MINA, S.D. — Repairs were made to the air pressurization system of pro golfer Payne Stewart's business jet two days before the plane crashed, a federal investigator said Wednesday.
The revelation that an air input modulator valve was replaced last week suggests another possible decompression scenario in Monday's crash of the Learjet 35 that killed six people.
Investigators believe the plane depressurized about 39,000 feet over Gainesville, Fla., incapacitating the two pilots and four passengers aboard. The jet--likely on autopilot--flew on another 1,400 miles before apparently running out of fuel and spiraling nose-first into a muddy pasture about a mile from this tiny rural town.
The National Transportation Safety Board said all those aboard the plane probably died of asphyxiation long before the crash.
Investigators said Wednesday that it was still too early to say what caused the apparent depressurization, but several possibilities have been suggested.
The latest to emerge involves the plane's two modulator valves, which control the flow of air pumped by the engines into the cabin to maintain adequate pressurization at high altitudes.
Bob Benzon, the investigator in charge at the crash site, said records show that one of these valves was replaced Saturday. If the replacement valve was defective, or installed improperly, that might have affected pressurization.
However, Benzon was quick to point out, the plane has a modulator valve on each engine. Should one fail, he said, the other should be adequate to maintain pressurization.
Other scenarios involve the possible failure of an interior pressure bulkhead on the plane, failure of the pilots to turn the pressurization system on and failure of the plane's outflow valve--which bleeds off excessive air pressure.
Aviation experts say that in most plane crashes, more than one thing has gone wrong.
Like all pressurized planes, the Learjet 35 has an emergency oxygen system for the pilots and passengers to use in the event of unexpected depressurization.
Assuming depressurization occurred on Stewart's plane, the experts wonder why the pilots apparently were unable to swiftly utilize emergency oxygen masks, stowed in their headrests, to avoid incapacitation and enable them to maintain control of the aircraft.
The experts say there could be several reasons--failure of the emergency oxygen system to work properly, failure of the pilots to turn it on and failure of the pilots to recognize an emergency situation before it was too late.
The oxygen masks for the passengers were among the items investigators recovered Wednesday. Benzon said investigators want to determine whether the masks had dropped from the ceiling as they are designed to do during a loss of cabin pressure. The pilots' oxygen masks had not been found.
At about 5 p.m., the plane's severely damaged cockpit voice recorder also was recovered from the 10-foot-deep debris crater. Officials said that despite the damage, they hoped to obtain useful information from the device--such as possible wind sounds that might indicate the source of the depressurization.
The recorder has a 30-minute loop that usually records over itself. Officials do not expect to hear anything about what happened when radio contact was lost and the plan veered off course, which happened hours before the crash.