Another A330 ADIRU Failure for Qantas

Air Safety Week, Jan 12, 2009

Qantas has suffered a second air data inertial reference unit (ADIRU) failure.

The Australian Transport Safety Bureau (ATSB) said it was advised on Dec. 27 of an occurrence that day involving a Qantas A330-300 cruising at 36,000 ft. on its way from Perth to Singapore. Some 260 nm. northwest of Perth, the autopilot disconnected and the crew received an Electronic Centralized Aircraft Monitor message (NAV IR 1 fault) indicating a problem with ADIRU No. 1.

The crew implemented the new Airbus Operations Engineering Bulletin procedure by selecting the IR 1 and ADR 1 pushbuttons to off. The aircraft returned to Perth without incident.

At the time that the autopilot disconnected, the aircraft was approximately 260 nautical miles northwest of Perth airport and approximately 350 nm south of Learmonth airport.

"It is very early in the investigation and too soon to draw any conclusions as to specific causal factors involved in this incident," says the ATSB.

"As it appears to be similar to a previous event involving an A330 aircraft, it will be included as part of the earlier investigation. The ATSB investigation will explore all aspects of the operation of the aircraft, including examination of recorded data, and any commonalities with past occurrences." ATSB added.

While the investigation is likely to take a number of months, the ATSB plans to release an interim factual report by about mid-February.

On Oct. 7, 2008, 70 of the 313 people onboard a Qantas A330 (also flying from Singapore to Perth) were injured when the aircraft pitched up and then dove twice after a failure of its ADIRU 1.

Australian air safety investigators said a fault in one of three Northrop Grumman air data inertial reference units is likely to have caused the Qantas A330-300 upset.

Flight QF72 from Singapore to Perth abruptly lost altitude. The A330 made an emergency landing in Manila. The ATSB said in its preliminary report, citing flight data recorder information, that the ADIRU likely fed incorrect information to the main flight computer.

The three ADIRUs were to undergo comprehensive testing at the manufacturer's factory.

But the ATSB continues to explore whether interference from a low- frequency U.S.-Australian naval submarine communications transmitter or even a passenger's laptop caused the Qantas jetliner upset. Another possible source of the interference is portable electronic devices, such as cell phones, Australian Transport Safety Bureau director Kerryn Macaulay said. "This is unlikely, especially if the problem is clearly identified during the ADIRU system testing, she added.

The ATSB said "most components on modern aircraft, including ADIRUs, are highly reliable and there has only been a small number of occasions where ADIRUs of different types made by varying manufacturers have had some form of failure. It is extremely rare for any such failures to have an effect on an aircrafts flight controls."

The technical glitch was dubbed a "unique event," but serious enough to prompt Airbus to issue emergency guidelines to airlines worldwide operating the Airbus A330-300s and A320s. Airbus issued an Operations Engineering Bulletin on 15 October, applicable to all A330 aircraft fitted with Northrop-Grumman ADIRUs, which detail a procedure for flight crew to follow in the event that specified fault indications were observed in order to reduce or eliminate the risk of a future similar in-flight upset event.

The glitch had never been encountered during any previous A330-300 flight, said ATSB air safety investigations director Julian Walsh. The device, one of three aboard the A330-300, and which supplies information, such as air speed, altitude and position, led the jetliner's Flight Control Primary Computers to incorrectly determine the aircraft was climbing when actually in level flight, he said. As a result, the aircraft's nose pitched down.

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