Air data inertial reference unit

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). 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. Air data reference The air data reference (ADR) component of an ADIRU provides airspeed, Mach number, angle of attack, temperature and barometric altitude data. 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. 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. ==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. 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. Alitalia A320 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. Malaysia Airlines 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. In that incident the incorrect data impacted all planes of movement while the aircraft was climbing through . The aircraft pitched up and climbed to around , 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. 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. Qantas Flight 68 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 and estimated position north of Learmonth, Western Australia, The ATSB has yet to confirm if this event is related to the other Airbus A330 ADIRU occurrences. ===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. Qantas Flight 71 On 27 December 2008, Qantas Flight 71 from Perth to Singapore, a different Qantas A330-300 with registration VH-QPG was involved in an incident at 36,000 feet approximately north-west of Perth and south of Learmonth Airport at 1729 WST. The autopilot disconnected and the crew received an alert indicating a problem with ADIRU Number 1. 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. While examining possibly related events of weather-related loss of ADIRS, the NTSB decided to investigate two similar cases on cruising A330s. On a 21 May 2009 Miami–São 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. Ryanair Flight 6606 On 9 October 2018, the Boeing 737-800 with registration EI-GJT, 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, 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==