The Bombardier Learjet 35 business jet carrying golfer Payne Stewart and five others crashed in October 1999 when the crew blacked out from lack of oxygen, the US National Transport-ation Safety Board (NTSB) has concluded. But the 13-month investigation has failed to pinpoint the depressurisation cause.
The NTSB's report says that during the flight there was "incapacitation of the flight crew members as a result of their failure to receive supplemental oxygen following loss of cabin pressurisation, for undetermined reasons".
Operated by Sanford, Florida-based charter operator Sunjet Aviation, the aircraft was destroyed when it crashed near Aberdeen, South Dakota, after flying almost 2,300km (1,250nm) across the USA on autopilot. The aircraft departed Orlando for Dallas, but controllers lost radio contact with the Learjet 35 north of Gainsville, Florida, after clearing the jet to climb to 35,000ft (11,000m). Military aircraft tracked the jet until it ran out of fuel, reporting that the forward windshields were fogged or frosted over. The military pilots could not see into the cabin, and noticed no problems with the jet.
Pressurisation and oxygen have to be turned on manually before take-off. If cabin altitude exceeds 10,000ft, an alarm sounds. If the pressurisation system fails, the pilot can manually select a back-up mode which directs windshield defog bleed air into the cabin. In newer models, this is automatic if cabin altitude exceeds 14,500ft.
One of two modulator valves which direct air into the cabin was replaced two days before the accident, but there is no evidence that the valve malfunctioned. The board was unable to determine why the cabin lost pressure, whether the occupants wore oxygen masks, or if a depleted or contaminated oxygen supply or carbon monoxide were involved.
The NTSB recommends that the Federal Aviation Administration order flight deck crews to don oxygen masks immediately after the first sign of cabin depressurisation, and that Learjet should modify the oxygen bottle regulator/ shutoff valve assembly to ease verification of valve position during preflight checks. Other recommendations include automating the emergency pressurisation system on this type of aircraft.
Sunjet's maintenance practices were under FAA and Department of Transportation scrutiny before the accident, and are subject to an FAA enforcement action.
Source: Flight International
