A computer glitch may have caused a Qantas jet to plunge suddenly mid-flight, an Australian air safety investigator has said.
The incident injured 40 passengers, in a terrifying airborne drama.
Qantas officials said it was too soon to say what had caused the sudden drop in altitude that forced the Airbus A330-300 into an emergency landing.
The plane, flying from Singapore to Perth, landed near Exmouth in Western Australia after making a mayday call.
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".
"They put two people in the seats in front of us - when I looked up they had their backs on the ceiling"
"The aircraft departed normal flight and climbed 300 feet," Mr Walsh said later on Australian radio.
"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.
Mr Walsh added that 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. 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.
Passengers were badly shaken by the incident.
One of them, Nigel Court, said the sudden change in altitude had caused mayhem on board.
"It is turning into something of an annus horribilis for Qantas, Australia's fabled Flying Kangaroo"
"They put two people in the seats in front of us. When I looked up they had their backs on the ceiling. My wife was the same - she crashed headfirst into the roof above us," he told the Reuters news agency.
"We had either the captain, or the first officer, in the area just behind us and he got caught unaware as well as, as well as a number of the cabin crew, one of who broke his leg as well."
The Australian flag-carrier prides itself on having a good safety record, but a spate of recent incidents has dented its image, correspondents say.
QANTAS INCIDENTS
Later that month, one of the airline's Boeing 737s returned to Adelaide after a landing gear bay door failed to close. Days later, a Boeing 767 bound for Manila needed to return to Sydney after a hydraulic fluid leak was detected.
The Australian Civil Aviation Safety Authority subsequently launched a review of the airline. Officials said they had no evidence of falling safety standards, but that it was "prudent" to take a closer look.
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