10.1 Display Technologies: CRT, LED and LCD

10.1 Display Technologies: CRT, LED and LCD

Current EASA Part-66 Appendix I heading 5.11 Electronic displays supplies only the title and category levels. The pre-12 June 2024 detailed description named cathode-ray tube (CRT), light-emitting diode (LED), and liquid-crystal display (LCD) principles, which this guide retains as historical study scope. Category A and B3 candidates sit at level 1 (familiarisation with the existence and role of each type). Categories B1 and B2 sit at level 2 (general knowledge of construction, operating principle, and the supplies the device needs). This section therefore explains how each historically listed type produces or controls light, which optical and electrical elements it contains, and which faults a certifying engineer should expect. Organic light-emitting diode (OLED) panels appear on some recent integrated flightdecks as a successor technology; they are useful modern context, but they were not one of the types named in the former detailed 5.11 description.

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CRT: Electron Gun, Deflection and Phosphor

A CRT is a high-vacuum glass envelope. The neck houses the electron gun. The faceplate carries a phosphor coating that converts electron kinetic energy into visible light.

The gun is a stack of electrodes with distinct jobs:

1. Heated cathode. A nickel or oxide-coated emitter, usually indirectly heated by a filament, releases electrons into the vacuum by thermionic emission. Emission current rises steeply with cathode temperature. If the heater is open, the screen stays dark even though extra-high tension may still be present.
2. Control grid (G1, Wehnelt cylinder). Held negative with respect to the cathode, it sets beam current and therefore spot brightness. A more negative grid reduces current; a sufficiently negative grid blanks the beam during raster flyback or between stroke-written symbols so that return traces are invisible.
3. Focus electrodes. Electrostatic lenses (sometimes aided by a magnetic focus coil) converge the beam to a small spot so that pitch ladders and airspeed tapes remain sharp.
4. Final anode / ultor. This electrode accelerates the electrons. Avionics colour tubes and high-brightness monochrome tubes typically run extra-high tension (EHT) of the order of 8 kV to 25 kV. A conductive graphite (aquadag) coating on the inside of the bell is bonded to this anode and collects secondary electrons so the face does not charge uncontrollably.

Deflection aims the beam. Electrostatic plates in the neck are common where the picture is drawn as vectors. Electromagnetic yoke coils around the neck are common where a television-style raster is required. First-generation electronic flight displays often used stroke (calligraphic) writing: the symbol generator steered the beam as a pen to draw the horizon, pointers and scales, unblanking the beam only while a symbol was being painted. Later combined-mode tubes superimposed raster weather-radar or map video behind stroke symbology. Raster timing still matters on remaining CRT heads: horizontal and vertical deflection amplifiers, flyback blanking, and a final-anode supply that must remain within limit or the picture blooms, dims, or produces excess radiation at the faceplate.

Light itself is cathodoluminescence. An electron arriving with kilovolt energy excites the phosphor crystal; the crystal relaxes by emitting a photon. Phosphor chemistry sets colour and persistence (how long the glow remains after the beam has moved). Colour tubes use a triad of red, green and blue phosphors with a shadow mask or aperture grille so that each of three guns strikes only its own colour. Monochrome avionics CRTs used a single high-efficiency green or white phosphor.

CRT virtues that kept them in service for decades are a wide viewing angle, high peak brightness, and the ability to overlay bright stroke symbols on sensor video. The maintenance penalties dominate line work: cabinet depth and mass; stored EHT charge after power is removed; possible soft X-radiation at the face if anode voltage and shielding are out of limit; geometric pincushion and barrel distortion; colour-purity errors from stray magnetic fields (hence degaussing coils); phosphor burn-in of static PFD tapes; heat; and implosion if the envelope is cracked. Before removing a CRT display unit, discharge the anode to chassis through a resistor in accordance with the aircraft maintenance manual. Never operate or inspect a live tube with the magnetic shields or face-plate filter omitted.

[!WARNING] CRT extra-high tension remains after switch-off. The ultor capacitance can hold a dangerous charge. Treat an un-discharged CRT as a live high-voltage component, not as an ordinary avionics box that is safe once the bus is dead.

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LED: Forward-Bias Emission

An LED is a PN junction engineered for electroluminescence. Under forward bias, electrons and holes are injected into the junction region and recombine. In a direct-bandgap III–V semiconductor (examples include GaAs, GaAsP, AlGaInP, GaN and InGaN) a useful fraction of those recombinations is radiative: the electron’s lost energy appears as a photon rather than only as lattice heat.

Photon energy, and therefore colour, follows the bandgap. A wide gap yields blue or near-ultraviolet light; a narrower gap yields red or infrared. White cockpit lighting and LCD backlights normally use a blue InGaN die plus a yellow phosphor. Typical forward voltages at rated current are about 1.6 V to 2.2 V (red/infrared), 2.0 V to 3.2 V (yellow/green) and 2.8 V to 3.6 V (blue/white). Reverse-voltage ratings are only a few volts, so a reverse bus spike without a series diode or resistor destroys the junction.

The LED is a current device. Luminous intensity is roughly proportional to forward current in the linear region, then saturates; die temperature reduces output and shortens life, so drivers derate current in a hot flightdeck. A ballast resistor or a constant-current driver is mandatory.

Worked example — ballast resistor on a 28 V DC essential bus. A red legend LED is specified at 15 mA forward current with a forward voltage of 2.0 V. The resistor must drop 28.0 minus 2.0, which is 26.0 V, so R = 26.0 V / 0.015 A = 1.73 kΩ.

Fit the preferred value 1.8 kΩ. Dissipation is I squared R = (0.015) squared times 1800 = 0.405 W; specify at least a 1 W aerospace resistor, not a 0.25 W commercial part. If two identical LEDs are placed in series, forward voltage doubles and the resistor is recalculated. Direct paralleling without individual ballast is poor practice because forward voltage has a negative temperature coefficient and one die will hog the current.

On the aircraft, discrete LEDs form warning, caution and advisory annunciators, sunlight-readable push-button legends, and seven-segment or dot-matrix readouts on radio and lighting panels. High-power white LED arrays have also replaced cold-cathode fluorescent lamps (CCFL) as LCD backlights, because they start instantly after cold soak, need no high-voltage inverter, and tolerate vibration. An LED by itself does not paint a PFD attitude sphere unless it is organised as a full matrix; the former detailed 5.11 description included LED as a display technology because the same forward-bias physics covers a single caption, a matrix readout and a backlight string.

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LCD: Polariser, Light Valve and Backlight

An LCD does not generate light. It is a light valve. A backlight (transmissive avionics panel) or ambient illumination (reflective cell) supplies the photons; the liquid-crystal layer and two polarisers decide whether those photons reach the pilot.

A typical transmissive stack, from the backlight forward, is:

1. LED array or legacy CCFL backlight, diffuser and brightness sensor.
2. Rear polariser (analyser).
3. Rear glass with transparent indium-tin-oxide (ITO) electrodes and a rubbed polymer layer that orients the molecules.
4. A few micrometres of nematic liquid crystal.
5. Colour-filter mosaic (red, green and blue sub-pixels) and front ITO on colour panels.
6. Front glass and front polariser.
7. On many aircraft active-matrix LCDs (AMLCDs), a transparent heater and temperature sensor so the fluid remains within its specified viscosity after a polar cold soak.

In the twisted-nematic (TN) teaching model the unpowered rod-like molecules twist through 90° from one glass to the other. Light that has passed the front polariser has its polarisation rotated by that twist and can leave through the crossed rear polariser, so the pixel appears bright (normally white). An applied electric field untwists the rods so they stand on end; rotation of polarisation ceases; the analyser blocks the light; the pixel goes dark. Other electro-optical modes exist (in-plane switching, vertically aligned), but the engineer’s model is unchanged: voltage sets molecular orientation; orientation sets polarisation; polarisers convert polarisation into brightness. Grey scale is a controlled RMS voltage, usually an alternating drive so that DC does not electrolyse the liquid.

Passive-matrix panels multiplex rows and columns and are limited in contrast and size. AMLCD (TFT) places a thin-film transistor at each sub-pixel so the voltage is stored for the whole frame. That is the architecture of modern primary flight and navigation displays. Colour is not generated in the liquid: it is the colour filter over each sub-pixel. Three sub-pixels make one pixel.

LCD advantages are shallow depth, low mass, no EHT, and comparatively low power. In-service limitations that generate defects are viewing angle (contrast and colour shift off axis), temperature (image smear when the fluid is viscous; clearing to an isotropic, image-free state if overheated), backlight or LED-string failure, polariser ultraviolet damage or delamination, and heater failure. A panel that is black but still glows at the edges usually has a cell, flex-cable or column-drive fault. A panel with no edge glow has a backlight, LED driver or (on CCFL types) inverter fault.

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OLED as Modern Context Only

An OLED stack injects holes and electrons into thin organic electroluminescent layers. Recombination emits light in the pixel itself, so there is no backlight and no vacuum envelope. Blacks are very deep and the package can be thin. Some recent integrated flightdeck units use OLED. For Module 05 it is enough to recognise OLED as a self-emissive successor to CRT and backlit LCD, not as a fourth type in the former detailed 5.11 list. Characteristic faults differ again: burn-in of static speed and altitude tapes, moisture sensitivity of organic films, and differential ageing of blue emitters. There is no separate backlight LRU to swap.

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Comparison for the Certifying Engineer

Category B1/B2 candidates should be able to state which physical process produces the light (or, for LCD, that none is produced in the cell), name the electron-gun electrodes and the polariser/backlight stack, size a simple LED ballast resistor, and match a high-voltage shock risk to CRT, a current-limit risk to LED, and a heater/backlight risk to LCD. Category A familiarisation is the same table without the numerical resistor example: CRT is a phosphor tube with an electron gun, LED glows when forward biased, LCD needs a backlight and polarisers.

[!NOTE] Do not confuse LED backlights with LED displays. Many AMLCD flight displays are LED-backlit LCDs. The picture is still formed by the liquid-crystal light valve. The LEDs only provide the photons that the valve passes or blocks. A failed backlight string makes the LCD go dark even though the cell and symbol generator are healthy.