SEATTLE — The Federal Aviation Administration has issued a warning regarding potential weaknesses in parts of the tail of Otter seaplanes, the same type of aircraft that crashed off Whidbey Island last month.
The emergency airworthiness directive, issued Tuesday, warns of potential cracks and corrosion in the movable surface of the horizontal tail that controls the plane’s pitch. The directive is not a result of the investigation into the fatal crash in Mutiny Bay, which killed 10 people, but does suggest one possible cause.
The directive warns that, if not addressed, the cracks and corrosion could lead to a structural failure in the tail and “loss of control of the airplane.”
A person close to the Mutiny Bay crash investigation said the directive was spurred when mechanics discovered a crack in the tail of another Otter during a routine inspection unrelated to the deadly crash.
The person spoke anonymously because they were not authorized to release information about the ongoing National Transportation Safety Board investigation.
The directive states that the FAA received “multiple recent reports” of cracks in the same part: the elevator, a movable surface at the rear of the horizontal tail.
In addition, there have been a series of other non-fatal Otter incidents involving elevator failure.
A sudden elevator failure can cause a plane to pitch immediately nose-down, similar to the trajectory reported by some witnesses of the Sept. 4 crash, said Douglas Wilson, a Seattle-based seaplane pilot and president of aviation consulting business FBO Partners.
The Mutiny Bay crash plane was a de Havilland Canada DHC-3 Otter turboprop operated by Renton-based Friday Harbor Seaplanes.
Todd Banks, president of Kenmore Air, which flies similar Otter seaplanes, said investigators could be examining as many as a dozen possibilities, ranging from structural failures of the airframe to a pilot health emergency.
However, he said the timing of the FAA directive is “interesting” and that something going wrong with the control surface on the tail is likely one focus of the investigation.
NTSB spokesman Peter Knudson said via email Friday that in the Mutiny Bay crash investigation “the wreckage examination and records review are still underway.”
“We will by looking at all the structure and systems on the airplane to determine if there were any malfunctions or failures that contributed to the accident,” he said.
An FAA spokesman said Friday “the investigation is ongoing. No cause has been determined.”
Tuesday’s FAA directive states that it was “prompted by multiple recent reports of cracks in the left-hand elevator auxiliary spar.”
This unsafe condition, if not addressed, could lead to “elevator failure, with consequent loss of control of the airplane,” the directive states.
The auxiliary spar is the trailing edge of the elevator, which flaps up or down to move the nose of the plane up or down.
The left elevator is interconnected with and moves in synch with the flaps on the wings to enhance stability.
The last DHC-3 Otters were built in the late 1960s. Today’s fleet of aging workhorse seaplanes have been modified and rebuilt over the years. They need constant maintenance to fight the corrosive effects of seawater.
The FAA directive mandates “repetitive detailed visual inspections of the entire left-hand elevator auxiliary spar for cracks, corrosion, and previous repairs, and depending on the findings, replacement of the left-hand elevator auxiliary spar.”
The wording requires urgent action, indicating the danger is considered serious.
Within three days of receipt of the directive all Otter operators are instructed to “remove the left-hand elevator tab from the elevator and perform a detailed visual inspection.”
Results of the inspections have to be reported back to the FAA within 10 days.
Wilson of FBO Partners said daily visual inspection of the elevator is not easy on a seaplane.
That’s because when a seaplane is docked, with one of its floats tight against the dock, the tail is at a height and a distance from the dock that makes a thorough visual inspection difficult. The only way to do so is to pull the plane out of the water and do a complete inspection on dry land, Wilson said.
Following a 1995 DHC-3 Otter in-flight vibration incident in Ketchikan, Alaska, caused by cracks in an elevator tab, the operator’s director of maintenance told NTSB investigators that “the airplane’s tail is approximately 10 to 11 feet above the ground and is very difficult to examine during pre-flight.”
The FAA did not detail where any of the recent multiple reports of cracks in Otter elevator spars occurred.
However, several past accident reports show elevator failure as a cause.
In May, a DHC-3 Otter crashed nose down into forested terrain while coming in to land in Yakutat, Alaska. The pilot had noticed the elevator not responding fully to his controls at various points in the flight and the airplane pitching up alarmingly.
No one died but the pilot and three passengers on board suffered serious injuries. The NTSB said that accident is still under investigation.
In 2014, an Otter experienced “an anomalous in-flight vibration and uncommanded nose down pitch during cruise flight in the vicinity of Homer, Alaska,” an NSTB report states.
No one was injured. But the airplane’s right elevator “sustained substantial damage.”
In 2015, another Otter in cruise flight near Skwentna, Alaska, also experienced unusual vibration. After landing, the tab on the right hand elevator, which had been poorly repaired, was found to be damaged.
With investigators not yet finished examining the wreckage from the Mutiny Bay crash, it’s impossible to definitively point to the cause. For sure, they will be looking for evidence of elevator failure as a serious focus of the inquiry.
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