An Air Data Inertial Reference Unit (ADIRU) is a key component of the integrated Air Data Inertial Reference System (ADIRS), which supplies air data (airspeed, angle of attack and altitude) and inertial reference (position and attitude) information to the pilots' electronic flight instrument system displays as well as other systems on the aircraft such as the engines, autopilot, aircraft flight control system and landing gear systems.[1] An ADIRU acts as a single, fault tolerant source of navigational data for both pilots of an aircraft.[2] It may be complemented by a secondary attitude air data reference unit (SAARU), as in the Boeing 777 design.[3]

This device is used on various military aircraft as well as civilian airliners starting with the Airbus A320[4] and Boeing 777.[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).[6] The No 3 ADIRU is a redundant unit that may be selected to supply data to either the commander's or the co-pilot's displays in the event of a partial or complete failure of either the No 1 or No 2 ADIRU. There is no cross-channel redundancy between the Nos 1 and 2 ADIRUs, as No 3 ADIRU is the only alternate source of air and inertial reference data. An inertial reference (IR) fault in ADIRU No 1 or 2 will cause a loss of attitude and navigation information on their associated primary flight display (PFD) and navigation display (ND) screens. An air data reference (ADR) fault will cause the loss of airspeed and altitude information on the affected display. In either case the information can only be restored by selecting the No 3 ADIRU.[1]

Each ADIRU comprises an ADR and an inertial reference (IR) component.[7]

Air data reference

See also: Pitot-static system

The air data reference (ADR) component of an ADIRU provides airspeed, Mach number, angle of attack, temperature and barometric altitude data.[8] Ram air pressure and static pressures used in calculating airspeed are measured by small ADMs located as close as possible to the respective pitot and static pressure sensors. ADMs transmit their pressures to the ADIRUs through ARINC 429 data buses.[9]

Inertial reference

The IR component of an ADIRU gives attitude, flight path vector, ground speed and positional data.[1] The ring laser gyroscope is a core enabling technology in the system, and is used together with accelerometers, GPS and other sensors to provide raw data.[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.

Complexity in redundancy

Analysis of complex systems is itself so difficult as to be subject to errors in the certification process. Complex interactions between flight computers and ADIRUs can lead to counter-intuitive behaviour for the crew in the event of a failure. In the case of Qantas Flight 72, the captain switched the source of IR data from ADIRU1 to ADIRU3 following a failure of ADIRU1; however ADIRU1 continued to supply ADR data to the captain's primary flight display. 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.[11]

Reliance on redundancy of aircraft systems can also lead to delays in executing needed repairs, as airline operators rely on the redundancy to keep the aircraft system working without having to repair faults immediately.[1][2][3][11]

Failures and directives

FAA Airworthiness directive 2000-07-27

On May 3, 2000, the FAA issued airworthiness directive 2000-07-27, addressing dual critical failures during flight, attributed to power supply issues affecting early Honeywell HG2030 and HG2050 ADIRU ring laser gyros used on several Boeing 737, 757, Airbus A319, A320, A321, A330, and A340 models.[2][12][13]

Airworthiness directive 2003-26-03

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]

Alitalia A320

I-BIKE, the aircraft involved in the A320 incident

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. While approaching London Heathrow Airport during deteriorating weather another ADIRU failed, leaving only one operable. In the subsequent confusion the third was inadvertently reset, losing its reference heading and disabling several automatic functions. The crew was able to effect a safe landing after declaring a Pan-pan.[15]

Malaysia Airlines Flight 124

9M-MRG, the aircraft involved as flight 124

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.[16] In that incident the incorrect data impacted all planes of movement while the aircraft was climbing through 38,000 feet (11,600 m). The aircraft pitched up and climbed to around 41,000 feet (12,500 m), with the stall warning activated. The pilots recovered the aircraft with the autopilot disengaged and requested a return to Perth. During the return to Perth, both the left and right autopilots were briefly activated by the crew, but in both instances the aircraft pitched down and banked to the right. The aircraft was flown manually for the remainder of the flight and landed safely in Perth. There were no injuries and no damage to the aircraft. 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.[18]

Qantas Flight 68

VH-QPA, the aircraft involved as both flight 68 and 72

On 12 September 2006, Qantas Flight 68, Airbus A330 registration VH-QPA, from Singapore to Perth exhibited ADIRU problems but without causing any disruption to the flight. At 41,000 feet (12,000 m) and estimated position 530 nautical miles (980 km) north of Learmonth, Western Australia,[19] NAV IR1 FAULT then, 30 minutes later, NAV ADR 1 FAULT notifications were received on the ECAM identifying navigation system faults in Inertial Reference Unit 1, then in ADR 1 respectively. 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. While investigating the problem, the crew noticed a weak and intermittent ADR 1 FAULT light and elected to switch off ADR 1, after which they experienced no further problems. There was no impact on the flight controls throughout the event. The ADIRU manufacturer's recommended maintenance procedures were carried out after the flight and system testing found no further fault.[19]

Jetstar Flight 7

VH-EBC, the aircraft involved as flight 7

On 7 February 2008, a similar aircraft (VH-EBC) operated by Qantas subsidiary Jetstar Airways was involved in a similar occurrence while conducting the JQ7 service from Sydney to Ho Chi Minh City, Vietnam. In this event - which occurred 1,760 nautical miles (3,260 km) east of Learmonth - many of the same errors occurred in the ADIRU unit. The crew followed the relevant procedure applicable at the time and the flight continued without problems.[19]

The ATSB has yet to confirm if this event is related to the other Airbus A330 ADIRU occurrences.[19]

Airworthiness directive 2008-17-12

On 6 August 2008, the FAA issued airworthiness directive 2008-17-12 expanding on the requirements of the earlier AD 2003-26-03 which had been determined to be an insufficient remedy. In some cases it called for replacement of ADIRUs with newer models, but allowed 46 months from October 2008 to implement the directive.[20]

Qantas Flight 72

On 7 October 2008, Qantas Flight 72, using the same aircraft involved in the Flight 68 incident, departed Singapore for Perth. 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). The accident injured up to 74 passengers and crew, ranging from minor to serious injuries. The aircraft was able to make an emergency landing without further injuries. The aircraft was equipped with a Northrop Grumman made ADIRS, which investigators sent to the manufacturer for further testing.[21][22]

Qantas Flight 71

VH-QPG, the aircraft involved as flight 71

On 27 December 2008, Qantas Flight 71 from Perth to Singapore, a different Qantas A330-300 with registration VH-QPG[23] was involved in an incident at 36,000 feet approximately 260 nautical miles (480 km) north-west of Perth and 350 nautical miles (650 km) south of Learmonth Airport at 1729 WST. The autopilot disconnected and the crew received an alert indicating a problem with ADIRU Number 1.[24]

Emergency Airworthiness Directive No 2009-0012-E

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. In the event of a NAV IR fault the directed crew response is now to "select OFF the relevant IR, select OFF the relevant ADR, and then turn the IR rotary mode selector to the OFF position." The effect is to ensure that the faulted IR is powered off so that it no longer can send erroneous data to other systems.[19]

Air France Flight 447

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.[25] While examining possibly related events of weather-related loss of ADIRS, the NTSB decided to investigate two similar cases on cruising A330s.[26] On a 21 May 2009 Miami-Sao Paulo TAM Flight 8091 registered as PT-MVB, and on a 23 June 2009 Hong Kong-Tokyo Northwest Airlines Flight 8 registered as N805NW each saw sudden loss of airspeed data at cruise altitude and consequent loss of ADIRS control.[27][28][29]

Ryanair Flight 6606

On 9 October 2018, the Boeing 737-800 operating the flight from Porto Airport to Edinburgh Airport suffered a left ADIRU failure that resulted in the aircraft pitching up and climbing 600 feet. 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. The remainder of the flight was flown manually with an uneventful landing. The UK's AAIB released the final report on 31 October 2019,[30] with the following recommendation:

It is recommended that Boeing Commercial Aircraft amend the Boeing 737 Quick Reference Handbook to include a non-normal checklist for situations when pitch and roll comparator annunciations appear on the attitude display.

See also


  1. ^ a b c d "The intricate complexity within an immaculate redundancy concern". Air Safety Week. August 14, 2006. Retrieved 2008-07-16.
  2. ^ a b c d "Safety concern". Air Safety Week. May 5, 2005. Retrieved 2006-09-16.
  3. ^ a b "In the grip of the gremlins". Air Safety Week. March 26, 2007.
  4. ^ "Honeywell's ADIRU selected by Airbus". Farnborough. 22–28 July 2002. Archived from the original on 2006-10-17. Retrieved 2008-07-16.
  5. ^ Digital Avionics Systems. IEEE, AIAA. 1995. ISBN 0-7803-3050-1. Retrieved 2008-10-16.
  6. ^ "738-3 Air Data and Inertial Reference System (ADIRS)". ARINC. 2008. Archived from the original on 25 May 2011. Retrieved 14 July 2008.
  7. ^ "Airline training guides, Aviation, Operations, Safety -Navigation A330". Archived from the original on 15 January 2007. Retrieved 12 June 2009.
  8. ^ "Erroneous flight instruments". Boeing Aero Magazine, Issue 08. Archived from the original on 12 June 2008. Retrieved 2008-07-14.
  9. ^ "Air Data Module" (PDF). Honeywell. Archived from the original (PDF) on 3 March 2016. Retrieved 25 December 2016.
  10. ^ International Aerospace Abstracts. Cambridge Scientific Abstracts, Inc, American Institute of Aeronautics and Astronautics. 1985. Retrieved 2008-10-16.
  11. ^ a b C.W. Johnson; C. Michael Holloway (2 February 2009). "The Dangers of Interaction with Modular and Self-Healing Avionics Applications: Redundancy Considered Harmful" (PDF). Retrieved 8 June 2009.
  12. ^ "Various transport category airplanes equipped with certain Honeywell Air Data Inertial Reference Units". US Federal Aviation Administration. April 18, 2000. Retrieved 2008-10-15.
  13. ^ "AD/INST/45 Honeywell Air Data Inertial Reference Units 6/2000 DM" (PDF). Australian Civil Aviation Safety Authority. April 27, 2000. Archived from the original (PDF) on August 5, 2008.
  14. ^ "Airbus model A318, A319, A320, and A321 series airplanes equipped with certain Northrop Grumman (formerly Litton) Air Data Inertial Reference Units". US Federal Aviation Administration. August 6, 2008. Retrieved 2008-10-15.
  15. ^ "AAIB Bulletin: 6/2006" (PDF). UK Air Accidents Investigation Branch. 2006. Archived from the original (PDF) on 2008-10-22. Retrieved 2008-10-15.
  16. ^ Accident description for Malaysia Airlines Flight 124 at the Aviation Safety Network. Retrieved on 2008-10-15.
  17. ^ "In-flight upset event, 240 km north-west of Perth, WA, Boeing Company 777-200, 9M-MRG, 1 August 2005" (PDF). Australian Transport Safety Bureau. 2007-03-13. Archived from the original (PDF) on 2021-03-15. Retrieved 2008-10-15.
  18. ^ "Emergency Airworthiness Directive (AD) 2005-18-51". Federal Aviation Administration. 2005-08-29. Retrieved 2008-10-15.
  19. ^ a b c d e "In-flight upset, 154 km west of Learmonth, WA, 7 October 2008, VH-QPA, Airbus A330-303 - Interim Factual" (PDF). Aviation Occurrence Investigation AO-2008-070. Australian Transport Safety Bureau. 2009-03-06. Archived from the original (PDF) on 2018-11-23. Retrieved 2009-03-07.
  20. ^ "AD 2008-17-12 Airbus" (PDF). US Federal Aviation Administration. 6 August 2008. Archived from the original (PDF) on 17 June 2009. Retrieved 2008-10-16.
  21. ^ "Computer error behind Qantas midair drama". Australian Broadcasting Corporation. 14 October 2008. Archived from the original on 16 October 2008. Retrieved 2008-10-15.
  22. ^ Steve Creedy (17 October 2008). "US tests on false data sent on Qantas jet over WA". The Australian. Archived from the original on 2008-10-17. Retrieved 2008-10-16.
  23. ^ Mike Walker. "Learmonth A330 pitch events" (PDF). Archived from the original (PDF) on 2014-12-17. Retrieved 2014-12-17.
  24. ^ "Qantas Airbus A330 incident, 480km North West of Perth on 27 December 2008" (Press release). Australian Transport Safety Bureau. 2 January 2009. Archived from the original on 10 January 2009. Retrieved 6 January 2009.
  25. ^ Simon Hradecky (2 June 2009). "Crash: Air France A332 over Atlantic on June 1st 2009, aircraft impacted ocean". The Aviation Herald.
  26. ^ "Air France 447–Two A330 airspeed and altitude incidents under NTSB scrutiny". aviationnewsrelease. 28 June 2009.
  27. ^ "NTSB investigating two recent incidents involving possible A-330 speed and altitude indication anomalies" (Press release). NTSB. 25 June 2009. Retrieved 14 October 2011.
  28. ^ "Brief of Incident". NTSB. 18 July 2011.
  29. ^ "Brief of Incident". NTSB. 27 June 2011.
  30. ^ "AAIB investigation to Boeing 737-8AS, EI-GJT".

Further reading