What is an obstruction light

Obstruction lights are aircraft warning lights installed on all obstructions that present a hazard to aircraft in flight. They are used for the identification of obstacles and to warn pilots of obstructions during hours of darkness and periods of limited daytime visibility. Any high-rising structure or geographical obstruction could represent an obstacle to air navigation. These obstacles include skyscrapers, chimneys, tower structures, smokestacks, cranes, bridges, catenary river crossings, radio/television masts, cable car and power line pylons, wind turbines, anchored balloons, hyperbolic cooling towers, radar and telecommunication antennas. The presence of an obstacle may be marked by low, medium, or high intensity obstruction lights. The luminous intensity as well as the position of lights, number of levels, and number of lights required to adequately reveal the obstruction to assure visibility depend on the height, size and shape of the obstruction. There are two major international standards that regulate the installation of obstruction lights: ICAO (International Civil Aviation Organization) and the FAA (Federal Aviation Administration). These two organizations give general recommendations, and their standards differ slightly.

Obstruction light categories

The FAA Advisory Circular (AC 70/7460-1M) that sets forth standards for marking and lighting obstructions divides obstruction lights into eight categories. They are aviation red obstruction lights, medium-intensity flashing white obstruction lights, high-intensity flashing white obstruction lights, dual lighting, lighted spherical markers, aircraft detection lighting system, obstruction lights during construction, and obstruction lights in urban areas. Aviation red obstruction lights are flashing lights or steady-burning lights used during nighttime. Flashing red lights are typically used to mark tower structures. Steady-burning red lights are used to identify buildings and smaller obstructions located near airports. Medium-intensity flashing white obstruction lights can be operated during daytime and twilight, with automatic reduced intensity selected for nighttime operation. These lights are not normally recommended on structures 200 feet (60.96 m) AGL (above ground level) or less. Obstruction lighting on structures exceeding 700 feet (213.36 m) AGL requires aviation orange and white paint for daytime marking. High-intensity flashing white obstruction lights are used on structures exceeding 700 feet (213.36 m) during daytime, with automatically selected reduced intensities for twilight and nighttime operations. A dual lighting system incorporates a red light for nighttime and a high- or medium-intensity flashing white lights for daytime and twilight. Lighted spherical markers are used to increase night conspicuity of high-voltage (69 kV or greater) transmission line catenary wires near airports, heliports, across rivers, canyons, lakes, areas of known risk to aviation, etc. Aircraft detection lighting systems (ADLS) are controlled by sensor based obstruction lights which can detect aircraft approaching a single obstacle or group of obstacles and automatically switch on the applicable obstruction lights until the departure of the aircraft and the lights are no longer needed. Any construction that could pose a hazard to air navigation should be marked with two or more temporary high- or medium-intensity flashing white lights until all permanent lights are in operation. In urban areas where there are numerous other white lights (e.g., streetlights), red obstruction lights with painting or a medium-intensity dual system are recommended to mark an obstacle that has a minimum height of 200 feet (60.96 m) or is located within 3 NM of an airport.

Regulatory lighting requirements

A red obstruction light is a flashing omnidirectional light or steady-burning light. Low structures, such as airport instrument landing system buildings, and also long horizontal structures, including perimeter fences and building roof outlines, are normally identified by single steady-burning lights. Single flashing or steady-burning lights may be used to mark skeletal and solid structures when more than one level of lights is installed and there are two or more single lights per level. Structures 150 feet (45.72 m) AGL or less should be marked with one or more double steady-burning lights which are installed at the highest point, they should operate simultaneously. When a malfunction of a single light could cause a hazard and in remote areas where immediate maintenance cannot be performed, double flashing or steady-burning lights should be installed at intermediate levels. Red obstruction lights should be operated by lighting controls (e.g., timers, photocells) which turn on the lights when the northern sky illuminance reaching a vertical surface falls below a level of 60 foot-candles (645.83 lux) but exceeds 35 foot-candles (376.73 lux). The control device should turn the red obstruction lights off when the northern sky illuminance rises above 60 foot-candles (645.83 lux). A medium intensity flashing white obstruction light is normally a flashing omnidirectional light. Twilight-to-night and night-to-day transition is controlled by a device (photocell) which adjusts the luminous intensity when the ambient light changes. High-intensity flashing white obstruction lights are also photocell-controlled to automatically change intensity steps to the day-to-twilight, twilight-to-night, or night-to-day mode based on the change in illuminance reaching a north-facing vertical surface. Dual lighting system may be used in lieu of operating a flashing high intensity white lighting system at night to reduce/mitigate environmental concerns from nighttime use of high-intensity lights. ADLS systems should be equipped with three-dimensional sensing capability to provide complete detection coverage for aircraft entering the airspace, or coverage area, around the obstruction(s). The detection system should activate the obstruction lighting system in sufficient time and allow to synchronize the lights so that they flash simultaneously prior to an aircraft penetrating the three-dimensional volume of airspace. Lighted markers should not be less than 36 inches (91.44 cm) in diameter and should be installed on the highest energized line. An alternating color scheme should be used to provide the most conspicuity against all backgrounds.

Lighting technology

In the past, red obstruction lights used incandescent filament bulbs and high intensity white lights used xenon but advances in LED technology bring obstruction lighting to the forefront of sustainability. LEDs are low voltage semiconductor devices that provide instant light at low power consumption. Typically, only 5% of the electrical power is converted to visible light for an incandescent light. LEDs convert up to 40% of the input energy into optical radiation. They reduce power consumption by 60-90% when compared to a conventional xenon flashtube. When overall energy and maintenance costs are considered in purchase decisions, the high efficacy and long-lasting LEDs drive a significantly better return on investment. LEDs are inherently narrow spectrum devices that have typical bandwidths of a few tens of nanometers. The narrow-band emission can be engineered for any color such as red, green, blue, or infrared. Broad spectrum white light can also be generated from phosphor-converted or color-mixed LED architectures. Monochromatic red output allows obstruction lights to eliminate the use of colored lenses which can cause a huge optical loss. Monochromatic LEDs in different colors can be integrated to enable multi-color signal generation from a single luminaire. The semiconductor nature lends LEDs excellent controllability for different lighting effects. LEDs can be controlled individually at the source- or module-level to create static, flashing and rotating effects within a single light engine.

Design and construction

A typical LED obstruction light consists of an LED module, a driver, a lens, a photocell, and a housing. The LED driver is configured to convert the AC line voltage (e.g., 120–277V, 50–60 hertz) into the constant DC (direct current) power compatible with the LED load requirement. It can also be designed to operate on low-voltage DC grids such as DC power converted by a solar photovoltaic system. It includes control circuitry that can dim or turn on/off the LEDs based on a digital or analog control signals initiated by the photocell or communicated through the network control system. The driver may provide multi-channel output to keep individual LEDs or groups of LEDs within the LED module operating independently of each other for dual lighting applications or visual effects of rotating a rotating beacon. Most LED drivers are switching power supplies. High-speed switching operation introduces a potential source of electromagnetic interference (EMI) that has to be suppressed by careful circuit board design, screening, and filtering. The lens is made from UV-stabilized polycarbonate or tempered glass. The housing is constructed from aluminum and typically forms with the heat sink as one piece to facilitate more efficient heat dissipation into the ambient. Chemical surface treatments are applied to provide corrosion resistance. The lens is fully gasketed to the housing to keep out rain, moisture, dirt and contaminants.