The Federal Aviation Administration has issued a warning about potential weaknesses in parts of the tails of Otter seaplanes, the same type of plane that crashed off Whidbey Island last month.
The emergency airworthiness directive, issued on Tuesday, warns of possible cracks and corrosion in the horizontal tail moving surface that controls the plane’s pitch. The directive is not the result of the investigation into the fatal Mutiny Bay crash, which killed 10 people, but suggests a possible cause.
The guideline warns that, if left untreated, cracks and corrosion could lead to tail structural failure and “loss of aircraft control”.
A person close to the Mutiny Bay crash investigation says the directive was spurred when mechanics discovered a crack in the tail of another Otter during a routine inspection unrelated to the accident deadly.
The person spoke anonymously because they were not authorized to release information about the National Transportation Safety Board’s ongoing investigation.
The directive states that the FAA has received “several recent reports” of cracks in the same part: the elevator, a moving surface aft of the horizontal stabilizer.
Additionally, there were a series of other non-fatal Otter incidents involving an elevator breakdown.
A sudden elevator failure can cause a plane to pitch down immediately, similar to the trajectory reported by some witnesses of the Sept. 4 crash, said Douglas Wilson, a Seattle-based seaplane pilot and president of the aviation consulting firm FBO Partners.
The Mutiny Bay accident plane was a de Havilland Canada DHC-3 Otter turboprop operated by Renton-based Friday Harbor Seaplanes.
Todd Banks, president of Kenmore Air, which operates similar Otter seaplanes, said investigators could be looking at up to a dozen possibilities, ranging from structural airframe failures to a pilot health emergency.
However, he said the timing of the FAA directive is “interesting” and that something wrong with the control surface on the tail is likely one focus of the investigation.
NTSB spokesman Peter Knudson said by email Friday that in the Mutiny Bay crash investigation “examination of wreckage and review of records are still ongoing.”
“We will examine the entire structure and systems of the aircraft to determine if there were any malfunctions or failures that contributed to the crash,” he said.
An FAA spokesperson said Friday that “the investigation is ongoing. No cause has been determined.
Several recent reports of cracks
Tuesday’s FAA directive says it was “prompted by several recent reports of cracks in the left elevator auxiliary spar.”
This dangerous condition, if left untreated, could lead to “elevator failure, resulting in loss of aircraft control,” the directive says.
The auxiliary spar is the trailing edge of the elevator, which flaps up or down to move the nose of the aircraft up or down.
The left riser is interconnected and moves in sync with the wing flaps to improve stability.
The last DHC-3 Otters were built in the late 1960s. The current fleet of aging seaplanes has been modified and rebuilt over the years. They need constant maintenance to combat the corrosive effects of seawater.
The FAA directive mandates “repetitive detailed visual inspections of the left elevator auxiliary spar assembly for cracks, corrosion, and previous repairs, and depending on the results, replacement of the left elevator auxiliary spar. ‘left riser’.
The wording requires urgent action, indicating that the danger is considered serious.
Within three days of receiving the directive, all Otter operators are instructed to “remove the left elevator tab from the elevator and perform a detailed visual inspection”.
Inspection results must be reported to the FAA within 10 days.
Wilson of FBO Partners said daily visual inspection of the elevator is not easy on a seaplane.
Indeed, when a seaplane is moored, with one of its floats tight against the quay, the tail is at a height and at a distance from the quay which makes a thorough visual inspection difficult. The only way to do that is to get the plane out of the water and do a full inspection on land, Wilson said.
Following a 1995 DHC-3 Otter in-flight vibration incident at Ketchikan, Alaska, caused by cracks in an elevator tab, the operator’s maintenance manager told NTSB investigators that “The tail of the plane is about 10 to 11 feet above the ground and very difficult to examine during pre-flight.
Past Elevator Failure Incidents
The FAA did not specify where one of multiple recent reports of cracks in Otter elevator spars occurred.
However, several previous accident reports point to elevator failure as the cause.
In May, a DHC-3 Otter crashed nose-down into wooded terrain while landing in Yakutat, Alaska. The pilot had noticed that the elevator did not fully respond to his commands at various times during the flight and that the aircraft pitched up alarmingly.
Nobody died but the pilot and the three passengers on board were seriously injured. The NTSB said the crash is still under investigation.
In 2014, an Otter experienced “abnormal in-flight vibration and inadvertent nose-down during a cruise flight in the vicinity of Homer, Alaska,” an NTSB report said.
No one was hurt. But the plane’s right elevator “sustained significant damage”.
In 2015, another Otter on a cruise flight near Skwentna, Alaska also felt unusual vibrations. After landing, the right elevator tab, which had been improperly repaired, was found to be damaged.
Investigators have not yet finished examining the wreckage of the Mutiny Bay accident, it is impossible to definitively determine the cause. Of course, they will be looking for evidence of elevator failure as a serious investigative goal.