On Monday 4 September 2000, a chartered Beechcraft 200 Super King Air departed Perth for a flight to the mining town of Leonora, Western Australia. The aircraft crashed near Burketown, Queensland, Australia resulting in the deaths of all 8 occupants. During the flight, the aircraft climbed above its assigned altitude. When air traffic control (ATC) contacted the pilot, the pilot's speech had become significantly impaired and he was unable to respond to instructions. Three aircraft intercepted the Beechcraft but were unable to make radio contact. The aircraft continued flying on a straight heading for five hours before running out of fuel and crashing 65 km south-east of Burketown. The accident became known in the media as the "Ghost Flight".
A subsequent investigation concluded that the pilot and passengers had become incapacitated and had been suffering from hypoxia, a lack of oxygen to the body, meaning the pilot would have been unable to operate the aircraft. Towards the end of the flight, the left engine began to be starved of fuel and the aircraft crashed into the ground. The accident report said that, due to extensive damage to the aircraft, investigators were unable to conclude if any of the eight aboard had used the oxygen system. The Australian Transport Safety Bureau (ATSB) final report could not determine what incapacitated the occupants. A number of safety recommendations were made following the accident.
The aircraft involved in the accident was a Beechcraft 200 Super King Air, registration VH-SKC, serial number BB-47, manufactured in 1975. The aircraft had 18,771 hours of service before the accident. The amount of air passed into the cabin is controlled by bleed air valves on the engines. The positions of the bleed air valves can be altered by the pilot. According to the accident report, "The aircraft was not ﬁtted with a high cabin altitude aural warning device, nor was it required to be." The aircraft was fitted with an emergency oxygen system—an oxygen tank which could supply oxygen to the crew through two masks located in the cockpit and to passengers through masks which drop from the ceiling of the cabin.
Accident investigators concluded the aircraft was airworthy at the time the accident occurred, and a pilot who flew the aircraft earlier in the day said the aircraft functioned normally. "The maintenance release was current and an examination of the aircraft’s maintenance records found no recurring maintenance problems that may have been factors in the accident," the accident report stated.
On 4 September 2000, the Beechcraft, chartered by mining company Sons of Gwalia, departed Perth, Western Australia, for the town of Leonora, Western Australia, On board were seven mine workers travelling to Gwalia Gold Mine. The aircraft took off from Perth at 6:09 pm local time (1009 Coordinated Universal Time (UTC)), and was cleared by ATC to climb to FL130 (13,000 feet (4,000 m)). Five minutes later, at 1015, it was further cleared to its cruising altitude of FL250 (25,000 feet (7,600 m)); with an instruction to be at FL160 by 36 nautical miles (67 km; 41 mi) from Perth. The pilot acknowledged this transmission.
Five minutes later, at 1020, as the aircraft climbed through FL156 (15,600 feet (4,800 m)) it was cleared to waypoint DEBRA. The pilot again acknowledged. At approximately 1033 the aircraft passed through its cleared level and at FL256 (25,600 feet (7,800 m)) ATC asked the pilot to confirm his altitude. “Sierra Kilo Charlie–um–standby," the pilot said. This was the final spoken transmission from the aircraft, and its climb continued. According to the accident report, several open-microphone transmissions followed, with the sound of background noise from the engines, a person breathing, "one unintelligible syllable", and "two chime-like tones, similar to those generated by electronic devices." During this time ATC attempted to regain contact with the pilot. "Sierra Kilo Charlie Sierra Kilo Charlie, Melbourne Centre, if receiving this transmission squawk ident" the controller said at 1040. At 1041, the controller asked again, "Sierra Kilo Charlie, only receiving open mike from you. Would you contact me on one two ﬁve decimal two."
The aircraft continued to climb and left radar coverage at 1102, passing FL325 (32,500 feet (9,900 m)). Thirty-one minutes later, Australian Search and Rescue asked the crew of a business jet to approach the Beechcraft. They reported that it was in level flight at FL343 (34,300 feet (10,500 m)), and they could see no movement on the flight deck or in the cabin. However, the conditions made it difficult to observe closely.
Two other aircraft which were asked to monitor the Beechcraft intercepted it over the Northern Territory, north-west of Alice Springs. The pilots reported that it was now in a steady descent, and both aircraft followed it as its airspeed increased. "Although its external lights were on, nothing could be seen inside the cabin" the accident report stated. "The crews of the chase aircraft attempted to contact the pilot of the Beechcraft by radio but they did not receive a response." At 1510, the aircraft turned left through 90 degrees as it descended through 5,000 feet (1,500 m). About 65 km south-east of Burketown, Queensland, it hit the ground and broke up. The pilot and seven passengers died.
The accident was investigated by the Australian Transport Safety Bureau (ATSB), a federal body responsible for investigating transportation accidents in Australia. The final accident report was published in March 2001. The report was unable to make a definitive conclusion as to the cause of the crash.
The report found all those aboard the aircraft, including the pilot, had likely become incapacitated and begun suffering from hypoxia.
The accident report stated, "After the aircraft climbed above the assigned altitude of FL250, the speech and breathing patterns of the pilot, evidenced during the radio transmissions, displayed changes consistent with hypoxia." Investigators were, however, unable to conclusively dismiss toxic fumes as the cause. "The incapacitation of the pilot and passengers was probably due to hypobaric hypoxia because of the high cabin altitude and their not receiving supplemental oxygen" the report said, adding, "The reasons for the pilot and passengers not receiving supplemental oxygen [from the oxygen tank aboard the aircraft] could not be determined."
The ATSB found it likely that the autopilot was engaged and this caused the aircraft to fly on a straight heading; the vertical path of the aircraft indicated climb power had been set before the occupants of the aircraft were incapacitated. "The design of the aircraft systems were such that, with the autopilot engaged, the engines would continue to operate and the aircraft would continue to ﬂy without human input until it was disrupted by other events, such as collision or fuel exhaustion," according to the accident report. It was suggested that, towards the end of the flight, the fuel tank for the left engine was almost empty. "The near exhaustion of fuel in the left wing tanks may have produced at least one, and probably several, momentary losses of left engine power shortly before all power was lost" the report said. "The aircraft yawed and rolled towards the left engine, as was observed shortly before the aircraft collided with the ground."
The accident report said due to the damage to the aircraft upon impact with the ground, investigators were unable to conclude if any of the eight aboard used the oxygen system. However, the report stated "The absence of a distress radio call, or an attempt to descend the aircraft, and the likelihood that the pilot did not don his oxygen mask, suggested that the pilot was unaware that the aircraft was unpressurised or depressurising." The passengers, the report added, were also likely not wearing their oxygen masks, as there was no noise recorded on the ATC transmissions indicating they were attempting to assist the pilot.
Investigators were not able to determine what caused the depressurisation of the aircraft, but stated likely causes included either an incorrect switch selection due to pilot error or a mechanical failure in the aircraft pressurisation system. The air traffic control recordings suggested it was unlikely a rapid decompression had occurred. "During an explosive or rapid depressurisation of a pressurised aircraft, however, the noise, pressure changes, temperature changes and draughts within the cabin would have alerted the occupants that a substantial failure had occurred," the report added. The document listed the two main factors in the accident as,
1. The aircraft was probably unpressurised for a signiﬁcant part of its climb and cruise for undetermined reasons.
2. The pilot and passengers were incapacitated, probably due to hypobaric hypoxia, because of the high cabin altitude and their not receiving supplemental oxygen.
In the aftermath of an incident in 1999 in which the pilot of a Beechcraft Super King Air suffered hypoxia, the ATSB published Report 199902928 and recommended an aural warning be fitted on the flight deck of all Australian Beechcraft Super King Air aircraft. The flight deck of the Beechcraft Super King Air only has visual warning of inadequate cabin pressure.
In the aftermath of the Ghost Flight, the Civil Aviation Safety Authority issued a Discussion Paper and a Notice of Proposed Rule Making (NPRM), both proposing aural warning in the Beechcraft Super King Air and other pressurised aircraft. The outcome of consultation on the NPRM was that the Civil Aviation Safety Authority did not mandate aural warning of inadequate cabin pressure and this angered the families of some of the victims. Instead, the Civil Aviation Safety Authority issued a notice to owners of pressurised aircraft registered in Australia recommending installation of an aural warning, but not making it mandatory. The notice said, "The benefit to your pilots and passengers lies in the reduction in risk of an uncommanded depressurisation leading to an incident or fatal accident. The benefit is much greater than the cost of purchase and installation of one of these low-cost systems."
The West Australian Coroner, Mr. Alistair Hope, conducted an inquest into the deaths of the eight occupants of the aircraft. The inquest determined that the deaths were accidental, but was unable to determine the cause of the crash. The Coroner recommended an aural alarm system for pressurised aircraft, and a low-cost black box flight recorder.
The Coroner was critical of the poor co-ordination between the ATSB, the Queensland Police and the Civil Aviation Safety Authority (CASA). He also criticised the ATSB for failing to take notes when interviewing witnesses, and for its poor presentation of evidence.
When the air traffic controller responsible for the Beechcraft received the open-microphone transmissions, he alerted his supervisor that he was concerned the pilot could be suffering from hypoxia. The controller and his supervisor completed the standard checklist which, at the time, did not include a procedure to follow in the case of incapacitation or hypoxia. In the aftermath of the accident, the checklist was changed to "incorporate procedures to be followed by air trafﬁc controllers, when a controller suspects that a pilot has been affected by hypoxia."