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 today, 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.
The accident happened Oct. 25, 1999 after Stewart's chartered
Learjet 35 left Orlando, Fla., headed for Dallas.
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 the
tank was used up during the flight or was empty at takeoff.
The agency also noted that there had been some 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.
