Air data inertial reference unit

Air data inertial reference unit (ADIRU)✈

In an Airbus A-320 family aircraft 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, flight control, and landing gear systems. An ADIRU acts as a single fault-tolerant source of navigational data for both pilots of an aircraft. It may be complemented by a secondary attitude air data reference unit (SAARU), as in the Boeing 777 design.

This device is used on various military aircraft as well as civilian airliners starting with the Airbus A320 and Boeing 777.

Description

An ADIRS consists of up to three fault-tolerant ADIRUs located in the aircraft electronic rack, 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.

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

Air data reference

See also: Pitot-static system

The ADR component of an ADIRU provides airspeed, Mach number, angle of attack, temperature, and barometric altitude data. 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. The ADMs transmit their pressures to the ADIRUs through ARINC 429 data buses.

Inertial reference

The IR component of an ADIRU gives attitude, flight path vector, ground speed, and positional data. 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. 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 ADIRU's can lead to counter-intuitive behavior 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 the redundancy of aircraft systems and 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.