FAA mandates third A330, A340 inertial navigation fault fix

By John Croft

The US FAA today issued a final airworthiness directive instructing operators of most Airbus A330 and A340 models to power down suspect air data inertial reference units (ADIRU) using one specific rotary dial on the panel.

The mandate follows two previous EASA directives in November and December issued after a Qantas Airways A330-300 pitched down unexpectedly while in cruise flight at 37,000ft on 7 October, seriously injuring 14 of the 303 passengers.

The uncommanded descent was traced to "abnormal behaviour" of one of three Northrop Grumman-built air data reference units that provide flight parameters to various aircraft subsystems, an early result of the investigation of the incident by the Australian Transport Safety Bureau and others.

Though the ADIRU system is designed to disregard out-of-family input data, the aircraft's flight control computer received and used the faulty inputs regardless, resulting in the pitch-down. The FAA says the unit "was providing erroneous and temporary wrong parameters in a random manner".

"Among the abnormal parameters," says the FAA, "the provided Angle of Attack (AoA) value was such that the flight control computers commanded a sudden [uncommanded] nose down aircraft".

A temporary fix issued by EASA in a 19 November emergency airworthiness directive called for pilots to isolate the faulty system by turning off its inertial reference and air data units via pushbutton switches.

Operators however reported cases in which the "OFF" light did not illuminate after shutting down the systems via the pushbuttons, resulting in a second EASA emergency AD.

That mandate called for de-energizing the components, a solution that resulted in the "OFF" light illuminating, but one that operators later found did not necessarily prevent the faulty inertial reference unit's data from being used by other aircraft systems.

The FAA's final rule, which must be implemented in flight manuals by mid-March, calls for de-energizing a faulty ADIRU by setting a specific rotary selector to the OFF position.

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