Air data inertial reference unit
[5] An ADIRS consists of up to three fault tolerant ADIRUs located in the aircraft electronic rack, an associated control and display unit (CDU) in the cockpit and remotely mounted air data modules (ADMs).
[10] The primary benefits of a ring laser over older mechanical gyroscopes are that there are no moving parts, it is rugged and lightweight, frictionless and does not resist a change in precession.
In addition, the master flight control computer (PRIM1) was switched from PRIM1 to PRIM2, then PRIM2 back to PRIM1, thereby creating a situation of uncertainty for the crew who did not know which redundant systems they were relying upon.
[2][12][13] On 27 January 2004 the FAA issued airworthiness directive 2003-26-03 (later superseded by AD 2008-17-12) which called for modification to the mounting of ADIRU3 in Airbus A320 family aircraft to prevent failure and loss of critical attitude and airspeed data.
[2][14] On 25 June 2005, an Alitalia Airbus A320-200 registered as I-BIKE departed Milan with a defective ADIRU as permitted by the Minimum Equipment List.
[15] On 1 August 2005, a serious incident involving Malaysia Airlines Flight 124 occurred when an ADIRU fault in a Boeing 777-2H6ER (9M-MRG) flying from Perth to Kuala Lumpur International caused the aircraft to act on false indications, resulting in uncommanded manoeuvres.
The ATSB found that the main probable cause of this incident was a latent software error which allowed the ADIRU to use data from a failed accelerometer.
[17] The US Federal Aviation Administration issued Emergency Airworthiness Directive (AD) 2005-18-51 requiring all 777 operators to install upgraded software to resolve the error.
The crew reported to the later Qantas Flight 72 investigation involving the same airframe and ADIRU that they had received numerous warning and caution messages which changed too quickly to be dealt with.
Some time into the flight, while cruising at 37,000 ft, a failure in the No.1 ADIRU led to the autopilot automatically disengaging followed by two sudden uncommanded pitch down manoeuvres, according to the Australian Transport Safety Bureau (ATSB).
[24] On 15 January 2009, the European Aviation Safety Agency issued Emergency Airworthiness Directive No 2009-0012-E to address the above A330 and A340 Northrop-Grumman ADIRU problem of incorrectly responding to a defective inertial reference.
[19] On 1 June 2009, Air France Flight 447, an Airbus A330 en route from Rio de Janeiro to Paris, crashed in the Atlantic Ocean after transmitting automated messages indicating faults with various equipment, including the ADIRU.
The left ADIRU was put in ATT (attitude-only) mode in accordance with the Quick Reference Handbook, but it continued to display erroneous attitude information to the captain.
