Hypoxia suspected in King Air accident

 - October 9, 2007, 9:30 AM

RAYTHEON BEECH KING AIR B200, WERNADINGA STATION, QUEENSLAND, AUSTRALIA, SEPT. 4, 2000–Investigators were unable to find the cause of the pilot’s apparent hypoxia incapacitation. However, investigators did have an eight-minute tape of open-microphone transmissions that revealed “the progressive deterioration of the pilot toward unconsciousness.” Along with the pilot, seven passengers perished, and it appears all were unconscious for most of the flight.

The King Air, registered as VH-SKC, departed Perth at 1009 UTC for what was to be a 25-min flight to Leonora. Controllers gave clearance for the climb to 13,000 ft after departure. As the aircraft passed through 7,500 ft the controllers asked the pilot if he could reach 16,000 ft by 36 nm from Perth. The pilot responded that he could and controllers cleared him to FL 250 with the climb restriction as a condition. At 1020, controllers cleared the pilot to debra intersection, and the pilot responded and complied. Some 13 min later controllers saw the aircraft climb through 25,600 ft and asked the pilot to confirm his altitude. His response was “Sierra Kilo Charlie-um-standby.”

The Australian Transportation Safety Board (ATSB) commissioned two analyses of the pilot’s captured speech, one by the NTSB and the other by two forensic phoneticians in Australia. NTSB investigators compared the pilot’s speech recorded in an August 2000 flight with his speech before the last communication and found his response times to ATC, the time between keying the microphone and speaking and the rate of the pilot’s speech were similar. During the last intelligible transmission, the results were significantly slowed. The forensic phonetic analysis showed muddied articulation of words, hesitancy in completing the transmission and “discernible jitter.” The analysis moved investigators toward the hypoxia theory.

Investigators suspect the pressurization loss was gradual rather than a rapid decompression, although the wreckage yielded no clues to the cause–human or mechanical. They were unable to conclude whether anyone on board used the oxygen system or even if it was armed. While arming the system is an item on the “Before Start” portion of the checklist, pilots have been known not to arm the system to avoid the slow escape of oxygen.

According to the final report, “As hypobaric hypoxia took effect, the pilot’s visual acuity and color discrimination would have reduced, making the red warning lights even less likely to be noticed by the pilot. An aural warning was more likely to have been detected and acted upon by the pilot than a visual warning alone.”

While Australia’s Civil Aviation Safety Authority (CASA) struggles with the ATSB’s recommendation for mandatory installation of aural warnings, the FAA–notified by ATSB in October 1999 as the certifying authority of the aircraft–responded in March 2000: “Although…adding an aural warning is a desirable enhancement of the system, requiring such a warning for the existing fleet is not considered necessary to meet the minimum airworthiness standards.”

After the accident, although not considered a factor in its cause, Airservices Australia amended the in-flight emergency response checklist to incorporate new procedures controllers can use if they suspect hypoxia has affected a pilot.