And Still, Virtually No Answers

It was a somber anniversary Monday for both the world of golf and the world of aviation. Five years ago, golfer Payne Stewart and five others were killed in a bizarre accident involving a Lear 35. Investigators believe that the aircraft lost cabin pressure shortly after taking off from Orlando (FL), headed to Dallas (TX). But, ultimately, what caused the loss of cabin pressure remains just as much a mystery as it was five years ago.

The End Of N47BA

The accident aircraft, N47BA, was owned by Sunjet Aviation, an on-demand air taxi operation based

Instead of landing in Dallas, the Lear 35 continued flying at altitude for four hours, a ghost ship with no one at the controls. The aircraft was intercepted twice -- first, by F-16s with the Oklahoma Air Guard, and then by a pair of Falcons from the North Dakota Air Guard. Finally, near Aberdeen (SD), the Lear's fuel supply was exhausted. The Lear lost power and spiraled into the ground.

The NTSB final report on the accident was released November 28th, 2000, more than a year later. In it, investigators listed the probable cause as "incapacitation of the flight crew members as a result of their failure to receive supplemental oxygen following a loss of cabin pressurization, for undetermined reasons."

History Of Pressurization Trouble

N47BA wasn't the first choice for Stewart's last flight. The 42-year old golfer and his three companions were to have taken a commercial flight from Orlando to Dallas. But fellow golfers Van Arden and Robert Fraley convinced Stewart that flying an air taxi was safer. Even though Stewart himself owned a piece of an aircraft, the tab for this ride was being picked up by a sponsor.

The aircraft had just come out of the shop, according to published reports. Pilots on recent flights had reported problems with the cabin pressurization, saying it sometimes failed to hold pressure at lower altitudes. A maintenance supervisor at Sunjet told investigators that, prior to its last flight, N47BA was being checked for a 'throttle problem.' During a visual inspection of the left engine, the supervisor spotted a problem with an engine modulation valve. He said "the spring [was] not functioning." The valve was replaced the next day.

The repair tag on the old valve read, "Reason removed: ITT (interstage turbine temperature) split at altitude and cabin pressurization loss with reduced power setting."

Sunjet executives said the aircraft was flown once before it was put back into service. However, investigators found that, during the test flight, the aircraft never flew above 13,000 feet.

The replacement valve was never officially blamed for the depressurization that led to the accident. But the NTSB report noted that it had been the object of scrutiny as early as four years before the crash.

On April 12, 1995, a prepurchase inspection performed by Learjet at its Wichita (KS), facility indicated the following:

Cabin pressure follows throttles - 2,000 feet bump both directions...R/H [right] engine mod...Valve does not shift when power is brought up...when moving cabin air switch to max flow you get no increase of air flow...with cabin pressure at 1 pound in auto, cabin will not up rate when selecting a higher altitude...should up rate depending on where rate knob is at...emergency exit seal...coming loose...main cabin door is smashed at split line area...O2 need serviced.

Investigators did find the valves in the wreckage of N47BA and noted, "On October 23, 1999, the left engine modulation valve, S/N P-247, was removed and replaced with one of the modulation valves that was discovered in the wreckage. The functional test of the replaced modulation valve revealed that the flow mixing poppet between the low- and high-pressure stages did not operate (open) at low bleed air pressures."

FMI: www.ntsb.gov