From the basic T to the glass cockpit
For most of aviation history the flight instruments were separate electromechanical dials. Since the 1930s they have been arranged in the basic T: airspeed top left, attitude top centre, altitude top right and heading bottom centre. A turn coordinator (or turn-and-slip) and a vertical speed indicator are added beside them. Radio navigation added a radio magnetic indicator (RMI) and a course deviation indicator or horizontal situation indicator (HSI). On transport aircraft the engines and systems needed dozens more gauges, and a flight engineer to watch them.
From the early 1980s, digital computers and colour cathode-ray tubes allowed these dials to be replaced by pictures drawn on screens — the electronic instrument system (EIS), or glass cockpit. The first generation drew an electronic ADI (EADI) and electronic HSI (EHSI) that mimicked the old instruments, to ease pilot conversion. The second generation combined everything the pilot flies by onto a primary flight display (PFD) and everything used to navigate onto a navigation display (ND). Flat-panel liquid crystal displays (Chapter 8) then replaced the heavy, hot CRTs.
- Advantages: fewer moving parts and much higher reliability, less weight, panel space and wiring; many data sources combined on one screen; colour and moving tapes give trends at a glance; alerts are prioritised; and display formats can be changed by software.
- Integration: each screen can show different formats, so a failed screen's picture can be moved to another — something a failed dial could never do.
- Glass cockpits range from one or two screens in a light aircraft to six screens on a Boeing 777, eight on an Airbus A380 and five very large screens on a Boeing 787. The display units are normally identical and interchangeable, which reduces the spares holding.
Cockpit layout
Most transport aircraft follow the same arrangement. Each pilot has a PFD outboard and an ND inboard, so the first officer's side is a mirror image of the captain's. The centre displays serve both pilots: the upper one shows primary engine indications and crew alert messages, and the lower one shows secondary engine data, system synoptic pages and maintenance pages.
| Position | Boeing | Airbus |
|---|---|---|
| Outboard, each pilot | PFD | PFD |
| Inboard, each pilot | ND | ND |
| Upper centre | EICAS — primary engine display and alerts | ECAM E/WD — engine/warning display |
| Lower centre | Secondary EICAS / multifunction display (MFD) | ECAM SD — system display |
| Glareshield | Mode control panel + EFIS control panels | Flight control unit (FCU) with EFIS control panels |
| Pedestal | CDUs (FMS) | MCDUs (FMS) and ECAM control panel |
The flight instruments of both pilots together are the EFIS (electronic flight instrument system). The engine and alerting displays are EICAS (engine indication and crew alerting system) on Boeing and ECAM (electronic centralised aircraft monitoring) on Airbus. How EICAS and ECAM prioritise alerts is covered in Chapter 13.
How the system is arranged
An electronic instrument system has three layers. Sources — air data computers, inertial reference units (often combined as ADIRUs), radio navigation receivers, the radio altimeter, the flight management computer, weather radar, TCAS, terrain awareness and the engine and system computers — send their data as digital words on data buses (Chapter 4). Symbol generators (Airbus: display management computers, DMCs; on integrated aircraft, display functions hosted in a cabinet) select the data, run the graphics and drive the display units (DUs).
- There are normally three generators: one for each pilot and a third as a standby or for the centre screens. A failed generator is replaced by the standby through a source-select switch (Airbus: EIS DMC switching) or automatically.
- Pilots can also choose which sensor feeds their displays — for example air data or inertial source switching — so that a fault on one side does not take out both pilots' information.
- Older display units were 'dumb' screens drawing what the generator sent. Many modern DUs are 'smart' and run the graphics themselves from data received on the bus.
- Each pilot's displays should come from independent sources, generators and power supplies. The system compares the two sides and raises a flag or comparator message if they disagree.
The primary flight display
The PFD shows everything needed to fly the aircraft: attitude, airspeed, altitude, vertical speed, heading, flight director commands, the autoflight modes, ILS deviation, radio altitude, decision height or minimums, and TCAS resolution-advisory guidance.
- Speed and altitude tapes move behind a fixed pointer box, showing the exact digital value and the trend. A speed trend vector shows the speed expected in about 10 seconds. Coloured bands mark the maximum and minimum manoeuvring speeds.
- The flight mode annunciator (FMA) across the top shows the engaged autothrottle, roll and pitch modes in green and the armed modes beneath them.
- Flags replace missing data — for example a red or amber ATT, SPD or ALT flag when the source fails, instead of a frozen pointer.
The navigation display
The ND replaces the HSI and the moving-map. It shows heading and track, the active route from the FMS with waypoints, distance and time to the next waypoint, wind, ground speed and true airspeed, VOR and ILS course deviation, and overlays such as weather radar, TCAS traffic, terrain, airports, navaids and waypoints.
- Modes (names vary): ROSE or full-compass VOR and ILS (APP) modes; an expanded ARC/MAP mode showing the sector ahead; and a north-up PLAN mode for reviewing the route.
- Range is selected on the EFIS control panel, for example 10 to 640 NM.
Controlling the displays
- EFIS control panel (one per pilot): ND mode and range, map overlays (weather radar, terrain, traffic, waypoints, airports, navaids), barometric setting and reference (QNH/QFE/STD), minimums (radio or barometric), flight path vector and VOR/ADF needle selection.
- Display select panel or ECAM control panel: chooses the system pages on the lower centre display.
- Brightness controls for each DU, often with automatic adjustment from ambient light sensors.
- Source and reversion switches: air data, inertial, FMS and display-generator source selection, and display transfer switches.
Reversion after a failure
Because every DU can show any format, the system reconfigures itself. Typical logic: the PFD has priority. If a PFD screen fails, the PFD picture moves automatically to the adjacent ND screen. If the upper centre screen fails, the engine and alert display moves to the lower screen; if both centre screens fail, a compacted engine format can be shown on an ND screen. Manual transfer switches give the crew the same choices. The exact rules are in the aircraft's flight crew operating manual.
Every glass cockpit keeps standby attitude, airspeed and altitude information that is independent of the main display system — separate sensors, power from the battery or essential bus, and its own display. Older aircraft had a standby attitude indicator, altimeter and airspeed indicator. Newer aircraft have one integrated standby instrument (ISIS), plus a standby magnetic compass.
The colour code
All electronic displays use colour consistently, and exam questions often ask for it. The exact convention is in each aircraft's manual, but the common rules are:
| Colour | Typical meaning |
|---|---|
| Red | Warnings, exceeded limits, failure flags — immediate crew action |
| Amber / yellow | Cautions and abnormal conditions — crew awareness and timely action |
| Green | Normal operating ranges, engaged modes, active data |
| White | Scales, current values, labels and armed modes |
| Cyan (blue) | Selected or pre-selected values (Airbus) and inactive or background information |
| Magenta | Targets and commands — the active FMS waypoint and route, flight director bars, target speeds and altitudes |
Red is never used for anything that is merely information. When a question asks which colour shows a caution, the answer is amber (yellow), not red.
Display units and generators are LRUs with built-in test. They need cooling air from the avionics ventilation system — overheating is a common cause of blank or flickering screens. DUs are usually interchangeable, so swapping two DUs is a quick fault-isolation check, but always follow the AMM. Screens with anti-reflective coatings must be cleaned only with the materials the manual specifies.