Aircraft Warning Lights - Lamp Types

Lamp Types

The lights generally come in two forms:

• Red lamps that are either constantly illuminated or turn on and off slowly in a cycle of a few seconds.
• White xenon discharge flashers.

Both types were in use in the United Kingdom until recently, however new regulations stipulate the use of red lamps at nighttime only. Xenon flashers are therefore gradually being phased out.

In the United States and Canada, there are several types of lights:

• Obstruction lights (that are constantly illuminated)
• Red beacons/red strobes
• High-intensity white (strobe) lights
• Medium-intensity white (strobe) lights

Traditionally, red lamps (or beacons) use incandescent filament bulbs. In order to improve the otherwise quite short lifespan, they are made with a ruggedised design and are run below normal operating power (under-running). A recent development has been the use of arrays of high-power red LEDs in place of incandescent bulbs, which has only been possible since the development of LEDs of sufficient brightness. LED-based lamps have a significantly longer lifespan than incandescent bulbs, thus reducing maintenance costs and increasing reliability. Several manufacturers have also developed medium-intensity white strobes based on LED technology to replace xenon. Xenon flashers, whilst more visually impressive, tend to require frequent replacement and so have become a less favoured option. However, with the advent of LEDs, white strobes are still somewhat desired.

It is common to find structures with white xenon flashers/white strobes during the daytime, and red lights at night. Red lights are commonly found to be used in urban areas, since it is easier for pilots to spot them from above. White strobes (that flash round the clock) may also be used in urban areas. However, it has been recommended that flashing white strobes should not be used in densely populated areas; the lights usually merge with background lighting at nighttime, making it difficult for pilots to spot them and thereby aggravating the hazard. In addition, residents near the lit structure will complain of light trespass. In rural areas, red beacons/strobes may also be used during nighttime. However, white strobes are (sometimes) preferred since they reduce the maintenance cost (i.e. no maintenance of painting, no red side lights) and there are no background lights that would blend with the strobes.

There are a medium-intensity white strobe and a high-intensity white strobe. Medium-intensity white strobes are usually used on structures that are between 200–500 feet (61-152.4 meters). If a medium white strobe is used on a structure greater than 500 feet (152.4 meters), the structure must be painted. The common medium white strobe flashes 40 times in a minute, at an intensity of 20,000 candelas for daytime/twilight, and 2,000 candelas at nighttime.

A high-intensity white strobe light is used on structures that are greater than 500 feet (152.4 meters). These lights provide the highest visibility both day and night. Unlike a medium strobe, a high-intensity strobe does not provide 360˚ coverage; this requires the use of at least 3 high strobes at each level. On the other hand, it reduces maintenance costs (i.e. no painting). If the structure has an antenna at the top that is greater than 40 feet, a medium-intensity white strobe light must be placed above it rather than below. The common high white strobe flashes 40 times in a minute, at an intensity of 270,000 candelas for daytime, 20,000 candelas at twilight, and 2,000 candelas at nighttime.

Dual lighting is a system in which a structure is equipped with white strobes for daytime use, and red beacons/strobes for nighttime use. In urban areas, these are commonly preferred since it usually exempts a structure from the requirement of having to be painted. One advantage to the dual system is that when the uppermost red lights fail, the lighting switches onto its backup lighting system, which uses the white strobes (at its night intensity) for nighttime. In the United States and Canada, red beacons are slowly being withdrawn from service and being replaced with red strobes. In addition, some medium strobes are equipped to flash the white light for daytime and red light for night in a single strobe (unlike the old type which had two different lights).

For high-tension power lines, the white strobes are equipped to flash 60 times per minute, using the same intensities as stated above. Unlike the common white strobes, these strobes are specified not to flash simultaneously. The FAA-mandated flash pattern is middle, top, and bottom to provide "a unique system display".