Tuesday, October 14, 2008

Systems fault identified in Qantas Flight QF72 in-flight upset accident

Qantas A330-300The Australian Transport Safety Bureau (ATSB) held another media conference earlier today to provide new details in the progress of its investigation of the Qantas Flight QF72 in-flight upset. In today's media conference, the ATSB described the role a faulty Air Data Inertial Reference Unit played in the accident involving the Airbus A330-300 aircraft.

To review briefly, the accident occurred on October 7, 2008, while Qantas Flight QF72 was en route from Singapore to Perth, Australia with 303 passengers and 10 crew on board. While in cruise at 37,000 ft., the pilots received electronic centralized aircraft monitoring messages in the cockpit relating to some irregularity with the aircraft's elevator control system, and the aircraft departed level flight. According to the ATSB, "the aircraft climbed about 200 feet from its cruising level of 37,000 feet, the aircraft then pitched nose-down and descended about 650 feet in about 20 seconds, before returning to the cruising level. This was closely followed by a further nose-down pitch where the aircraft descended about 400 feet in about 16 seconds before returning once again to the cruising level."

The in-flight upset injured dozens of people in the aircraft's cabin. The crew ultimately declared a MAYDAY and diverted to Learmonth, Australia where they made an emergency landing.

Systems Fault Identified

Today the ATSB update reported a preliminary sequence of events based on further analysis of the accident aircraft's Flight Data Recorder data, Post Flight Report data and Built-in Test Equipment. Quoting from the ATSB's October 14, 2008 statement:
The aircraft was flying at FL 370 or 37, 000 feet with Autopilot and Auto-thrust system engaged, when an Inertial Reference System 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. However, it is important to note that in fly by wire aircraft such as the Airbus, even when being flown with the Autopilot off, in normal operation, the aircrafts flight control computers will still command control surfaces to protect the aircraft from unsafe conditions such as a stall.

The faulty Air Data Inertial Reference Unit continued to feed erroneous and spike values for various aircraft parameters to the aircrafts 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 Centralised Aircraft Monitoring system warnings.
About 2 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.
The crew's timely response led to the recovery of the aircraft trajectory within seconds. During the recovery the maximum altitude loss was 650 ft.

The Digital Flight Data Recorder data show that ADIRU 1 continued to generate random spikes and a second nose-down aircraft movement was encountered later on, but with less significant values in terms of aircraft's trajectory.

At this stage of the investigation, the analysis of available data indicates that the ADIRU 1 abnormal behaviour is likely as the origin of the event.
The ATSB officials went on to say that as far as they 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.

Action by Airbus

Airbus has issued an Operators Information Telex reflecting the preliminary findings of the ATSB investigation of this accident. The ATSB anticipates that Airbus also will issue Operational Engineering Bulletins and provide "information relating to operational recommendations to operators of A330 and A340 aircraft fitted with the type of ADIRU fitted to the accident aircraft. Those recommended practices are aimed at minimising 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."

ATSB Investigation Continues

The ATSB reported that its investigation is ongoing and will include:
  • Download of data from the aircraft's three ADIRUs and detailed examination and analysis of that data. Arrangements are currently being made for the units to be sent to the component manufacturer's facilities in the US as soon as possible and for ATSB investigators to attend and help with that testing, along with representatives from the US National Transportation Safety Board, The French Bureau d'Enquêtes et dAnalyses (BEA) and Airbus.
  • 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 ATSB expects to publish a Preliminary Factual report in about 30 days from the date of the accident.

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