Investigators are looking for evidence that depressurisation caused the 26 October crash of a Bombardier Learjet 35 business jet, which killed golfer Payne Stewart and four others.

The cause may be difficult to determine, warns the US National Transportation Safety Board, as the aircraft was destroyed when it crashed near Mina, South Dakota, after flying almost 2,300km (1,250nm) across the USA on autopilot. No flight data recorder was fitted and the cockpit voice recorder, if recovered intact, can have recorded only the last 30min of the 4h flight.

The last radio contact with the aircraft, built in 1976 and owned by charter operator Sunjet Aviation, was 25min after take-off from Orlando, Florida, when the aircraft was passing through 37,000ft (11,300m). Instead of turning toward its intended destination, Dallas in Texas, the Learjet continued on its initial heading, porpoising between 51,000ft and 22,000ft, before it ran out of fuel.

The pilots of US Air Force fighters that intercepted the aircraft reported the cabin windows fogged or frosted over. The Learjet 35 is certificated to a ceiling of 45,000ft, with pressurisation maintaining a cabin altitude of 8,000ft.

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 has failed, the pilot can manually select a back-up mode which directs windshield defog bleed air into the cabin. In the later Learjet 35A this is automatic if cabin altitude exceeds 14,500ft.

In 1995, Learjet 35 operators were directed to replace pressurisation valves after AlliedSignal discovered a manufacturing defect in units produced after 1989. It has emerged that one of two modulator valves, which direct air into the cabin, was replaced just two days before the accident.

A pressurisation or oxygen problem is suspected in the January 1990 fatal crash of a Learjet 23 at Ansonia, Ohio.

Source: Flight International