Loss of cabin pressure and failure to obtain oxygen incapacitated the crew of golfer Payne Stewart's plane, leading to the crash last year that killed all six aboard the chartered Learjet.
But while the National Transportation Safety Board reached that conclusion Tuesday, it was unable to say why the plane lost pressure.
The yearlong investigation was hampered by the plane's extensive damage, its lack of a flight data recorder and the short half-hour duration of the cockpit voice recorder, Board Chairman Jim Hall said.
"All of us wish we had more answers than we have out of this report," Hall said at the end of a four-hour session in which board members questioned investigators about what they had been able to learn.
Flying at 23,000 feet, the pilot acknowledged permission to climb to 39,000 feet in the last contact with the plane. It eventually climbed to more than 40,000 feet and flew on autopilot for four hours before running out of fuel and crashing near Aberdeen S.D.
Military pilots sent to observe the unresponsive craft reported that the cockpit windows were iced up.
The loss of cabin pressure could cause this, as well as the loss of enough oxygen to cause unconsciousness. Emergency oxygen was available, but in the older-style plane it had to be activated manually by the crew.
Dr. Mitchell Garber, the board's medical officer, said that many pilots believe that when pressure fails they have a minute or two to take action before they need oxygen.
But impairment begins within seconds, he said, and the longer the crew waits to activate the oxygen the less likely they are to make the right decision. In a depressurization, he said, the first thing a pilot should do is reach for the oxygen mask.
Aircraft systems investigator Kevin Pudwill told the board that some parts of the pressurization system were too badly damaged to determine if they failed.
But, he said, a flow control valve was found turned off and that would have cut off the incoming warm air from the engines that is used to pressurize the cabin.
Robert Benzon, investigator in charge for the accident, said it could not be determined if the valve had been turned off before the flight, if the crew had turned it off as part of switching to an emergency pressurization system or it was off for some other reason. Turning that valve off is part of the switch to the emergency pressure system, he said, but the emergency system had not been turned on.
In addition, he said it could not be determined why the crew didn't obtain emergency oxygen, since a backup oxygen tank was in the plane.
The oxygen tank was empty and its flow valve was open, Pudwill told the board. He said the investigators can't tell whether he tank was used up during the flight or was empty at takeoff.
The agency also noted that there had been pressure problems reported with the plane in the days before the flight. The day before the accident maintenance workers fixed an engine power problem by replacing a valve that also could have affected pressurization.
The morning of the crash the plane flew to Orlando at altitudes of 12,000 feet to 13,000 feet, with no pressure problems reported.
Airplanes are pressurized so that the atmosphere inside never feels higher than 8,000 to 10,000 feet, even if the aircraft is flying much higher.
Planes of this type are not required to have flight data recorders, which track actions of the engine, instruments and so forth, so investigators lacked that data. Most recorders, however, do not measure cabin pressure.
It had a cockpit voice recorder, but that had only a 30-minute loop, meaning investigators heard only the last half hour of the long flight and could not hear anything said hours earlier when the actual depressurization occurred.
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