The National Transportation Safety Board on Thursday issued a formal and urgent safety recommendation to the Federal Aviation Administration and its Canadian counterpart that they mandate an immediate one-time inspection of a critical component on all DHC-3 Otter seaplanes.
In an investigation update, the NTSB cited "an immediate safety hazard because it could result in a reduction or loss of pitch control during flight."
In an interview, NTSB Chair Jennifer Homendy stressed that the safety agency wants operators of the aircraft "to conduct these inspections immediately before the planes fly again."
"We are concerned about the potential for a catastrophic loss of control," she said. "And so we need some immediate action by the FAA and Transport Canada to ensure safety."
The operators are also asked to report back to the FAA if they find any anomalies in the inspections.
"If they find there are issues, we want to know about them," Homendy said.
The urgent request for action, after a vote of the full board, flows from the investigation into the Sept. 4 crash of a de Havilland Canada DHC-3 Otter turboprop off Whidbey Island that killed the pilot and nine passengers.
That seaplane, operated by Renton-based Friday Harbor Seaplanes, crashed into Puget Sound about half an hour into a flight from Friday Harbor to Renton.
Viking Air of Canada, which controls the maintenance and certification of the DHC-3, issued Wednesday a service letter to all operators of the plane detailing the required inspection and advising that it be done "upon receipt of this letter."
The FAA said in a statement Thursday it "is contacting DHC-3 operators in the United States to ensure they are acting on the aircraft manufacturer's service letter."
The FAA outreach began Thursday morning after it obtained Viking Air's service letter.
Because Transport Canada was the regulator that originally certified the DHC-3, it's up to that jurisdiction to decide on a formal airworthiness directive, after which the FAA would typically follow suit.
"The FAA remains in close communication with Transport Canada," the FAA said Thursday.
The cause of the seaplane's nosedive
Thursday's NTSB report said multiple witnesses and security camera video showed "that the airplane was in level flight before it entered a slight climb and then pitched down in a nose-low, near-vertical descent until water impact."
The NTSB recommendation follows the discovery last week by crash investigators who examined the wreckage retrieved from the sea that a component controlling the movement of the plane's horizontal tail had come apart.
The details discovered are clear evidence that this separation was the likely cause of the tragedy.
"We are very focused on it," Homendy said. "We certainly have teams looking at other parts of the investigation, from company operations, to FAA oversight and other components on the plane. But this is a very big focus."
The critical component is the actuator that moves the plane's horizontal tail, also called the stabilizer. The stabilizer swivels to control the pitch of the aircraft, moving the nose of the plane up or down.
A cable from the cockpit wraps around the barrel of the actuator, rotating it to move the stabilizer.
When the wreckage was retrieved, the upper portion of the actuator was found still attached to the horizontal stabilizer. Beside it, but disconnected, the lower portion was "attached to its mount in the fuselage."
This separation broke the mechanical connection between the pilot and the stabilizer.
Investigators discovered that a clamp nut threaded into the barrel of the actuator and that connects the top of the assembly to the stabilizer had unscrewed from the barrel.
Crucially, a wire lock ring designed to secure the clamp nut in place was missing.
The NTSB update states that "given the findings thus far ... we are concerned that a missing or an improperly installed lock ring on other DHC-3 airplanes has the potential to result in a catastrophic loss of control."
The NTSB therefore recommends that the FAA and Transport Canada require "all operators of De Havilland Canada DHC-3 airplanes to conduct an immediate one-time inspection of the horizontal stabilizer actuator lock ring" following the instructions in the Viking Air service letter.
Doug Brazy, the NTSB investigator in charge, said the required inspection of the clamp nut and lock ring is "very short."
"It does not require any disassembly of the airplane. There's an inspection panel that's removable by design to provide access," Brazy said. "There's no heavy maintenance."
Last week, local DHC-3 Otter operator Kenmore Air conducted the inspections on its fleet of 10 airplanes and found no issues.
The final reports and recommendations from investigations into fatal airplane accidents typically take a year and often longer.
Following the tragedy Sept. 4, this call to action is on a fast track.
Homendy said once the safety risk was identified a week ago, her agency pulled together all the data — including photos of the failed component from the wreckage and technical drawings of the assembly — to make sure the aviation community had a complete justification for urgent action.
"We pushed that through as fast as possible and I think that is pretty record time for us," Homendy said.
