Former Alaska Sen. Ted Stevens Plane Crash: NTSB Report Finds No Definitive Cause

ABC News' Lisa Stark ( @lisastark) reports: What caused the plane carrying former Alaska Senator Ted Stevens and eight others to crash into an Alaska mountain in August last year, killing the pilot and four passengers, and injuring the others on board?

After months of extensive investigation, the National Transportation Safety Board was left with no definitive answer.

The Board determined that the probable cause was “the pilot’s temporary unresponsiveness for reasons that could not be established from the available information.” That says the Board did determine that there was no mechanical problem with the plane and that weather was not likely a factor in the crash.

It is very rare for staff to work this exhaustively and not be able to come up with a cause, said Board Chairman Deborah Hersman.

Investigators did determine that the pilot, for some unexplained reason, turned off course as he was flying the passengers to a fishing lodge. Sixty-two-year-old Theron (Terry) Smith was highly experienced and very familiar with the area he was flying in. Was Smith somehow incapacitated just prior to the crash?

Investigators noted the pilot had had a stroke in 2006, and had suffered impairment after the stroke. He worked hard to recover, and was given medical clearance by the FAA to continue flying.

Autopsies revealed no indication of another stroke during the accident flight but medical experts testified that some types of strokes may not have left a trace.

The Board determined FAA inappropriately reissued Smith’s medical license based just on an examination by a local neurologist. They said a more extensive examination should have been required, and recommend the agency rework its requirements for issuing a pilot’s medical certificates after a stroke.

The FAA said that it last changed its procedures for recertifying pilots following stroke in March 2010, mandating an extensive neurological examination. The agency now says it will clarify that guidance even further.

Other factors that may have affected his performance include fatigue: the pilot had nine hours of sleep the night before the accident, less than his usual 12 to 13 hours, but investigators said they had no evidence that he nodded off.

Finally, was he distracted by grief? The pilot’s son-in-law had died in a plane crash just 12 days before the accident.

Dr. Malcolm Brenner, NTSB Human Performance Investigator, told the board, “The pilot’s life events placed him at an elevated risk for stress at the time of the accident.”

The evidence suggest that the pilot -- if he was incapacitated for a brief time -- did realize something was terribly wrong just seconds before the accident. After the unexplained left turn, as the de Havilland Otter airplane approach the mountain an alert would have gone off in the cockpit indicating the plane is too close to the ground.

That alarm would have sounded just 4 to 6 seconds before impact. Investigators concluded at that point Smith then tried to sharply pull up the nose of the plane. It was too late to avoid the crash.

Another terrain warning system, that would have provided 30 seconds warning was turned off. The board said this alarm system is routinely turned off by pilots flying in the rough Alaska terrain because they go off so often as the planes weave through the mountains.

The plane was not equipped with a flight data or voice recorder. It was not required to have the equipment. The NTSB has long recommended video recorders for plane without flight or voice recorders.

Pilots have been adamantly opposed and the FAA has told the board “we are not going to require this,” according to Chairman Hersman.

“We have equipment that is off the shelf, available and affordable," she said. "We think it is far past time for the FAA to move forward on this recommendation."

Without those critical recorders this accident remains a mystery. “What we do not know –- and may never know -- is what happened in the last three minutes of that fatal flight," said Hersman.

“Perhaps one of the lessons of this tragedy is the need for onboard computers,” she added. As Board Member, Robert Sumwalt put it, “It could have been a medical event, but we can’t prove it. It could have been fatigue, but we can’t prove it.”

The lack of clear information is clearly frustrating to the NTSB and no doubt to the family members of those who died and were injured, about 30 of whom attended today’s hearing.

Hersman offered the board’s “deepest condolences “ to those “whose lives were forever changed when the crash occurred.”

She added, “I am disappointed that we don’t have a probable cause, mainly to provide that to family members.”

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