Electronic Displays
Principles of operation of display types used in modern aircraft, cathode ray tube, light emitting diode, liquid crystal display including active matrix, and plasma.
Electronic displays, summary notes
- CRT (cathode ray tube): an electron beam from a heated cathode is deflected onto a phosphor screen. Bright and proven, but bulky, heavy, power-hungry and running at several kilovolts, a shock hazard long after power-down because the tube stores charge.
- LED: a forward-biased semiconductor junction emits light directly. Rugged, low voltage and long-lived; used for annunciators, segment displays and increasingly as the BACKLIGHT of an LCD.
- LCD: liquid crystal molecules twist under an applied voltage and, with polarising filters, either pass or block light. An LCD emits no light of its own, it must be backlit (or reflective) to be seen.
- AMLCD (active matrix LCD) puts a thin-film transistor at every pixel, giving fast response, high contrast and full colour. This is the standard modern flight-deck display technology.
- Plasma: small gas cells are ionised to emit ultraviolet light that excites phosphors. Wide viewing angle but high power and heat, so it is rare in aircraft.
- Flight-deck display requirements drive the choice: sunlight readability, wide viewing angle, dimming down to night levels, and compatibility with night vision imaging systems where fitted.
- ⚠ Exam trap: an LCD does not generate light, a failed BACKLIGHT gives a dark but still-driven display. And a CRT can hold a lethal charge after shutdown.
A flight-deck AMLCD goes dark but the system still reports the display unit as serviceable and its brightness control has no effect. What is the likely failure?
The backlight assembly or its inverter/driver. The liquid-crystal matrix is still being addressed correctly, which is why no fault is annunciated, but with no light behind it there is nothing to modulate, so the screen appears black.
Why is a CRT display unit treated as a hazard during removal even after the aircraft is de-powered?
The tube and its EHT circuit store a high-voltage charge that can persist for a long time after shutdown. The unit must be handled per the maintenance manual's discharge procedure and never opened casually.
Electronic displays concept map
Electronic Displays
Electronic displays quiz
Electronic Displays, quiz
1. An active-matrix LCD controls each pixel with:
Its own transistorA single shared switchA mechanical shutter2. Compared with a CRT, a flat-panel LCD offers:
Lower weight, volume and powerHigher weight but better colourNo difference3. Modern glass-cockpit displays are predominantly:
LCDCRTPlasma4. An LCD requires a backlight because it:
Emits no light of its ownOperates at high voltageUses a phosphor coating5. AMLCD stands for:
Active matrix liquid crystal displayAmbient modulated LCDAutomatic multi-colour LCD6. A particular hazard when handling a CRT display unit is:
Stored high voltage remaining after power-downLiquid crystal leakageUltraviolet emission7. A display goes dark but is still reported serviceable and brightness control has no effect. The likely failure is the:
Backlight or its driverSymbol generatorLiquid crystal matrix8. In a CRT the image is produced when:
An electron beam strikes a phosphor screenA gas is ionised in sealed cellsA junction is forward biased