N47BA prior to its final flight.
|Date||October 25, 1999|
|Site||Aberdeen, South Dakota|
|Aircraft type||Learjet 35|
|Flight origin||Orlando, Florida|
On October 25, 1999, a chartered Learjet 35 flew between Orlando, Florida, and Dallas, Texas. Early in the flight, the aircraft was cruising at altitude on autopilot and gradually lost cabin pressure. As a result, all on board died of hypoxia, a lack of oxygen. The aircraft continued its flight over the southern and midwestern United States for almost four hours and 1,500 miles (2,400 km), until it ran out of fuel and crashed into a field near Aberdeen, South Dakota after an uncontrolled descent. Among the six people on board were golf star Payne Stewart and Bruce Borland, a highly regarded golf architect with the Jack Nicklaus golf course design company.
On October 25, 1999, Learjet 35, registration N47BA, operated by Sunjet Aviation, Inc., of Sanford, Florida, departed Orlando (IATA: MCO, ICAO: KMCO) at 1319Z (0919 EDT) on a two-day, five-flight trip. Before departure, the aircraft had been fueled with 5,300 lb (2,400 kg) of Jet A, enough for 4 hours and 45 minutes of flight. On board were two pilots and four passengers.
At 1327:13Z, the controller from the Jacksonville ARTCC instructed the pilot to climb and maintain flight level (FL) 390 (39,000 feet (12,000 m) above sea level). At 1327:18Z (0927:18 EDT), the pilot acknowledged the clearance by stating, "three nine zero bravo alpha." This was the last known radio transmission from the airplane, and occurred while the aircraft was passing through 23,000 feet (7,000 m). The next attempt to contact the aircraft occurred six minutes, 20 seconds later (14 minutes after departure), with the aircraft at 36,500 feet (11,100 m), and the controller's message went unacknowledged. The controller attempted to contact N47BA five more times in the next 4½ minutes, again with no answer.
About 1454Z (now 0954 CDT due to the flight's crossing into the Central Time zone), a U.S. Air Force F-16 test pilot from the 40th Flight Test Squadron at Eglin Air Force Base, Florida, who happened to be in the air nearby, was directed by controllers to intercept N47BA. When the fighter was about 2,000 feet (610 m) from the Learjet, at an altitude of about 46,400 feet (14,100 m), the test pilot made two radio calls to N47BA but did not receive a response. The F-16 pilot made a visual inspection of the Lear, finding no visible damage to the airplane. Both engines were running, and the plane's red rotating anti-collision beacon was on (standard operation for aircraft in flight). The fighter pilot could not see inside the passenger section of the airplane because the windows seemed to be dark. Further, he stated that the entire right cockpit windshield was opaque, as if condensation or ice covered the inside. He also indicated that the left cockpit windshield was opaque, although several sections of the center of the windshield seemed to be only thinly covered by condensation or ice; a small rectangular section of the windshield was clear, with only a small section of the glare shield visible through this area. He did not see any flight control movement. About 1512Z, the F-16 pilot concluded his inspection of N47BA and broke formation, proceeding to Scott AFB, Illinois.
At 1613Z, almost three hours into the flight of the unresponsive Learjet, two F-16s from the Oklahoma Air National Guard (ANG), flying under the call-sign "TULSA 13 flight", were directed by the Minneapolis ARTCC to intercept. The TULSA 13 lead pilot reported that he could not see any movement in the cockpit, that the windshield was dark and that he could not tell if the windshield was iced. A few minutes later, a TULSA 13 pilot reported, "We're not seeing anything inside, could be just a dark cockpit though...he is not reacting, moving or anything like that he should be able to have seen us by now." At 1639Z, TULSA 13 left to rendezvous with a tanker for refueling.
The aircraft's altitude varied from 22,000 feet (6,700 m) to 51,000 feet (16,000 m).
About 1650Z, two North Dakota ANG F-16s with the identification "NODAK 32 flight" were directed to intercept N47BA. TULSA 13 flight also returned from refueling, and all four fighters maneuvered close to the Lear. The TULSA 13 lead pilot reported, "We've got two visuals on it. It's looking like the cockpit window is iced over and there's no displacement in any of the control surfaces as far as the ailerons or trims." About 1701Z, TULSA 13 flight returned to the tanker again, while NODAK 32 remained with N47BA.
The Learjet's cockpit voice recorder (CVR), which was recovered from the wreckage, contained an audio recording of the last 30 minutes of the flight (it was an older model which only recorded 30 minutes of data; the aircraft was not equipped with a flight data recorder). At 1710:41Z, the Learjet's engines can be heard winding down, indicating that the plane's fuel had been exhausted. In addition, sounds of the stick shaker and autopilot disconnect can be heard (with the engines powered down, the autopilot would have attempted to maintain altitude, causing the plane's airspeed to bleed off until it approached stall speed, at which point the stick shaker would have automatically engaged to warn the pilot and the autopilot would have switched itself off).
At 1711:01Z, the Lear began a right turn and descent. One NODAK 32 airplane remained to the west, while one TULSA 13 airplane broke away from the tanker and followed N47BA down. At 1211:26 CDT, the NODAK 32 lead pilot reported, "The target is descending and he is doing multiple aileron rolls, looks like he's out of control...in a severe descent, request an emergency descent to follow target." The TULSA 13 pilot reported, "It's soon to impact the ground; he is in a descending spiral."
Impact occurred approximately 1713Z, or 1213 local, after a total flight time of 3 hours, 54 minutes, with the aircraft hitting the ground at a nearly supersonic speed and an extreme angle. The Learjet crashed just outside of Mina, South Dakota, in Edmunds County on relatively flat ground, and left a crater 42 feet (13 m) long, 21 feet (6.4 m) wide and 8 feet (2.4 m) deep. None of its components remained intact.
The 42-year-old captain, Michael Kling, held an airline transport pilot certificate and type ratings for Boeing 707, Boeing 720, and Learjet 35. He also had Air Force experience flying the KC-135 and E-3 Sentry. He was also an instructor pilot on a Maine ANG KC-135E. According to Sunjet Aviation records, the captain had accumulated a total of 4,280 hours of flight time (including his military flight time) and had flown a total of 60 hours with Sunjet, 38 as a Learjet pilot-in-command and 22 as a Learjet second-in-command.
The first officer, Stephanie Bellegarrigue, age 27 at the time of the accident, held a commercial pilot certificate and type ratings for Learjet and Cessna Citation 500. She was also a certified flight instructor. She had accumulated a total of 1,751 hours of flight time, of which 251 hours were with Sunjet Aviation as a second-in-command and 99 as a Learjet second-in-command.
The National Transportation Safety Board (NTSB) has several levels of investigation, of which the highest is a "major" investigation. In the last ten years, the NTSB has conducted 35 major investigations of aircraft crashes. Because of the extraordinary circumstances in this crash, a major investigation was performed.
The NTSB determined that:
|“||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.||”|
In addition, the Board concluded that:
|“||Following the depressurization, the pilots did not receive supplemental oxygen in sufficient time and/or adequate concentration to avoid hypoxia and incapacitation. The wreckage indicated that the oxygen bottle pressure regulator/shutoff valve was open on the accident flight. Further, although one flight crew mask hose connector was found in the wreckage disconnected from its valve receptacle (the other connector was not recovered), damage to the recovered connector and both receptacles was consistent with both flight crew masks having been connected to the airplane's oxygen supply lines at the time of impact. In addition, both flight crew mask microphones were found plugged in to their respective crew microphone jacks. Therefore, assuming the oxygen bottle contained an adequate supply of oxygen, supplemental oxygen should have been available to both pilots' oxygen masks.
[A] possible explanation for the failure of the pilots to receive emergency oxygen is that their ability to think and act decisively was impaired because of hypoxia before they could don their oxygen masks. No definitive evidence exists that indicates the rate at which the accident flight lost its cabin pressure; therefore, the Safety Board evaluated conditions of both rapid and gradual depressurization.
If there had been a breach in the fuselage (even a small one that could not be visually detected by the in-flight observers) or a seal failure, the cabin could have depressurized gradually, rapidly, or even explosively. Research has shown that a period of as little as 8 seconds without supplemental oxygen following rapid depressurization to about 30,000 feet (9,100 m) may cause a drop in oxygen saturation that can significantly impair cognitive functioning and increase the amount of time required to complete complex tasks.
A more gradual decompression could have resulted from other possible causes, such as a smaller leak in the pressure vessel or a closed flow control valve. Safety Board testing determined that a closed flow control valve would cause complete depressurization to the airplane's flight altitude over a period of several minutes. However, without supplemental oxygen, substantial adverse effects on cognitive and motor skills would have been expected soon after the first clear indication of decompression (the cabin altitude warning), when the cabin altitude reached 10,000 feet (3,000 m) (which could have occurred in about 30 seconds).
Investigations of other accidents in which flight crews attempted to diagnose a pressurization problem or initiate emergency pressurization instead of immediately donning oxygen masks following a cabin altitude alert have revealed that, even with a relatively gradual rate of depressurization, pilots have rapidly lost cognitive or motor abilities to effectively troubleshoot the problem or don their masks shortly thereafter. In this accident, the flight crew's failure to obtain supplemental oxygen in time to avoid incapacitation could be explained by a delay in donning oxygen masks of only a few seconds in the case of an explosive or rapid decompression or a slightly longer delay in the case of a gradual decompression.
In summary, the Safety Board was unable to determine why the flight crew could not, or did not, receive supplemental oxygen in sufficient time and/or adequate concentration to avoid hypoxia and incapacitation.
The NTSB report showed that the plane had several instances of maintenance work related to cabin pressure in the months leading up to the accident. The NTSB was unable to determine whether they stemmed from a common problem - replacements and repairs were documented, but not the pilot discrepancy reports that prompted them or the frequency of such reports. The report gently chides Sunjet Aviation for the possibility that this would have made the problem harder to identify, track, and resolve; as well as the fact that in at least one instance the plane was flown with an unauthorized maintenance deferral for cabin pressure problems.
There was some speculation in the media that military jets were prepared to shoot down the Lear if it threatened to crash in a heavily populated area. Officials at the Pentagon strongly denied that possibility. Shooting down the plane "was never an option," Air Force spokesman Captain Joe Della Vedova said. "I don't know where that came from."
Stewart was ultimately headed to Houston for the 1999 Tour Championship, but planned a stop in Dallas for discussions with the athletic department of his alma mater, Southern Methodist University, about building a new home course for the school's golf program. Stewart was memorialized at the Tour Championship with a lone bagpipe player playing at the first hole at Champions Golf Club prior to the beginning of the first day of play.
In 2001, Stewart was posthumously inducted into the World Golf Hall of Fame.
On June 8, 2005, a Florida state court jury in Orlando found that Learjet was not liable for the deaths of Stewart and his agents Robert Fraley and Van Arden, who had also been aboard the plane.
While this type of crew incapacitation is extremely rare, it is not unknown. Several similar accidents have occurred in the last thirty years.
Robert Edward "Bo" Rein, a noted football coach for North Carolina State and then Louisiana State University (where he had been hired only six weeks before) was flying with a pilot from Shreveport, Louisiana to Baton Rouge on January 10, 1980 in a Cessna 441 Conquest twin-turboprop, registered N442NC. The flight was supposed to last 40 minutes, but after flying east and climbing to avoid a thunderstorm, the plane lost contact with air traffic control, and was seen on radar to climb to 40,000 feet (12,000 m).
The Conquest was eventually intercepted by two Michigan Air National Guard F-4C Phantoms from Seymour-Johnson AFB and a pair of F-106 Delta Dart interceptors from the 48th Fighter Interceptor Squadron at Langley AFB aircraft in Virginia. When intercepted, the Cessna was over a thousand miles off course, and flying at an altitude of 41,600 feet (12,700 m), 5,000 feet (1,500 m) higher than its maximum certified ceiling. The fighter pilots could not see anyone in the cockpit. The plane continued out over the Atlantic Ocean, east of Langley, Virginia, where it ran out of fuel, descended to 25,000 feet (7,600 m) and then entered a spin, crashing into the water. The military pilots spotted some debris, but no wreckage was ever recovered. The bodies of Rein and pilot Lou Benscotter were never found. The most likely reason given was that the two men apparently lost consciousness due to slow depressurization of the cabin. Cessna later settled out of court with Rein's widow for an undisclosed amount.
On May 18, 1983, a Learjet 25B flying from Wien-Schwechat International Airport, Vienna, Austria to Hamburg-Fuhlsbüttel Airport, Hamburg, West Germany, did not respond to controllers' requests 40 minutes after takeoff. The aircraft, registered D-CDPD and operated by Air Traffic Executive Jet, was carrying two crew and one passenger. It continued in flight at 39,000 feet (12,000 m) until its fuel was exhausted. It crashed into the Atlantic 350 miles (560 km) northwest of Scotland.
A Lear 24B, N234CM, departed Memphis International Airport on December 16, 1988, heading for Addison, Texas with two crew aboard, including NASA astronaut-candidate Susan Reynolds. After it flew past its destination, the aircraft was intercepted by an Air Force T-38 Talon from the 560th Flying Training Squadron at Randolph AFB, but the pilot was unable to contact the crew. He reported that the cockpit windows appeared to have frost on the inside. The Lear entered Mexican airspace. After exhausting its fuel supply, it entered a spin and crashed near Cuatro Ciénegas in the northern state of Coahuila. The two pilots were killed.
On September 4, 2000 a Beechcraft 200 Super King Air departed from Perth, Australia, destined for Leonora, a mining town 370 miles (600 km) away, carrying seven passengers plus the pilot. After 22 minutes of flight, the aircraft had passed through its assigned altitude. It was at this point that Air Traffic Control noticed that the pilot's speech had become significantly impaired and he was unable to respond to instructions. Communications continued with the pilot for a further eight minutes before he lapsed into unconsciousness and no sounds of life on board could be heard. The plane continued flying on a constant heading for five hours, covering 1,760 miles (2,840 km), before crashing in northwest Queensland, at 0210 local time. The accident became known in the Australian media as the "Ghost Flight".
The aircraft was completely destroyed in the impact and the post-crash fire, making the investigation difficult. The Australian Transport Safety Bureau (ATSB) and coroner investigating the crash concluded that depressurization was likely the cause of incapacitation, but were unable to conclusively rule out the presence of toxic fumes, possibly from the air conditioning system.