The Australian Transport Safety Bureau (ATSB) has confirmed the sudden drop in altitude of a Qantas Flight 72 over Western Australia last year was due to a computer error.
Moments after the pilot of Qantas Flight 72 had switched off autopilot, one of the Airbus A330-303's air data inertial reference units (ADIRUs) "started providing erroneous data" spikes, according to an ATSB interim report released today.
The ADIRU is a core component of the aircraft's navigational systems, and is used to capture and relay air data sensor information such as position and altitude which is then fed into the aircraft's flight control computers. The Airbus A330-303 was equipped with three ADIRUs.
While the two other ADIRU's continued to function correctly, the plan's flight control computers had failed to filter data relating to the "angle of attack" of the aircraft — an aviation phrase used to describe a wing's motion while in flight.
QF72 damage (left), Air Data Inertial Reference Unit (right)
"The computers subsequently commanded the pitch-down movements," the report said.
Failure warnings had started as soon as the autopilot system was disconnected, the report states. But as flight staff were evaluating the situation, the aircraft suddenly pitched nose-down and fell around 650 feet, according to the report's timeline of events.
Shortly after correcting the fall in altitude to 37,000 feet, the aircraft pitched nose down again, that time descending 400 feet.
The two sudden drops in altitude had resulted in 11 passengers being seriously injured.
The ATSB had also investigated whether the glitches were caused by electromagnetic interference affecting the aircraft's wiring, as well as interference that could have come from nearby communication systems, however, these have been ruled out.
The next phase of the investigation will further interrogate the aircraft's primary flight control computer and two secondary computers. The ATSB had identified problems with the secondary computers' ability to action commands sent from the primary. The investigation will examine the computers' software development cycle, such as design, hazard analysis, testing and certification.
The US manufacturer of the ADIRU units, not disclosed in the report, is also conducting a theoretical analysis of the device's software and hardware to identify possible fault origins, the ATSB said.