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CANBERRA – It was a computer glitch that caused the terrifying nosedive by a Qantas plane flying from Singapore to Perth last week, Australian investigators said on Tuesday.
The Australian Transport Safety Bureau (ATSB) said incorrect information from the faulty computer triggered a series of alarms and then prompted the Airbus A330-300's flight control computers to put the jet into a 650-foot (197-metre) nosedive.
At least 51 passengers and crew were hurt, many suffering broken bones and spinal injuries, when the plane carrying 313 people climbed suddenly before plunging downwards on October 7.
The plane was cruising at 37,000 feet when a fault in the air data inertial reference system caused the autopilot to disconnect.
Even with the autopilot off, the plane's flight control computers still command key controls in order to protect the jet from dangerous conditions, such as stalling, the ATSB said.
The faulty unit fed "erroneous and spike values" to the plane's primary computers, said ATSB investigation director Julian Walsh. "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 unit) generated very high, random and incorrect values for the aircraft's angle of attack," Walsh said.
"These very high, random and incorrect values of the angle attack led to the flight control computers commanding a nose-down aircraft movement, which resulted in the aircraft pitching down to a maximum of about 8.5 degrees."
The pilots quickly regained control of the jet, issued a mayday emergency call and requested an emergency landing at the Learmonth air force base in remote Western Australia where passengers received medical treatment.
"The crew's timely response led to the recovery of the aircraft trajectory within seconds. During the recovery, the maximum altitude loss was 650 feet," Walsh said.
The plane's French-based manufacturer has issued an advisory on the problem and will also issue special operational engineering bulletins to airlines that fly A330s and A340s fitted with the same air data computer, the ATSB said.
Walsh said any consideration of grounding aircraft fitted with the unit would be a matter for regulatory authorities, but said all signs indicated that "this is a fairly unique event".
"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," he said.
"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."
Passengers on board the jet described being slammed up against the ceiling when the plane dived suddenly as objects were hurled around the cabin.
The inside of the plane suffered significant damage including smashed panels. Qantas has described last week's incident involving flight QF72 as a "sudden change in altitude" but said it would cooperate fully with the ATSB.
The incident was the fourth involving Qantas planes in two-and-a-half months.
On July 25, an exploding oxygen bottle punched a huge hole in the side of a Qantas Boeing 747-400 during a flight from Hong Kong to Melbourne, forcing an emergency landing in the Philippines. No one was injured in the mid-air drama.
Just three days later, a Qantas Boeing 737-800 returned to Adelaide after a landing gear door failed to retract.
And in early August, a Boeing 767 bound for Manila turned back to Sydney after developing a hydraulic fluid leak. - AFP/vm
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