10.2 Electronic Flight Instrument System Architecture

10.2 Electronic Flight Instrument System Architecture

Current Appendix I heading 5.15, Typical electronic/digital aircraft systems, is level 1 for every licence category, and heading 5.1, Electronic instrument systems, is likewise level 1. EFIS, general arrangement, associated BITE, systems arrangements, and cockpit layout come from the pre-12 June 2024 detailed descriptions and are retained here as historical study scope. This section therefore describes what an Electronic Flight Instrument System is, which boxes generate the picture, how a two-crew flightdeck is usually organised, and how the crew selects data sources. No single airframe is treated as the official layout; the teaching model is the arrangement that repeats across transport and large turboprop types.

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What EFIS Replaced

On a classic electromechanical panel the pilot’s primary attitude reference was an Attitude Director Indicator (ADI) driven by a vertical gyro or inertial unit, and the primary navigation compass was a Horizontal Situation Indicator (HSI) driven by a directional gyro, flux valve or inertial unit, with VOR or ILS deviation superimposed. EFIS replaces those two mechanical instruments with electronic display units (DUs) that paint the same information as computer-generated symbology, usually in colour, and that can overlay weather radar, terrain, traffic and flight-plan maps.

The two primary EFIS formats are:

• Primary Flight Display (PFD), also called the Electronic ADI (EADI) on older systems. Attitude (blue-sky / brown-ground sphere, pitch ladder, roll scale) occupies the centre. Airspeed, altitude and vertical speed appear as vertical tapes or, on some regional types, as round-dial images. A flight-mode annunciator (FMA) across the top shows autothrust and autopilot or flight-director modes. Flight-director command bars or a flight-path vector sit on the attitude sphere. Approach deviation (localiser and glideslope, or FMS lateral and vertical deviation) appears as scales or diamonds.
• Navigation Display (ND), also called the Electronic HSI (EHSI). A heading-up or track-up compass rose or arc, selected heading and track bugs, VOR/ADF bearing pointers, ILS course, map range rings, the FMS active route, navaids, airports, weather-radar returns, TCAS traffic and, on later systems, terrain. Crew-selected modes typically include MAP or ARC, VOR, ILS or APP, PLAN (north-up plan view for procedure review) and sometimes a centred ROSE.

EFIS does not remove the requirement for independent standby attitude, airspeed and altitude instruments. Those remain as a reversionary path if both electronic channels are lost. Engine and aircraft-system alerting (EICAS on many Boeing types, ECAM on Airbus types) occupy the centre display units; they are neighbouring electronic instrument systems, not the PFD/ND pair itself, and are treated in their own Module 05 sections.

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Functional Architecture: From Sensor to Pixel

A typical EFIS chain has four layers.

1. Sensors and source systems. Air-data computers (ADC) or air-data reference units (ADR); inertial reference systems, AHRS or ADIRUs; ILS, VOR, DME and ADF receivers; radio altimeter; FMS or flight-management guidance computers; weather radar; TCAS/ACAS; terrain-awareness computers. These output digital words (commonly ARINC 429) and, for some sensors, analogue voltages or dedicated radar video (historically ARINC 708 / 453 weather-radar buses).

2. Display processing / symbol generation. Named symbol generators (SG), display processing computers, EFIS computers, or, on Airbus Electronic Instrument System architecture, Display Management Computers (DMC). Each computer concentrates the selected sources, runs validity and comparator logic, generates the PFD and ND picture (stroke, raster, or a rasterised command list) and outputs a video, LVDS or similar drive to the DUs. This is also where BITE lives: power-up and cyclic tests of the processor, memory, power supply, DU link and often the control panel, with faults written to a maintenance memory or reported on a central maintenance computer (CMC) or centralised fault display system (CFDS).

3. Display units. The CRT or AMLCD heads on the panel. A DU is largely a monitor: backlight or EHT, heater, power supply and a display-drive card. It does not usually contain the navigation database or the air-data computers. On many types the DUs in the PFD and ND positions share a part number, so a failed head can be swapped with a lower-priority screen in accordance with the AMM.

4. Controls and source select. EFIS control panels set ND range (typical steps such as 10, 20, 40, 80, 160, 320 NM), mode, and overlays (weather, traffic, terrain, airports, waypoints, navaid data). Source-select or reversion switches labelled AIR DATA, ATT/HDG (or IRS), NAV and FMS tell the symbol generator which data bus to use, not which DU to illuminate. A PFD/ND transfer push-button, where fitted, swaps the two formats on that pilot’s pair of screens.

Independence is the safety idea behind the two-crew layout. The captain’s PFD and ND are driven by a left-side computer and left-side sensors; the first officer’s displays are driven by a right-side computer and right-side sensors. A comparator on attitude, heading, airspeed and altitude warns if the two sides disagree. A third computer or spare DMC is often fitted so that a failed processor can be replaced by switch logic without swapping boxes in the air.

[!NOTE] Display computer versus display unit. Replacing a DU because the attitude sphere is missing will not help if the symbol generator has lost its inertial source or has failed BITE. Ask whether both formats on one side are lost (computer, power or a shared source) or only one head is blank (that DU, its connector, cooling, or that head’s video output).

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Typical Two-Crew Cockpit Layout

A transport-category electronic instrument panel is organised around each pilot’s primary field of view and the centre engine/system panel.

Not every type uses six identical DUs — some regional aircraft use four — but the arrangement principle is the same: each pilot has a dedicated attitude display and a dedicated navigation display, and engine/system information is shared on the centreline where both can see it. The glareshield flight-control unit / mode-control panel / flight-control unit sits above the DUs; it is not itself an EFIS DU, but the modes it engages are written on the PFD FMA. Clocks, standby instruments and landing-gear indications remain outside the EFIS cluster.

Category A familiarisation stops at which screen is which and the fact that the two sides are independent. B1/B2 candidates should also be able to name the LRU classes (DU, SG/DMC, EFIS control panel, source-select panel) and state that BITE in the display computer is the first interrogation point when a DU shows an invalid-display caption or a blank raster with a status message.

Cooling air at each DU is part of the arrangement. An AMLCD that overheats may dim, show a temperature caption, or shut its backlight down; a CRT that loses cooling will drift in focus and colour purity. A display fault after a pack or avionics-fan failure is therefore not automatically a failed symbol generator.

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Source Selection, Comparators and BITE

Source selection is part of 5.1 typical arrangements. If ADC 1 fails, the captain’s EFIS computer can be switched to ADC 2 (or ADR 3 on an ADIRU aircraft). If IR 2 fails, the first officer may use IR 1 or the spare IR. Switching is electrical selection of a data bus, not a mechanical link inside the DU. The PFD flags (IAS, ALT, ATT, HDG, LOC, G/S) appear when the selected source is invalid or when the comparator trips.

Associated BITE typically covers:

• Internal computer memory, processor and power-supply monitors.
• DU communication (lost synchronisation, overtemperature, backlight or EHT current out of range).
• Control-panel interface (stuck range knob, open mode selector).
• Input-bus activity (no activity on the selected ARINC 429 label set).

A fault may be reported as a status message on the engine display, as a local EFIS maintenance page, or as a CMC/CFDS failure message identifying the LRU. Level 1 knowledge is the arrangement: EFIS has its own computers and displays, they are dual, they take selectable sources, and they report faults through BITE — not pin-level troubleshooting of a particular aircraft.

Worked arrangement example. A two-crew EFIS with three display computers is powered. Computer 1 drives the captain PFD and ND; computer 2 drives the first-officer PFD and ND; computer 3 is spare. Weather-radar video is a shared sensor but is selected independently on each EFIS control panel, so the captain can display weather on the ND while the first officer displays terrain. If computer 1 fails BITE in flight, a DISPLAY SWITCHING or similar selector lets computer 3 (or computer 2, depending on type) drive the captain’s DUs. Both pilots may then share processing capacity — a degraded but flyable state — and a caution tells the crew that independence is reduced. The DUs themselves have not been swapped; only the symbol generator assignment has changed.

Range and mode selections never travel through the standby instruments. If both EFIS channels are lost, the crew flies the standby attitude indicator and the standby airspeed and altitude instruments, not a compact PFD that no longer exists. That is why the cockpit layout still reserves a place for those standbys even on a fully electronic panel.