Integral Panel Lighting in Airbus Aircraft
Airbus panel use different kinds of back light technology depending on the age of the light plate and the manufacturer. All panels have to meet strict requirements regarding brightness, even distribution of light and color. To achieve this, it often seems to be necessary to manually rework light plates during the manufacturing process. Regarding color, the aim is to create a color similar to a yellowish/amber glowing lightbulb.
The classic way to back light aircraft panels are incandescent light bulbs. This type of backlight is used for all older generation A318-A321 aircraft. The bulbs used are usually of the type MS24367-715 5V/0.115A (0.6W). This bulbs are not commonly used anymore and therefore relatively expensive. This is an issue because bulbs have a high failure rate and need to be replaced regularly. There are also other problem involved by using bulbs for backlighting. The efficiency of bulbs is low, which means that most of the electric power is converted into heat and not light. This also means that a relative large amount of electric power is required to light a cockpit full of bulbs. The excessive heat is also heating up the surface of the panels. This can get so extreme that the surfaces are too hot to touch at full brightness. An good example for this are the old type Radio Management Panels, which can reach surface temperatures of above 60° Celsius! This can not also burn your fingers, but it also damages the light plate itself in the long run. Older light plates commonly have discolorations and damage at the places where bulbs are located.
An old light plate with 6 incandescent bulbs. Discoloration damage caused by heat is clearly visible where the light bulbs are installed (circled)
Newer generation panels use LED backlight. Most A330/A340 aircraft panels use this technology. A large number of amber LEDs is replacing the bulbs in these panels to generate an even light distribution. This LEDs are very reliable and usually don't fail. I single failure would also not be noticeable because of the large number of LEDs. However, over many years the LEDs gradually loose their brightness, and an older panel will appear noticeable dimmer than a new one. It is also difficult to replace LEDs. Because of their large number it is not feasible to replace all LEDs of a panel manually. It is also difficult to find a compatible replacement type to the A330/340 OEM LEDs because they are older generation LEDs with a low efficiency. That means that most modern LEDs that are commonly available today will to be much too bright and can't be used as a direct replacement. It is also very difficult to find the right shade of "amber" used by the OEM LEDs. There is no standardization of LED colors and every manufacturer has is own ideas how a amber color should look like. Most LED types that are specified as amber are actually either too yellow or too orange compared to the OEM LEDs. This color deviation is very noticeable, so it is important to find an exactly matching color when replacing OEM LEDs.
The third way to generate panel backlighting are white LEDs. White LEDs that have not been available at all until about the year 2000 are now common and cheap. A number of new generation panels now use white LEDs in combination with an color filter to generate amber panel backlight. An example for panels using this technology are new generation ECAM MCPs, which is shown in the photos below:
New generation ECAM ECP Panel with white LED back light
A light Plate of an ECAM ECP Panel. A color filter is used to change the white LED color to amber. Note the manual touch up work with white paint and a black pen below the color filter. This has been done during manufacturing to correct uneven light distribution caused by varying light output of individual LEDs and tolerances of the thickness of the light plate engravings
Power supply for aircraft lighting
In aircraft, the standard power supply for back lighting is 5 V AC with 400Hz, which varies between 0 and 5V for dimming. The voltage is generated by aircraft lighting controllers from the standard 115V/400Hz aircraft supply. This lighting controllers use an old and uncommon technology called magnetic amplifier or mag amp to regulate the output voltage. This technology uses inductors that are driven into magnetic saturation to regulate the output power. The result is that the lighting controllers don't need any power semiconductors and can regulate a large output current with very little waste heat generation. The output waveform of the aircraft lighting controller is not a pure sine wave. Instead it looks similar to the output of a TRIAC AC voltage dimmer, as shown in the picture below.
The output of the lighting controller is shown above in the upper waveform. The amount of the original sine waveform that remains in the output is varied to regulate the brightness
Light plates with LED backlight also need 28V DC in addition to the variable 0-5V AC. Actually, they only use the 0-5V AC as a control input to regulate the brightness. They will also accept a 0-5V DC signal for this purpose. The main power for the LEDs is provided from the 28V DC input by internal circuits on the light plate. The reason for this setup is that the LED light plates need to compatible to older incandescent light plates, which means they need to regulate their brightness the same way as bulbs controlled by the 0-5V AC voltage. They can not use the 0-5V AC directly as their only power source because LEDs would not produce any light when the panel is dimmed and the supply voltage drops below approximately 2V, whereas light bulbs still produce light at this voltage. To make them behave the same way as light bulbs the additional 28V voltage and electronics is necessary.
A330 light plate PCB with SMD LEDs and circuits for power supply and LED drivers. Note the large number of LEDs used for this small light plate. This provides a more even light distribution compared to bulbs.
In case of panels with incandescent light bulbs, the 0-5V AC voltage is the only power source and directly provided to the bulbs via a 2 pin circular connector on the back side of the light plates. The LED light plates have a 3 pin connector the provides GND, 0-5V AC and additionally +28V DC. These connectors are not very reliable, and it is common that a panel back light fails because of contact problems with the connector. This is true even for newer A330 aircraft that are only a few years old and have LED light plates. The light plates have a little blue cross printed at the location of the connector. It is often enough to press on this part of the light plate to fix the contact problem, at least temporally.
A330 LED light plate with a blue cross printed where the connector is located
The 3 pin circular connector is visible when the light plate is removed
The matching male connector on the backside of the light plate with a pinout diagram
The light plate with the LED PCB removed. Note the touch up work with white paint to achieve an even light distribution and to compensate for uneven engravings.