Qantas flight dive due to software limitation

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A Qantas plane plummetted more than 1,000 feet in 38 seconds, flinging passengers around the cabin, partly because of a design limitation in the flight control software, an Australian probe found on Monday.

More than 100 passengers and crew were injured, 12 seriously, during the two nose-down drops by the Airbus A330-303, en route from Singapore to Perth, as it flew at 37,000 feet off Australia's northwest coast on October 7, 2008.

In its final report on the matter, the Australian Transport Safety Bureau (ATSB) said the dives were caused by the combination of a design limitation in the flight control primary computer software and another misfiring instrument.

That instrument was one of the Airbus' three air data inertial reference units (ADIRUs) -- devices which provide information on airspeed and other critical flight parameters to the plane's main computer.

"This limitation meant that, in a very specific situation, multiple AOA (angle of attack) spikes from only one of the three air data inertial reference units could result in a nose-down elevator command," it said.

In the 2008 incident, the plane automatically dived after the ADIRU sent a series of incorrect data to other aircraft systems, prompting the plane to drop 150 feet in two seconds before the pilot regained the situation.

"Although the pitch-down command lasted less than two seconds, the resulting forces were sufficient for almost all the unrestrained occupants to be thrown to the aircraft’s ceiling," the report said.

The plane plummetted 690 feet over 23 seconds in the first occurrence and five minutes later, after the pilot had brought it back to altitude, dropped another 400 feet over 15 seconds.

The report found that when the aircraft manufacturer became aware of the problem, it issued flight crew procedures to manage any future occurrence of the same ADIRU failure mode and improved its algorithms for processing data.

"As a result of this redesign, passengers, crew and operators can be confident that the same type of accident will not reoccur," the ATSB said.

But the report said with aircraft systems becoming increasingly complex, more research was needed into how design engineers and safety analysts evaluated system designs.

And it said that while in-flight upsets were very rare, the 2008 accident was a "salient reminder to all passengers and crew of the importance of wearing their seat belts during a flight whenever they are seated".

At least 110 of the 303 passengers and nine of the 12 crew were injured in the incident which forced an emergency landing at a remote Australian air force base.