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Air data inertial reference unit

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The Air Data Inertial Reference Unit (ADIRU) is a redundant aircraft component.[1] It is a part of the integrated Air Data Inertial Reference System (ADIRS) which consists of up to three ADIRUs located in the aircraft electronic rack, an associated Control and Display Unit (CDU) in the cockpit and remotely mounted Air Data Modules (ADMs).[2] The ADIRS system supplies air data (air speed and altitude) and inertial reference (position and attitude) information to the pilots' Electronic Flight Instrument System displays and other user systems on the aircraft, including, but not limited to, the engines, autopilot, flight control and landing gear systems.[3] An ADIRU acts as a "single, fault tolerant" source for both pilots of an aircraft.[4] This device is used on the new Airbus aircraft.[5]

The No 3 ADIRU is a standby unit which 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 ADR (Air Data Reference) 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.[3]

Working principle

Each ADIRU is divided into two parts, either of which can operate independently in case of a failure of the other. The Air Data Reference (ADR) part provides airspeed, angle of attack, temperature and barometric altitude data. The Inertial Reference (IR) part gives attitude, flight path vector, ground speed and positional data.[3]

Air Data Reference

The pitot and static pressures are measured by small Air Data Modules (ADM) located as close as possible to the respective pressure sources (pitot and static probes). The ADMs transmit their pressures to the ADIRUs through ARINC 429 data buses.[5]

Failures and directives

  • May 3, 2000 - Airworthiness directive AD2000-07-27 was issued, addressing dual critical failures during flight, attributed to shocks simultaneously affecting all the ADIRU ring laser gyros.[4]
  • January 27, 2004 - Airworthiness directive AD2003-26-03 called for modification to the mounting of ADIRU3 to prevent failure and loss of critical attitude and airspeed data.[4]
  • 25 June 2005 - An 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 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.[6]
  • 1 August 2005 - Malaysia Airlines Flight 124, a Boeing 777-2H6ER departed Perth, Western Australia for Kuala Lumpur. Climbing through 38,000 feet a faulty accelerometer caused the aircraft's ADIRU and autopilot to command changes of altitude. The flight crew overrode the system and manually returned to land the aircraft at Perth. The subsequent ATSB investigation led the US FAA to issue emergency airworthiness directive 2005-18-51 on the fly-by-wire software. [7]

References

  1. ^ "ADIRU". The Free Dictionary. 2005. Retrieved 2006-09-16.
  2. ^ "738-3 Air Data and Inertial Reference System (ADIRS)". ARINC. 2008. Retrieved 2008-07-14.
  3. ^ a b c "The intricate complexity within an immaculate redundancy concern". Air Safety Week. August 14, 2006. Retrieved 2008-07-16.
  4. ^ a b c "Safety concern". Air Safety Week. May 5, 2005. Retrieved 2006-09-16.
  5. ^ a b "Honeywell's ADIRU selected by Airbus". Aviation International News. 2002. Retrieved 2008-07-16. Cite error: The named reference "art3" was defined multiple times with different content (see the help page).
  6. ^ "AAIB Bulletin: 6/2006" (PDF). Air Accidents Investigation Branch.
  7. ^ "Incident 20050801-1". Aviation Safety Network. Retrieved 14 October 2008.
 

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