Chapter 12: Digital Aircraft Systems I — BITE, ACARS, FMS, IRS and Fly-by-Wire

Built-in test equipment (BITE)

Every digital LRU contains built-in test equipment (BITE) — hardware and software that check the unit and its interfaces, detect faults, isolate them to an LRU (or a wire), and store them in non-volatile memory.

• Power-up BIT: a full self-test when power is applied — memory checksums, processor, inputs and outputs.
• Continuous BIT: monitoring in the background during operation — watchdog timers, input reasonableness, comparisons between channels, data-bus activity.
• Initiated BIT: a test started by the technician from the MCDU, a maintenance terminal or a test button, often only allowed on the ground with certain conditions met (engines off, hydraulics pressurised, and so on). The AMM uses it for fault confirmation and as the operational test after replacing a part.
• Faults are stored with the flight leg, date and time, and an ATA reference or fault code. Many are 'intermittent' — logged in flight but not present on the ground — and the history helps decide whether to act.

• A central maintenance computer gathers the BITE results from all systems: the CFDS with its CFDIU on the Airbus A320, the CMS with central maintenance computers on the A330/A340, the onboard maintenance system (OMS) on the A380 and A350, the CMC on the Boeing 747-400 and the central maintenance computing function (CMCF) on the 777 and 787.
• It correlates faults with the flight-deck effects (ECAM or EICAS messages) the crew saw, produces a post-flight report, gives access to each system's history and tests, and sends fault messages to the ground in flight via ACARS, so spares and staff can be ready on arrival.
• An aircraft condition monitoring system (ACMS) records engine and system parameters for trend monitoring and reports them by ACARS or quick-access recorder.

ACARS

ACARS — Aircraft Communications Addressing and Reporting System (CAR-66 calls it the ARINC Communication Addressing and Reporting System) — is a digital data link for short text messages between aircraft and ground, in use since the late 1970s.

• What it sends: OOOI times (Out of the gate, Off the ground, On the ground, In at the gate — triggered automatically by door, parking-brake and air/ground sensors); flight plans, weather, load sheets and free text; engine reports and maintenance fault messages; and, on FANS aircraft, air traffic services such as CPDLC controller-pilot messages, oceanic and departure clearances and digital ATIS.
• Airborne units: a management unit (MU), communications management unit (CMU) or Airbus air traffic services unit (ATSU); the MCDU as the crew interface; a cockpit printer; and the radios — normally one VHF transceiver in data mode (often VHF 3), HF data link, and SATCOM for oceanic and polar areas.
• Ground side: networks of VHF and HF ground stations and satellite earth stations operated by data-link service providers (ARINC — now part of Collins Aerospace — and SITA), routing messages to the airline's host computers and to ATC.
• Original VHF ACARS sends about 2.4 kbps; VDL Mode 2 about 31.5 kbps.
• The MU's message formats and routing tables are often user-modifiable software controlled by the airline (Chapter 10).

Flight management system (FMS)

• Components: one or two flight management computers (FMC) and two or three MCDUs (control and display units). Twin FMCs normally run in a dual mode, cross-checking each other; either can operate alone.
• Databases: a navigation database of airports, runways, navaids, waypoints, airways and procedures, updated every 28-day AIRAC cycle; and a performance database for the airframe and engines.
• Navigation: computes the best estimate of position from the IRS, GNSS and radio aids (DME/DME, VOR/DME), auto-tuning the navigation radios, and shows its accuracy (actual navigation performance against the required navigation performance, RNP).
• Flight planning: the route from origin to destination, with SIDs, airways, STARs and approaches, built from the database and entered or uplinked.
• Guidance: lateral (LNAV) and vertical (VNAV) commands to the autopilot and flight director, and speed or thrust targets to the autothrottle — 'managed' modes on Airbus.
• Performance: optimum speeds (by cost index), cruise altitude, step climbs, top of descent, fuel and time predictions.

Inertial reference system (IRS)

• An IRS is self-contained: it needs no outside signals. Modern units are strapdown — three gyros (normally ring laser gyros, or fibre-optic gyros) and three accelerometers fixed to the case along three axes. Their computer keeps track of the unit's attitude mathematically and integrates the accelerations, once to get velocity and again to get position.
• Ring laser gyro: two laser beams travel in opposite directions round a closed triangular path in a glass block. When the block rotates, one beam's path becomes effectively longer, so the two frequencies differ (the Sagnac effect); the fringe pattern counts the rotation. A small dither vibration stops the beams locking together at low rotation rates. There are no spinning parts.
• Outputs: pitch and roll attitude, true and magnetic heading, body rates and accelerations, ground speed, track, vertical speed, present position, and — with air data — wind. They feed the displays, FMS, autopilot, flight controls, weather radar stabilisation and more. Air data and inertial functions are often combined in an ADIRU.
• Alignment: on the ground, with the aircraft stationary, the IRU measures gravity to find the vertical and the Earth's rotation to find true north (gyrocompassing). It takes about 10 minutes (longer at high latitudes). The crew enter the present position, which the IRU checks. Moving the aircraft during alignment spoils it.
• Modes: OFF, ALIGN, NAV (normal) and ATT (attitude-only reversion, used if navigation alignment is lost in flight; heading must then be entered manually). An ON BAT light shows the unit is running on its battery back-up.
• Position drift is typically within about 2 NM per hour; GNSS updating (hybrid GPS/IRS) removes it.

Fly-by-wire (FBW)

• In a fly-by-wire system the pilot's controls are not mechanically connected to the control surfaces. Transducers (RVDTs or LVDTs) measure the sidestick or column and pedal positions, and flight control computers command the surface actuators electrically.
• Examples: the Airbus A320 family (ELAC, SEC and FAC computers), A330/A340 and A350/A380 (PRIM and SEC computers), the Boeing 777 and 787 (primary flight computers and actuator control electronics), and most new business jets.
• Redundancy and dissimilarity: several computers, each with command and monitor channels, different processor types and independently written software, powered from different electrical and hydraulic sources, with segregated wiring — so that no single failure or common design error can cause loss of control.
• Control laws shape the aircraft's response. In normal law the computers provide envelope protection — limits on angle of attack, load factor, pitch attitude, bank angle and overspeed. After certain failures the system reverts to alternate law (reduced protection) and then direct law, where stick deflection maps straight to surface deflection. Some types keep a limited mechanical backup.
• Benefits: lower weight and fewer mechanical parts, envelope protection, consistent handling across a family of aircraft, load alleviation, and easier maintenance — BITE and computer-based rigging instead of cable tensions.
• Maintenance: initiated tests through the central maintenance system, transducer and actuator checks, strict software configuration control, and particular care with bonding, shielding and HIRF protection.