CANBERRA - Air investigators say a faulty computer system was responsible for a terrifying mid-air plunge on a Qantas flight between Singapore and Perth last week.
A fault in the Airbus A330-300's air data inertial reference system is believed to have led to erroneous information being sent to its flight control computer, causing the autopilot to shut down.
The aircraft was cruising at 37,000 feet when the fault occurred, causing it to descend up to 650 feet in seconds.
More than 70 people were injured when the plane, carrying 303 passengers and 10 crew, suddenly dropped altitude, hurling people around the cabin and forcing the pilot to make an emergency landing at Learmonth in Western Australia.
Australian Transport Safety Bureau investigation director Julian Walsh said the faulty unit continued to feed "erroneous and spike values" to its primary computers.
"This led to several consequences, including false stall and overspeed warnings," he said.
"About two minutes after the initial fault (the air data inertial reference system) generated very high and incorrect values for the aircraft's angle of attack."
This led to the flight control computers commanding the aircraft to pitch down, Mr Walsh said.
"The crew's timely response led to the recovery of the aircraft's trajectory within seconds, and during the recovery, the maximum altitude lost was 650 feet."
Mr Walsh said analysis of the digital flight recorder showed the faulty air data system continued to generate false information, leading to a second, less serious "nose down aircraft movement".
The ATSB is expected to provide a preliminary factual report within three weeks.
There had been suggestions the incident may lead to the grounding of Airbus A330-300 models.
Mr Walsh today said that would be a matter for regulatory authorities.
"However, the information we have at hand indicates that this is a fairly unique event," he said.
"These aircraft have been operating over many hundreds of thousands of hours over many years, and this type of event has not been seen before."
"It's probably unlikely there will be a recurrence, but obviously we won't dismiss that, and it's important that we investigate to find out what led to the (fault) and reduce the chance of that happening in the future."
Mr Walsh said Airbus had provided advice to airlines operating the A330-300 that would minimise risk in the very unlikely event of a similar incident occurring again.
"It's a telex that will go out to all operators around the world, and that provides guidance for the crews to take certain action when they observe certain warnings and indications in the cockpit."
The mid-flight plunge over Western Australia is the latest in a spate of incidents involving Qantas planes.
On July 25, a faulty oxygen bottle blew a hole in the fuselage of a Qantas Boeing 747-400 flying from Hong Kong to Melbourne.
The blast caused the aircraft, with 365 people on board, to depressurise and it rapidly descended several thousand feet before making an emergency landing in Manila.
On July 29, an Adelaide-Melbourne flight returned to Adelaide when a wheel bay door failed to close, while on August 2 a hydraulic fluid leak forced a Boeing 767 to return to Sydney for an emergency landing.
On August 13, a Qantas Boeing 747-300 from Melbourne was grounded in New Zealand after an engine shut down on approach to Auckland.