16.2 Built-In Test Equipment and Central Maintenance Computers
Key Takeaways
- The former detailed 5.15 description paired associated BITE with its system examples; BITE is the system's own test and fault-memory function, not a separate aircraft system beside FBW or ACARS.
- Three BITE modes are standard teaching: power-up at energisation, continuous monitoring in operation, and initiated tests started by a technician or by a central maintenance function.
- Typical central names are the Boeing Central Maintenance Computer (CMC) and the Airbus Centralised Fault Display System (CFDS) or later Central Maintenance System (CMS).
- Class 1 / 2 / 3 (or similar) fault reporting is typical manufacturer teaching for cockpit effect and maintenance urgency; it is not a single official EASA regulatory fault-class table.
- Initiated BITE on the ground reads stored continuous-BITE history, correlates it with an LRU, and runs AMM tests — it does not replace wiring inspection or configuration control.
16.2 Built-In Test Equipment and Central Maintenance Computers
Current Appendix I gives only the broad 5.15 heading and level. The pre-12 June 2024 detailed description asked for the general arrangement and associated BITE of its named typical electronic/digital aircraft systems. Built-In Test Equipment (BITE) is therefore not a separate aircraft-system example sitting beside FBW or ACARS; it is the self-test, monitoring and fault-memory function inside those systems, collected for the technician by a central maintenance computer. Knowledge level 1 is familiarisation: you must recognise the three usual BITE modes, the typical central names, and how fault reporting is taught — without treating any manufacturer class table as if it were a single EASA regulation.
What BITE actually does
A modern LRU contains processors, memory, input/output and often a dedicated test program. BITE uses those resources to ask, in effect, three questions: did I start correctly, am I healthy now, and can a technician command a more thorough test on the ground? The answers are stored as fault words, class or level codes (manufacturer-specific), timestamps or flight legs, and sometimes freeze-frame analogue values. Those data travel on the same avionics buses already met in Module 5 (ARINC 429 on many older fleets, AFDX/ARINC 664 on later networks) to a central maintenance function, and they may also drive an operational cockpit effect on ECAM or EICAS.
BITE is not a substitute for the Aircraft Maintenance Manual (AMM), the Troubleshooting Manual (TSM) or the Minimum Equipment List (MEL). A green BITE pass does not prove that a connector is dry, that a hydraulic actuator is mechanically free, or that the loaded software part number is the approved one. A stored fault does not, by itself, authorise an LRU swap without the isolation steps the AMM specifies.
Three BITE modes (standard teaching)
Power-up BITE (often called PBIT) runs when the equipment is energised — typically at aircraft electrical power-up, after a computer reset, or when an LRU is plugged in and powered on the bench. It checks memory, processors, power-supply monitors and often the presence of required bus inputs. Power-up tests can last from a few seconds to several minutes; interrupting them by pulling the breaker early can leave a spurious fault. Some power-up tests are inhibited in flight so that a reset in the air does not command actuator cycling or radio transmissions.
Continuous BITE (often called CBIT) monitors the system while it is operating. It watches internal watchdog timers, sensor-range and reasonableness, actuator follow-up errors, bus activity and cross-talk with the opposite channel. Continuous BITE is how an ELAC, an ACARS management unit or a cabin director records a fault during cruise that the crew may or may not see as a cockpit message. The memory typically holds a rolling history of current (still present) and intermittent (occurred, then cleared) faults, often tagged by flight leg.
Initiated BITE (often called IBIT) is started on purpose, usually on the ground, from a central maintenance page, an MCDU/CFDS screen, a BITE connector, or a dedicated test switch as the type provides. Initiated tests can be more thorough than power-up: they may cycle actuators, light every segment of a display, key a VHF transceiver into a test loop, or poll every cabin decoder. They must be run only with the precautions in the AMM (doors, hydraulics, radio silence, passengers off the aircraft as required). Initiated BITE is a technician tool, not a crew in-flight drill.
[!NOTE] The three mode names are classroom teaching used across many types. Exact menus, inhibit logic and test duration are aeroplane-specific. Do not treat a Boeing CMC screen and an Airbus CFDS screen as identical.
Central maintenance computers — typical names
The local BITE in each system would be unusable if the technician had to open every equipment bay to read it. Transport types therefore concentrate fault data in a central function:
- Central Maintenance Computer (CMC) is the usual Boeing teaching name. Dual CMCs are common. Access is through a control-display unit, a maintenance access terminal, or dedicated CMC pages.
- Centralised Fault Display System (CFDS) is the usual Airbus teaching name on earlier fly-by-wire types. The crew and the technician may use the MCDU to read POST, last-leg and previous-leg reports, and to launch system tests.
- Central Maintenance System (CMS) is the later Airbus teaching name as the architecture became more networked. The job is the same: collect BITE, present it, and command initiated tests.
These are manufacturer names, not three official EASA LRU titles. A question that asks for "the EASA-standard Class-1 Fault Computer" is inventing a box that Appendix I does not define. For familiarisation study, recognise that typical systems report associated BITE to a central maintenance computer and that the technician reads that computer rather than guessing from a single cockpit memo.
Data flow is straightforward at familiarisation level. Each system LRU runs local BITE and sends fault and status words to the CMC/CFDS/CMS. The central function correlates messages (so that one failed bus does not appear as twenty unrelated LRU deaths), stores them by flight, and presents them on a maintenance display. Correlation is why a single sensor failure can produce both an operational ECAM/EICAS caution and a shop-level BITE identity for the acquisition computer that lost the input.
Fault reporting classes — typical teaching, not an EASA table
Manufacturers group BITE results so that the operator can see the difference between "this affects the next flight" and "log it for the night stop". Airbus-style classroom teaching often uses Class 1, Class 2 and Class 3 (or equivalent numbered levels). Boeing-style teaching often speaks of flight-deck effect versus maintenance message only, and of status versus advisory versus warning as the crew already saw on EICAS.
A typical Airbus-style teaching split, used here only as classroom language, is:
| Typical class (teaching) | Cockpit effect (typical) | Maintenance meaning (typical) |
|---|---|---|
| Class 1 | Operational warning or caution; dispatch may be affected | Failure with safety or operational consequence; isolate and rectify or apply the MEL before further flight as the documentation requires |
| Class 2 | Often a status or deferred item; may be time-limited | Failure recorded for scheduled rectification; not usually an immediate no-go by itself |
| Class 3 | Frequently no cockpit effect | Shop or trend item; recorded in BITE memory for maintenance, not an operational alert |
[!WARNING] Do not treat Class 1 / 2 / 3 as a single official EASA regulatory fault-class table. Appendix I does not publish those numbers as law. Exact names, colours and MEL interactions are type-specific. The MEL and the AMM remain the dispatch and isolation documents; BITE class is a manufacturer reporting aid.
The same caution applies to any similar three-level scheme you meet in a type course. Learn the idea: some faults have a flight-deck effect and operational urgency, some are deferred maintenance, and some are recorded only in the central computer. Do not memorise a universal EASA class-to-colour chart that does not exist.
Using BITE without creating no-fault-found
A familiarisation-level troubleshooting pattern is:
- Read the operational effect (ECAM/EICAS, status, crew report) and the maintenance effect (CMC/CFDS/CMS current and last-leg faults).
- Note whether the BITE mode that captured the fault was continuous (in service) or power-up (at energisation). Intermittent continuous faults that are not present on the ground are a classic no-fault-found trap if the LRU is swapped blindly.
- Apply the AMM/TSM isolation, which may include an initiated test after ground power, hydraulics or radios are set up safely.
- After an LRU replacement, run the required initiated or power-up test, confirm software identity where Chapter 12 applies, and check that the central computer no longer shows the fault as current.
Nuisance and correlated messages matter. A bus-off or a single air-data disagreement can light several systems at once. Replacing the first LRU named on the screen, without reading the correlated list, is how serviceable computers are removed. Continuous BITE that sets only in icing, vibration or a particular electrical transient may never repeat on jacks in a warm hangar; the initiated test then passes, and the unit returns from the shop as no-fault-found.
Power-up BITE that fails only when a particular bus is missing points at power or interface, not at a dead processor. Initiated BITE that fails a cabin decoder only when that decoder is disconnected is a wiring or addressing problem. Associated BITE is powerful when it is read as evidence, not as an automatic parts list.
Level 1 does not require you to navigate every CMC submenu from memory. For study, know that typical systems carry associated BITE, that the three modes exist, that CMC/CFDS/CMS are usual central names, and that class-style reporting is manufacturer teaching rather than an EASA-issued table.
Which statement correctly distinguishes power-up, continuous and initiated BITE?
Which naming of the central maintenance function is correct as typical manufacturer teaching?
How should Class 1, Class 2 and Class 3 (or similar) fault reporting be treated when studying Module 5?
A continuous-BITE maintenance message was stored in flight with no cockpit effect. What is the usual familiarisation-level use of initiated BITE on the ground?