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Aircraft obstacle avoidance systems From Wikipedia, the free encyclopedia
Aviation obstruction lighting is used to enhance the visibility of structures or fixed obstacles which may conflict with the safe navigation of aircraft. Obstruction lighting is commonly installed on towers, buildings, and even fences located in areas where aircraft may be operating at low altitudes. In certain areas, some aviation regulators mandate the installation, operation, color, and/or status notification of obstruction lighting. For maximum visibility and collision-avoidance, these lighting systems commonly employ one or more high-intensity strobe or LED devices which can be seen by pilots from many miles away from the obstruction.
The lights generally come in two forms:
Both types were in use in the United Kingdom until recently. New regulations stipulate the use of red lamps at nighttime only. Xenon flashers are gradually being phased out.
In the United States and Canada, there are several types of lights:
Traditionally, red lamps (or beacons) use incandescent filament bulbs. 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. With the advent of LEDs, white strobes are still somewhat desired.[citation needed]
It is common to find structures using 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) (60 per minute for flashing marking "a unique signal that pilots should interpret as a warning that catenary wires are in the vicinity of the lights") may also be used in urban areas. It has been recommended that flashing white strobes should not be used in densely populated areas, as the lights usually merge with background lighting at nighttime, making it difficult for pilots to spot them, 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. 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.[citation needed]
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 and 500 feet (61 and 152 meters). If a medium white strobe is used on a structure greater than 500 feet (150 meters), the structure must be painted. The common medium white strobe flashes 40 times per minute, at an intensity of 20,000 candelas for daytime/twilight, and 2,000 candelas at nighttime.
A high-intensity white strobe is used on structures that are taller than 700 feet (210 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 per minute, at an intensity of 270,000 candelas for daytime, 20,000 candelas at twilight, and 2,000 candelas at nighttime.[1]
Dual lighting is a system in which a structure is equipped with white strobes for daytime use, and red lights for nighttime use. In urban areas, these are commonly preferred since it usually exempts a structure from painting requirements. 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 US and Canada, red incandescent beacons are slowly being withdrawn from service and being replaced with red strobes or red LEDs.
For high-tension power lines, white strobes are equipped to flash 60 times per minute, using the same intensities as stated above. Unlike 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 signal that pilots should interpret as a warning that catenary wires are in the vicinity of the lights."[2]
On electric transmission towers, lights may be energized either from the electric field surrounding the energized conductor, or the magnetic field produced by current through the conductor.[citation needed] The first approach takes advantage of the high electric potential gradient around conductors. The second approach is based on Faraday's law of induction involving magnetic flux flowing through a circuit which energizes the warning light.
These lights can generally be found attached to any tall structure such as broadcast masts and towers, water tanks located on high elevation, electricity pylons, chimneys, tall buildings, cranes and wind turbines. Shorter structures that are located close to airports may also require lighting; an example of such is the south scoreboard at Lambeau Field in Green Bay, Wisconsin built in 2013, which is the tallest structure in the general area of nearby Austin Straubel International Airport. The International Civil Aviation Organization (ICAO) sets standards, usually adopted worldwide, for the performance and characteristics of aviation warning lamps.
Lights are usually arranged in clusters of two or more around the structure at specific heights on the tower. Frequently there will be a set at the top, and then one or more sets equally spaced down the structure. The United Kingdom's Belmont mast has nine clusters of red lamps spaced equally along the full height of the mast.
Conductor marking lights are sometimes used (in conjunction with overhead wire markers) to make overhead power lines more visible. Because power lines are often suspended between widely spaced masts, they present a particularly grave hazard to low flying aircraft. A simple and cost-effective solution to this problem is installing marking lights directly on the wires, but there are significant technical difficulties to low-cost power extraction from a distribution system which carries high voltages and wide-range AC current.
The ideal warning light must be able to power itself while clamped to a single wire of the line. Lights may be powered either from the electric field surrounding the energized wire, or the magnetic field produced by current through the wire. The first approach takes advantage of the high electric potential gradient between conductors, but a strong enough capacitive coupling is requested to allow capacitive extraction of the power required from the warning light. This means that long conductors must be suspended parallel to the line using glass/ceramic isolators: in fact several meters of suspended conductor are generally required, total length being inversely proportional to the line voltage. The second approach is based on Faraday's law of induction involving magnetic flux flowing through a circuit which powers the warning light.
On some tall structures there are or were non-standard aircraft warning lights installed.
Aviation laws also require towers and masts to be painted with equal length stripes of international orange and white paint along their length to improve daytime visibility. This paint scheme is usually required on towers over 200 feet tall, but may vary from state to state and near airports internationally. Because such a paint scheme is expensive to apply, antenna towers and masts are often built to heights just below the level of requirement. Antenna towers and masts often also have other aircraft warning features located at equal distances along their height and at their tops. These may include high powered strobe lights or LED lights in either red, white, or both colors in an alternating pattern. In such a case red is employed at night, while a white strobe is usually used during daylight hours.[2][3]
Warning lights on ground-based towers and buildings contribute to both light pollution and towerkill of birds[4].
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