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The Mystery of Learjet N47BA

On Oct. 25, 1999, celebrated professional golfer Payne Stewart died in a mysterious airplane crash that captured headlines and posed a difficult challenge for investigators at the National Transportation Safety Board. Those investigators included Anna Cushman '91, who analyzed the data from cockpit voice recorder recovered from the crash site and chaired the committee that produced the official transcript of the recording.

The ill-fated flight began at Orlando International Airport when the Learjet 35, tail number N47BA, rose from the runway and headed for Texas, where Stewart was due to play in a tournament. About eight minutes into the flight, air traffic control in Jacksonville instructed the jet's pilot to climb to 39,000 feet, and the pilot acknowledged the transmission. That was the last anyone heard from the plane. Six minutes later, the plane was contacted again, but there was no reply. Several more attempts to contact the plane were also unsuccessful.

Not long after the last attempt, an F-16 from Elgin Air Force Base in northwest Florida flew close to the aircraft and observed what appeared to be icy condensation on the windshield (an indication that normal airflow to the cabin-which includes a steady flow of warm air to defrost the windshield-had ceased). The inside of the plane seemed dark and the fighter pilot could see no movement in the cockpit. The pilots of other fighter jets that followed the plane as it traveled more than 1,500 miles said they observed no movements of any of its control surfaces.

Finally, three hours after it took off, the plane made a right turn and began a steep descent. The descent turned into a spin, and the plane, traveling at more then 600 miles per hour, slammed nose first into a field near Aberdeen, S.D. Stewart, the pilot and co-pilot, and three other passengers were all killed.

Almost immediately, investigators suspected that the Learjet's cabin had depressurized, rendering the crew and passengers unconscious, and that the plane had continued, probably on autopilot, until it ran out of fuel and crashed. A careful examination of the wreckage yielded no concrete clues about what might have caused the plane to lose pressure or whether the crew was able to turn on emergency oxygen.

The plane's cockpit voice recorder (CVR), badly damaged, was pulled from the debris field and transported to the NTSB, where Cushman, with help from the manufacturer, was able to extract the data from the unit's digital memory chips. Two out of the four channels-one for the pilot's headset and one for the cockpit-area microphone-contained usable information.

As expected, there were no voices on the 30-minute recording, just sounds from the plane's cabin. But the noises added credibility to the theory that the plane had depressurized, then continued with an incapacitated crew, crashing only after nearly depleting its fuel.

The dominant sounds as the tape starts are what appears to be the drone of the plane's two turbofan engines running at high speed and the cabin altitude warning, which alerts the flight crew of the need to begin using supplemental oxygen masks when the cabin loses pressure at high altitudes. (Without supplemental oxygen, the crew and passengers would be expected to lose consciousness quickly at the altitude the Learjet was flying at when it was intercepted by that first F-16.)

A sound spectrum analysis completed by Cushman shows that 28 minutes later, one of the engines winds down while the other continues to turn. At the same time, other sounds-the "stick shaker" on the control yoke and the autopilot disconnect warning-indicate that the plane is beginning to fly out of control.

In this voiceprint, part of a sound spectrum study Anna Cushman completed on the data from the cockpit voice recorder from the Payne Stewart Learjet, the so-called N1 drone, the sound of the Learjet's two turbofan engines operating, is seen as a yellow line at about 350 Hz. A second and a half into this plot, a second yellow line, sloping down, diverges from the first, indicating that one engine was spooling down – probably as a result of its fuel running out – while the other continued to operate at high speed.

Less than a minute after the engine noise ceased, the plane rolled to the right, as it might if only one engine were running-and began its fatal 85-second descent from an altitude exceeding 41,000 feet. Just 14 seconds before the plane hit the ground, as the plane passed through lower altitudes, the cabin altitude warning stopped, Cushman's analysis shows.

This voiceprint shows the signature of the engine overspeed warning (the long, upward-sloping pattern that continues throughout the plot) and the cabin altitude warning, which ceases at about the 26.1 second mark, or just a few seconds before the jet crashed. The altitude warning signal tells the crew to put on their oxygen masks when the cabin loses pressure at high altitudes. As the plane dived toward the ground, the alarm eventually sensed the changing altitude and turned off.

The NTSB investigation team spent a great deal of time trying to determine what may have caused the cabin to lose pressure, whether emergency procedures were taken by the crew to re-pressurize the cabin, and whether the crew was able to breath emergency oxygen (and if so, whether the supply was adequate). No definitive answers could be found to any of these questions, although an analysis by Cushman of CVR recordings made during test flights of a Learjet similar to the one that crashed showed that the emergency oxygen system was not operating during the flight's final 30 minutes.

More than a year after Learjet N47BA met its end in a field in South Dakota, the NTSB issued its final report on the accident, summarizing its conclusions in just one sentence: "The National Transportation Safety Board determines the probable cause of this accident was incapacitation of the flight crewmembers as a result of their failure to receive supplemental oxygen following a loss of cabin pressurization, for undetermined reasons."

To learn more about the crash of Payne Stewart's Learjet and the NTSB investigation, visit the investigation home page at the National Transportation Safety Board.

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Last modified: Sep 02, 2004, 13:53 EDT
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