inspections of the lock ring. But it also wants a secondary protection installed.

“To adequately protect safety, we must build in the necessary redundancies across the entire aviation system,” Homendy said.

Specifically, the NTSB recommends that the FAA and Transport Canada mandate that operators of the DHC-3 Otter must retrofit to their aircraft “a secondary retention feature,” an additional locking device to prevent the components of the actuator from separating.

This secondary protection must be attached to “the actuator clamp nut to ensure it remains secured to the barrel in the event the lock ring is not installed or otherwise fails,” the report recommends.

A ‘belt and suspenders’

The Otter, seating 10 passengers, is an aging workhorse of the Puget Sound transportation system. It was first built in the 1950s and a total of 466 were produced through 1967.

Though this aircraft type has suffered a rash of accidents in Alaska over seven decades of service, those have almost exclusively been tied to poor pilot decisions in rough terrain and bad weather.

Generally considered safe, the Otter is a familiar sight even to Seattleites who’ve never flown in it. Another local floatplane operator, Kenmore Air, uses it, along with a smaller six-passenger model called the DHC-2 Beaver, on the scenic flights that take off from Lake Union and top the downtown skyscrapers as they fly out.

Well before the NTSB recommendation, Kenmore has already designed and installed on its fleet of 10 Otters a secondary locking fix for the horizontal tail actuator.

When the actuator separation was discovered and pinpointed as the likely cause less than eight weeks after the crash, the NTSB recommended on October 26 that all Otters be grounded until their actuators were inspected and declared safe. On November 2, the FAA issued an emergency directive to operators mandating the inspections.

Kenmore, which worked closely with the NTSB on the investigation, inspected and cleared its fleet, then 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 this modification less than two months after the crash, and the FAA quickly approved it.

The solution provides a safety wire between a new 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.

Old plane fell short of new safety standards

The NTSB noted Thursday that aviation safety regulations were amended in 1996 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.”

However, there was no requirement to retrofit such secondary protections to previously designed airplanes. The design of the DHC-3 Otter was certified by the FAA in 1952.

Representatives for the 10 people who died have filed lawsuits – against the plane’s operator, Friday Harbor Seaplanes and associated companies; the DHC-3 Otter manufacturer, de Havilland Aircraft of Canada; and the plane’s certificate holder, Viking Air – saying these entities are responsible for their loved ones’ deaths.