The Harold E Holt naval communications base near Exmouth on WA’s North West Cape is about to get worked over again as a menace to airliners, at least in the excitable media.

A short while ago the Australian Transport Safety Bureau released this brief but important statement concerning a Qantas A330-300 that experienced an unexplained autopilot disconnection in the early stages of operating QF 71, the Perth to Singapore service on 27 December. The flight returned to Perth without incident other than making a routine ‘overweight’ landing, which meant that it required an additional inspection before being returned to service.

On 7 October a similar Qantas A330-300 operating QF 72 from Perth to Singapore was not so fortunate when it experienced an autopilot disconnection without warning on the final stage of its flight in the opposite direction. A series of bewildering malfunctions ensued, including a brief uncontrolled climb, and two short uncontrolled dives, the first of which was so violent it injured 74 passengers or crew, 14 of them seriously, and lead to an emergency landing at Learmonth.

The Australian Transport Safety Bureau says that “as it appears to be similar…to a previous event…it will be included as part of the earlier investigation.”

That continuing investigation is the most important the ATSB has ever embarked on as it affects the safe operation of hundreds of Airbus A330s in service world wide, and involves the US and French air safety authorities as well as Airbus and Qantas.

The core concern is one of three vital flight data computers called Air Data Inertial Reference Units or ADIRUs which provide the pilots and the autopilot functions with speed and attitude information. The same unit, ADIRU number 1, was the prime source of this vital data when each flight experienced autopilot failure.

On QF 71, on 27 December, the pilots followed the latest revised instructions for dealing with a failure of the unit. That advice, which has itself been revised several times since the QF 72 inquiry began, may have been material in avoiding another serious incident, although confirmation of this will depend on the course of the investigation.

Electromagnetic interference with the A330’s systems by the VLF or very long frequency antenna array at the Harold E Holt base was all but completely ruled out by the ATSB early in the QF 72 inquiry.

The loss of control aboard QF 72 occurred when the A330 was 154 kilometres west of Learmonth. QF 71 was more than four times further away at a point 630 kilometres south of Learmonth, or nearly as remote from the vicinity of the base as Melbourne is from Sydney.

The A330 operating QF 71 was not the same jet that was flying QF 72, although that particular A330 has been repaired and returned to service. Thousands of jets have flown as close if not closer to the naval base than either QF 71 or QF 72 since it was opened, including more than 14 A330s each week in recent years.

A Malaysia Airlines 777 experienced a serious ADIRU unit failure in the general area on 1 August 2005 while flying from Perth to Kuala Lumpur. However in that incident the unit was of a different design and manufacture, and the problem that the pilots had to overcome before making an emergency landing was an uncommanded climb to 42,000 feet at which point the jet exhibited stall warnings.

To paraphrase agent Mulder in the X-Files, the truth about these incidents is out there, somewhere, and unlikely to get in the way of a good tabloid headline or two.

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