A Qantas Airways Airbus A330 that descended suddenly appears to have received faulty data from one of its units and this then played havoc with the aircraft's flight control system.

"At this stage of the investigation, the analysis of the available data indicates that the air data inertial reference unit (ADIRU) 1 abnormal behaviour is the likely origin of the event," the Australian Transport Safety Bureau (ATSB) says in a statement today, referring to an incident that occurred on 7 October while the Qantas A330 was enroute from Singapore to Perth.

"The faulty ADIRU unit continued to feed erroneous and spike values, for various aircraft parameters, to the aircraft's flight control primary computers."

This "led to several consequences including: false stall and over-speed warnings, loss of altitude information on the captain's primary flight display and several centralised aircraft monitoring system warnings."

Because the ADIRU 1 generated very high, random and incorrect angles of attack it meant that "the flight control computers commanded a nose-down aircraft movement, which resulted in the aircraft pitching down to a maximum of 8.5 degrees."

It also "triggered a flight control primary computer pitch fault".

The ATSB says the crew responded in a timely fashion and helped prevent the aircraft's rapid descent from being even greater.

In its preliminary review released on 9 October the ATSB says the A330 descended about 650ft in about 20s, before returning to the cruising level of 37,000ft.

Then about 70s later the A330 descended about 400ft in about 16s before returning to the cruising level. In both instances the aircraft was pitched nose-down.

Of the 303 passengers and 10 crew on board 14 people were seriously injured, an additional group of up to 30 had serious enough injuries to receive medical treatment in hospital and up to a further 30 required first aid treatment, says the ATSB.

The Qantas pilots responded by making an emergency landing at Learmonth, a remote airport in northwest Western Australia and from there the passengers were put on other aircraft and flown to Perth.

In today's statement the ATSB says Airbus a few moments ago issued an operators information telex providing information about the incident along with recommendations to A330 and Airbus A340 operators that have aircraft fitted with the same type of ADIRU as on the Qantas aircraft.

The recommendations include "guidance and checklists for crew response in the event of an inertial reference system failure".

ATSB says it will issue a preliminary factual report within 30 days of the incident.

ADIRUs provide data with regards to the aircraft's air speed, altitude, position and altitude.

Source: Air Transport Intelligence news