Electronic Flight Instrument System (EFIS)

An electronic flight instrument system (EFIS) draws the flight instruments on screens. Instead of a separate electromechanical instrument for each quantity, display units show pictures composed by computers from the aircraft's sensors: attitude, airspeed, altitude and flight director commands on the primary flight display, and heading, track, route and radio navigation on the navigation display. A flight deck built this way is called a glass cockpit.

EFIS shows more, and more combined, information than the instruments it replaced, so its symbols and colours have to be learned. And because each picture depends on a chain of sensors, computers and screens, the pilot must know what each failure removes, how it is flagged and which switches restore the picture. The layout of the two main displays is covered in primary flight display and navigation displays and indicators.

Glass cockpit overview

Early airline EFIS installations replaced only two instruments on each side. The attitude director indicator became the electronic attitude director indicator (EADI) and the horizontal situation indicator the electronic horizontal situation indicator (EHSI), while airspeed, altitude and engine indications stayed on dials. Later designs merged the EADI with speed and altitude tapes into the primary flight display (PFD) and developed the EHSI into the navigation display (ND). Engine and system information moved to screens of their own: EICAS on Boeing aircraft, ECAM on Airbus aircraft (see flight warning and alerting systems).

A current airliner panel typically carries six identical display units. The A320's are full-colour liquid crystal displays: a PFD and an ND for each pilot, and in the centre the ECAM engine/warning display above the system display. The 737 NG also has six: each pilot's PFD outboard and ND inboard, with an upper and a lower display in the centre for the engines. The computers behind the screens sit in the avionics bay below the flight deck, which keeps their heat, weight and electromagnetic interference away from the panel.

Light aircraft followed with integrated suites such as the Garmin G1000, whose PFD combines attitude, air data and an HSI, and whose multi-function display (MFD) beside it shows a moving map, engine instruments and other pages. In every installation the screens are only the last link: attitude and heading come from an attitude and heading reference system (AHRS) or an inertial reference system, and speed and altitude from the air data computer.

Symbol generators and display units

The classic EFIS described in ATPL instrumentation texts has, for each pilot, an EADI, an EHSI, a control panel, a symbol generator (SG) and a remote light sensor. A third symbol generator in the centre can be switched to either side if one fails, which gives redundancy without duplicating the whole system.

The symbol generator is the heart of EFIS. It provides the analogue, discrete and digital interfaces between the data sources, the displays and the control panel; it generates the symbols, scales and text; it monitors the system for failures and controls power. Its inputs include the inertial reference system, air data computer, flight management computer, VOR, DME, ILS and ADF receivers, TCAS and the weather radar. The display unit (DU) only renders the picture it is sent.

Modern systems keep the principle under other names. The A320 has three identical display management computers (DMCs); each can drive one PFD, one ND and both ECAM screens at the same time, and DMC 3 is the spare that the crew can switch in for DMC 1 or DMC 2. The 737 NG has two display electronic units (DEUs): DEU 1 drives the captain's outboard and inboard units and the upper centre unit, DEU 2 the first officer's units and the lower centre unit, and either can drive all six if the other fails.

CRT and LCD technology

Early display units were cathode ray tubes (CRT), in which an electron beam scans a phosphor screen from the back of a deep glass tube: bulky, heavy and hot. Liquid crystal displays (LCD) replaced them. An LCD is thinner and lighter, generates less heat and so needs less cooling, and is more reliable under vibration and at extreme temperatures. Modern transports use LCDs almost exclusively, and the PFD and ND units are usually identical, which simplifies spares and lets one unit take over another's picture.

A display must stay readable from direct sunlight to a dark night. The remote light sensor, a photodiode on the glareshield, measures the ambient light and adjusts screen brightness automatically, while the pilot's brightness knob remains available to trim it. On the A320 the automatic adjustment for ambient light is superimposed on the manual brightness setting of the ECAM screens, and the integrated standby instrument sets its own brightness from a photosensitive cell, which the crew can then trim with its + and − keys.

EFIS control panel

Each pilot has an EFIS control panel that decides what that pilot's displays show. On the A320 the two panels form the outer parts of the flight control unit on the glareshield, either side of the autoflight panel; on the Boeing 737 they sit on the glareshield either side of the mode control panel. Typical controls are:

• the ND mode and range: MAP, VOR, APP and PLAN on the 737 NG, ROSE, ARC and PLAN on the A320;
• map overlays such as weather radar, terrain, airports, waypoints, navaids and route data;
• the source of each ND bearing pointer, VOR or ADF;
• the barometric reference, with STD selected by a push or pull, in hectopascals or inches of mercury;
• the minimums reference, radio or barometric;
• on some types the flight path vector or a metric altitude readout.

The two panels are independent, so one pilot can keep the map while the other shows raw radio data in a rose mode. Airbus procedures use this: on an approach without GPS PRIMARY, if the navigation accuracy check shows LOW accuracy, at least one ND must be in ROSE LS or ROSE VOR mode. If a panel fails, the 737 NG's CONTROL PANEL switch lets both sides run from the serviceable one (BOTH ON 1 or BOTH ON 2).

Colour coding conventions

Colour tells the pilot what kind of information a symbol carries. CS-25 guidance and the ATPL texts give a common scheme, and EFIS colour coding follows it with some differences between manufacturers.

Engine indication colour coding keeps the older convention of conventional gauges: green for the normal operating range, yellow or amber for a caution range, red for a limit. The Airbus ECAM uses red for a configuration or failure that needs immediate action, amber for one the crew must be aware of without immediate action, green for normal operation, white for titles and remarks, blue for actions to be carried out and for limitations, and magenta for particular messages.

View full schematic

Multi-function and synoptic displays

A multi-function display is a screen whose content the crew chooses. In a light-aircraft suite it normally shows a moving map with an engine strip and pages such as flight plan, weather and terrain; in airliners the engine and system displays fill the role.

A synoptic display draws an aircraft system as a diagram: pumps, valves, lines and tanks, with quantities and states coloured by condition. On Boeing aircraft EICAS shows the primary engine parameters and the warning and caution messages on the upper display, and the secondary engine parameters and system status on the lower one. On Airbus aircraft the lower ECAM screen, the system display, shows one of the synoptic display pages or the status page. A system synoptic display page appears automatically for the flight phase, when a failure affects that system, or as an advisory when a parameter drifts out of its normal range, and the crew can call any page on the ECAM control panel. The flight control page, for example, shows the surface positions that older aeroplanes showed on mechanical indicators. On the A310's ECAM the two screens stood side by side, the left for warnings and checklists and the right for the synoptic diagrams.

Source selection and comparators

Each pilot's displays normally take their data from on-side sources, the captain's from air data and inertial unit 1, the first officer's from unit 2. Data source selection switches let a pilot connect that side's displays to an alternate source after a failure: the other air data computer, the other or a standby inertial unit, the other flight management computer or flight control computer, or the spare display computer. On the A320 the ATT HDG and AIR DATA selectors switch either side to ADIRU 3, and the EIS DMC selector switches it to DMC 3. When both sides use the same source, the independence of the cross-check is lost, and the displays say so: on the 737 NG an amber INSTR SWITCH annunciation appears when both pilots' displays use the same IRU.

Instrument comparison monitoring is the display system's automatic cross-check of the two sides. With two sources a comparison monitor can detect a disagreement but not decide which side is wrong; the crew settles it with the standby instruments or a third source. With three sources, as in a triple inertial installation, a voting system identifies the odd one out. Typical thresholds:

• 737 NG: PITCH or ROLL when the two attitude displays differ by more than 5°; IAS DISAGREE above 5 kt for 5 seconds; ALT DISAGREE above 200 ft for 5 seconds.
• A320: CHECK ATT above 5° in pitch or roll; CHECK HDG above 5°; CHECK ALT above 250 ft with QNH set, or 500 ft with STD.

The A320's display computers also check that each screen actually shows what they sent. A discrepancy gives an amber message such as CHECK CAPT PFD.

Failure flags and display reversion

An EFIS removes a parameter it cannot trust rather than show it wrong. Classic installations paint an amber instrument failure flag at the place of the lost data, a flag in the speed area when airspeed fails, and add text messages such as WXR/MAP RANGE DISAGREE when two inputs are inconsistent. Light-aircraft glass cockpits cover the affected indication with a red X (invalid data flag). The red X is not a caution about accuracy: the system has judged the data invalid and removed it. The pattern of what has gone identifies the failed unit. Attitude and heading lost with speed, altitude and vertical speed intact points to the AHRS; the three air data tapes lost with attitude intact points to the air data computer.

EFIS display reversion moves a picture to a working screen when a display unit or display computer fails:

• A320: a failed PFD unit's image transfers automatically to the ND unit beside it, and the pilot can make the same swap with the PFD/ND XFR pushbutton or by turning the PFD brightness knob to OFF. A failed DMC leaves INVALID DATA on its screens until the crew selects DMC 3. If the lower ECAM screen fails, the ECAM/ND XFR selector puts the system pages on an ND.
• 737 NG: if an outboard unit fails, the PFD appears automatically on the inboard unit; if the upper centre unit fails, the primary engine display moves automatically to the lower one. There is no automatic switching for a lower unit failure, and selectors allow manual changes.
• Light aircraft: the reversionary mode, selected with a display backup button or automatically, puts the PFD with a compressed engine strip and map on the MFD.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.