Before the electronic flight deck can be studied screen by screen, two smaller questions come first: what the screens are physically made of, and what they replaced. The answer to the second is not the six separate dials of the classic panel but a pair of mechanical instruments that had already done the integrating: the attitude and director indicator and the horizontal situation indicator. Learn these two faces well; the next lesson's electronic displays are these instruments, redrawn.
Display technologies
Two display technologies have carried the electronic flight deck:
the cathode ray tube (CRT) — the first generation: an electron gun fires a beam at a phosphor-coated screen, deflection coils steering it to draw the picture. CRTs draw a bright, sharp image, but the tube behind the screen makes the unit deep and heavy; it needs a high-voltage supply, and it dissipates considerable heat.
the liquid crystal display (LCD) — the current generation: a thin layer of liquid crystal cells sits between polarising filters and gates the light of a backlight, each cell twisting or blocking the light as a voltage commands. The LCD unit is flat, light and far cooler, which is why it displaced the CRT panel by panel.
Whatever the technology, three limitations come with putting a screen on a flight deck:
Cockpit temperature — the liquid crystal layer is sluggish and dim when cold-soaked and degrades in heat, so the units must work across the temperature range a parked and a powered flight deck can reach, while the CRT adds to that problem by heating the panel around it;
Glare — direct sunlight can wash the picture out and reflections can hide it, so the units carry anti-reflective coatings and a wide brightness range to stay readable from a dark night to sun on the screen;
Resolution — the screen can only draw as finely as its dots allow, and the resolution sets how much symbology fits on the unit before the picture stops being legible at arm's length.
From dials to integrated instruments
The classic panel gave every measurement its own dial, and the pilot's eyes did the integrating, sweeping a scan across six instruments and assembling the picture mentally. An integrated instrument moves that work into the instrument: it combines several related measurements onto one face, so one look replaces a scan. Two mechanical integrated instruments defined the pattern:
the attitude and director indicator (ADI) — which gathers everything about controlling the aircraft's attitude;
the horizontal situation indicator (HSI) — which gathers everything about where the aircraft is going over the ground.
Memorise the pairing, because the electronic flight deck kept it: the primary flight display (PFD) of the next lesson descends from the ADI, the navigation display (ND) from the HSI.
The attitude and director indicator
The ADI is built around the attitude indicator module 4 taught: an artificial horizon behind a fixed aircraft symbol, showing pitch and bank at a glance. Onto that face the designers layered the flight director command bars, the crossbar and V-bar presentations the flight director lesson drew, so the instrument shows both the attitude the aircraft has and the attitude the flight director wants. Around the edges sit the approach and housekeeping data: localiser and glideslope deviation scales for the instrument landing system (ILS), a speed deviation scale showing fast or slow against the target, a rate-of-turn scale and a slip indicator, and on many units the radio altitude with a decision height light for the final feet of an approach.
Listed for the exam, the information an ADI can display:
pitch and bank attitude — the artificial horizon behind the fixed aircraft symbol;
flight director commands — command bars superimposed on the attitude;
localiser and glideslope deviation — ILS scales at the edges of the face;
speed deviation — fast/slow against the commanded speed;
rate of turn and slip — the turn scale and the slip ball;
radio altitude and decision height — the height readout and its light on approach-equipped units.
The horizontal situation indicator
The HSI is the plan view: the aircraft seen from above, at the centre of its own navigation picture. The face is a compass card driven by the remote-reading compass or its AHRS successor, read against a fixed lubber line at the top; a heading bug set by the crew rides on the card. Across the card lies the course arrow, turned with the course knob to the selected radial or final approach course, and its centre section is the course deviation bar: the bar displaces sideways from the arrow's line, against a scale of dots, to show how far the aircraft sits from the selected course, and the aircraft is flown toward the bar to re-intercept. A TO/FROM indicator resolves which way along the course the selected station lies, a glideslope pointer and scale at the side shows vertical deviation on an ILS, a DME (distance measuring equipment) range window counts the distance to the tuned station, and warning flags drop when the heading or navigation signal fails.