obstruction light[əb′strək·shən ‚līt]
A light indicating the presence of an obstruction.
Distinctive lighting to provide a uniform means for indicating the presence of obstructions, especially at night or in poor visibility conditions. An obstruction light, or one group of lights, is usually red or white, visible 360°, and frequently mounted on top of structures or natural terrain to warn pilots of the presence of obstructions. An obstruction may be marked with flashing aviation red beacons (20 to 40 flashes per minute) and steady burning aviation lights during nighttime operations and with aviation orange and white paint as daytime markings. Obstructions also may be marked with high-intensity white obstruction lights during the daytime, with reduced intensity at twilight and during nighttime operations. In this case, the use of orange and white markings for daytime operations may be omitted. Medium-intensity flashing white obstruction lights may be used during the daytime and twilight, with automatically selected reduced intensity for nighttime operation. When this system is used on structures 500 ft (150 m) AGL (above ground level) or less in height, other methods of marking and lighting the structure may be omitted. Aviation orange and white paint is always required for daytime marking on structures exceeding 500 ft (150 m) AGL. This system normally is not installed on structures less than 200 ft (60 m) AGL. In certain cases, dual lighting and flashing high-intensity lights may be used during the daytime. Similarly, at night, flashing aviation red beacons and steady burning aviation red lights may be used. Lighted markers are available for increased night visibility of high-voltage (69 kV or higher) transmission line catenary wires. Lighted markers provide visibility both day and night. See obstruction.