By Dave Tonks 

A week after the in-flight accident on 7th October 2008 involving a QANTAS Airbus A330 on Flight QF72 from Singapore to Perth, the Australian Transport Safety Bureau (ATSB) issued a Media Release outlining the results of the preliminary investigation.

The Airbus A330 is fitted with a sophisticated fly-by-wire control system that functions by various sensors on the aircraft sending information to Air Data Inertial Reference Units (ADIRUs). This information is then sent to the Flight Control Primary Computers (FCPCs) and that data is cross-referenced against autopilot and joystick/rudder inputs before any commands are sent to the flight actuation devices.

In other words, if an aircraft is flying straight and level (with the autopilot on or off) and a gust hits the nose of the aircraft from below and the nose starts to rise, the computer immediately checks to see if there is any input from either the flight controls or autopilot. If there is no input, the computer sends a command to the elevator control actuators to cancel out the rising nose. These commands are carried out in milliseconds and flight controls are constantly moving to ensure the aircraft adheres to parameters programmed by the flightcrew. 

The Media Release stated that the aircraft was cruising at Flight Level 370 (FL 370), or 37,000 feet with the autopilot and auto-thrust systems engaged. It appears that a fault occurred within the Number 1 ADIRU, resulting in the autopilot disconnecting. The faulty ADIRU then supplied incorrect values in relation to various aircraft parameters to the FCPCs, resulting in numerous false messages being communicated to the flightcrew, including false stall and overspeed warnings. Two minutes after autopilot disconnection, spurious data reporting a high angle of attack of the aircraft was sent to the FCPCs, resulting in the computers commanding a significant nose-down pitch of the aircraft. Information recovered from the Flight Data Recorders indicated the nose-down pitch at 8.5 degrees and a height loss of 650 feet.

This initial nose-down pitch was the first and most serious, with the faulty ADIRU continuing to feed erroneous data to the computers during the remainder of the flight, resulting in a later uncommanded nose-down pitch of lesser value.  It is obvious that this incident could have ended up very differently – continuing spurious data from the faulty ADIRU may have resulted in even higher uncommanded responses by the elevator. Under those circumstances, the aircraft could have been placed in an unrecoverable position, or suffered structural failure from excessive uncommanded control inputs.

After the Media Release was issued, Airbus released an Operators’ Information Telex to operators of Airbus A330 and A340 aircraft with the type of ADIRU fitted to the accident aircraft. The information in the release includes recommended practices aimed at minimising risk in the unlikely event of a similar occurrence and includes guidance and checklists for flightcrews in relation to Inertial Reference System failures.

Further investigation of the recent Airbus incident has revealed a disturbingly similar event that occurred to a Malaysian Airlines Boeing 777-200 on the evening of 1st August 2005. In this instance, as the aircraft was climbing through FL 380, a LOW AIRSPEED message was displayed on the Engine Indication and Crew Alerting System (EICAS) screen. As this happened, the Primary Flight Display (PFD) was giving erroneous readings with relation to slip/skid indication and displayed data that indicated the aircraft was simultaneously approaching both the stall and overspeed limits. The aircraft then pitched up and climbed to FL 410, with instruments indicating the airspeed had reduced from 270 knots to 158 knots. The Flight Data Recorder subsequently showed that during this stage of the flight the aircraft ascended at up to 10,580 feet per minute.

According to the ATSB report, the primary cause of the incident was listed as, ‘erroneous acceleration values sourced from the Air Data Initial Reference Unit (ADIRU) and flagged as valid to the aircraft precipitated an in-flight upset as the aircraft climbed through FL 365.’ The investigation further concluded that faulty accelerometers had contributed to the erroneous data being transmitted to the FCPCs by the ADIRU and went on to note that the output of an accelerometer is, ‘a voltage signal’.

Taking all of the above into consideration, it remains to be seen whether any further link will be made with respect to the high energy radio transmitters utilised at the Exmouth military establishment and the subsequent failures of electronic equipment on two separate aircraft operating at or around FL 370 in the vicinity.

 Copies of the Media Release on the QANTAS Flight QF72 accident and the Report on the Malaysian Airlines Boeing 777-200 ‘event’ can be downloaded from www.atsb.gov.au.