CANBERRA, Oct 8, 2008 (AFP) - A computer glitch may have caused a Qantas jet to plunge suddenly mid-flight, an Australian air safety investigator said Wednesday after 36 passengers were injured in a terrifying mid-air drama.

The Australian airline however said it was too early to speculate on what had caused the "sudden change in altitude" that forced an emergency landing on Tuesday of the Airbus A330-300 carrying 313 passengers and crew.

The Australian Transport Safety Bureau's (ATSB) director of aviation safety, Julian Walsh, said the plane was cruising at 37,000 feet (11,200 metres) when pilots received an automated warning of an "irregularity."

"The pilots received electronic centralised aircraft monitoring messages in the cockpit relating to some irregularity with the aircraft's elevator control system," Walsh told a press conference in Canberra.

"The aircraft departed normal flight and climbed 300 feet," Julian Walsh, director of aviation safety investigation for the Australian Transport Safety Bureau (ATSB), he said later on the Australian Broadcasting Corporation.

"The aircraft did that of its own accord and then, whilst the crew were doing the normal actions in response to that not normal situation, the aircraft then pitched down suddenly and quite rapidly," he said.

News reports had speculated that clear air turbulence may have caused flight QF 72 bound from Singapore to Perth to plunge, leaving 36 passengers injured, including 20 passengers with broken bones, spinal injuries and cuts.

Walsh however said it was not known how far the plane had fallen and cautioned that it was too early in the probe to draw any concrete conclusions as to what happened.

ATSB investigators have begun arriving at an air force base in Exmouth, in remote northwest Western Australia, where the battered plane put down on Tuesday.

An investigator from the French Bureau d'Enquetes et d'Analyses (BEA), which investigates air incidents, and another from the French-based aircraft manufacturer Airbus will also join the probe, he said.