Culprit in Qantas Upset Identified

Air Safety Week, Oct 20, 2008

A faulty air data inertial reference unit (ADIRU) caused a Qantas Airbus A330-300 jetliner to plummet Oct. 7, injuring scores of passengers on a flight from Singapore to Perth, according to the Australian Transportation Safety Bureau (ATSB).

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.

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.

The A330 was flying at 37, 000 feet with Autopilot and Auto-thrust system engaged when the IRU fault occurred within the Number-1 Air Data Inertial Reference Unit (ADIRU 1), which resulted in the Autopilot automatically disconnecting. From this moment, the crew flew the aircraft manually to the end of the flight, except for a short duration of a few seconds, when the Autopilot was reengaged.

The faulty Air Data Inertial Reference Unit continued to feed erroneous and spike values for various aircraft parameters to the aircraft's Flight Control Primary Computers, which led to several consequences including:

* false stall and overspeed warnings

* loss of attitude information on the Captain's Primary Flight Display

* several Electronic Centralized Aircraft Monitoring system warnings

About two minutes after the initial fault, ADIRU 1 generated very high, random and incorrect values for the aircrafts angle of attack.

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 triggering of a Flight Control Primary Computer pitch fault

"At this stage of the investigation, the analysis of available data indicates that the ADIRU 1 abnormal behavior is likely as the origin of the event...As far as we can understand, this appears to be a unique event and Airbus has advised that it is not aware of any similar event over the many years of operation of the Airbus." The ATSB stated.

But Airbus issued an Operators Information Telex/Flight Operations Telex reflecting the ATSB findings. Operational Engineering Bulletins will follow, providing safety recommendations to operators of A330 and A340 aircraft fitted with the same type of ADIRU.

"Those recommended practices are aimed at minimizing risk in the unlikely event of a similar occurrence. That includes guidance and checklists for crew response in the event of an Inertial Reference System failure, the ATSB added.

Meanwhile, the ATSB's investigation is ongoing and will include:

* Download of data from the aircraft's three ADIRUs and detailed examination and analysis of that data. Arrangements were made for the units to be sent to the component manufacturer's facilities in the U.S. as soon as possible.

* In addition, investigators have been conducting a detailed review of the aircraft's maintenance history, including checking on compliance with relevant Airworthiness Directives, although initial indications are that the aircraft met the relevant airworthiness requirements.

* Work is also ongoing to progress interviews, which will include with injured passengers to understand what occurred in the aircraft cabin. The ATSB plans to distribute a survey to all passengers.

The ADIRU in question was supplied by Northrop Grumman's Navigation Systems unit located in Woodland Hills, CA, according to the Airbus telex issued to affected airlines. Carriers can choose ADIRUs made by Litton or Honeywell for the A330/A340. The fault hasn't occurred with Honeywell equipment.

The air data system involved, the LTN-101, was certified for operation in 1993 and the nearly 7,000 units sold since then have logged millions of flight hours.

Other air carriers who have selected the Northrop Grumman LTN-101 for their Airbus A320/A330 aircraft include US Airways and India's Jet Airways.

"The dependability of our LTN-101E ADIRU was an important consideration for US Airways," said Jim Myers, vice president and general manager of Northrop Grumman's Navigation Systems Division. "It provides the aircraft with high dispatch reliability and lower cost of ownership demanded by today's carriers," he added in announcing the US Airways contract this past July.

Northrop Grumman's LTN-101E inertial reference unit replaces ring-laser gyro technology with fiber-optic gyros and micro-electro mechanical systems silicon accelerometers that are more reliable and easier to maintain. Enhancements over its predecessor, the LTN-101, include a more than 50 percent reduction in electronic modules, faster processors, and a new interface bus that speeds data transfer within the aircraft.