Obstruction & Beacon Lighting

Obstruction lighting marks fixed structures that pose a hazard to aircraft — chimneys, broadcast masts, cooling towers, high-rise buildings, and even wind turbines. They are classified by intensity — low, medium and high — with the class chosen mainly by the height of the structure: low intensity up to about 45 m, medium intensity from 45 m to 150 m, and high intensity above 150 m or wherever an aeronautical study calls for it. Low-intensity units are red and usually steady-burning; medium intensity is either flashing white by day or flashing/steady red by night; high intensity is flashing white, stepped down sharply at twilight and again at night.

Airport beacons are the rotating or flashing lights that identify the airport itself. The standard civil land airport beacon produces alternating white and green flashes (white-green-white-green). A water aerodrome uses alternating white and yellow; a heliport uses a green–yellow–white sequence; a military airfield uses two quick white flashes between green flashes. Flash rate carries information too: beacons marking airports, landmarks and points on federal airways run at 24–30 flashes per minute, while heliport beacons run faster, at 30–45. The beacon is usually mounted on the control tower or a dedicated structure, visible from long range.

Unlike runway or taxiway lighting, obstruction and beacon systems are usually independently powered and locally controlled — mains-fed, with solar and battery options for remote sites — but they are still part of the overall visual aids suite and must meet the photometric and colour requirements of ICAO Annex 14 Chapter 6, or the corresponding FAA or national specifications. Note that obstruction lighting frequently sits outside the aerodrome boundary and outside the aerodrome operator’s control: the structure owner is normally responsible for it, which is what makes the reporting and notification arrangements around a failed light as important as the hardware.

Runway and taxiway lighting guides an aircraft that is already following a known, planned path; obstruction and beacon lighting exists for the opposite case — warning aircraft that are not necessarily headed anywhere near a runway about a fixed hazard, or an airport's presence, that they might otherwise not see in time. Obstruction lights on masts, towers, and tall structures have to be effective for any aircraft passing nearby regardless of its intended route, which is why they are designed for all-round, long-range visibility rather than aimed toward a specific approach direction. The airport beacon serves a related but distinct purpose: it lets a pilot identify and locate the airport itself from a distance, well before the runway's own lighting or markings are visible, using an internationally recognised colour pattern (white-green for civil land airports) so the signal is unambiguous regardless of which country or airfield the pilot is approaching. Running these systems independently of the runway/taxiway circuits — sometimes on solar power at remote sites — ensures that this hazard-warning and identification function keeps working even if the airfield's main lighting circuits are down.

Obstruction lighting is installed on any structure that penetrates the obstacle-limitation surfaces or is tall enough to be a hazard:

• Low-intensity (red): Structures up to about 45 m, and as the intermediate levels of a layered scheme on taller structures. ICAO Type A is a minimum of 10 cd fixed red; Type B a minimum of 32 cd fixed red; Type E a flashing 32 cd red, which flashes in step with the other lights on the structure.
• Medium-intensity (white or red): Structures from 45 m to 150 m. ICAO Type A is flashing white at 20,000 cd by day, reducing to 2,000 cd at night, 20–60 fpm; Type B is flashing red at 2,000 cd, 20–60 fpm; Type C is steady red at 2,000 cd. Dual A/B and A/C units switch white by day to red by night on a photocell.
• High-intensity (white): Structures above 150 m, and cases where an aeronautical study requires it. ICAO Type A: 200,000 cd by day, 20,000 cd at twilight, 2,000 cd at night, flashing at 40–60 fpm. Type B (used for tower and catenary marking) is 100,000 cd by day with the same twilight and night steps.
• FAA equivalents: L-810 low-intensity red; L-864 flashing red (20–40 fpm); L-865 medium-intensity flashing white (40 fpm); L-856/L-857 high-intensity flashing white (40 and 60 fpm respectively).
• Aerodrome beacon: Mounted at the highest practical unobstructed point — control tower roof, dedicated mast or tip-down pole — alternating white and green for a lighted land aerodrome, 24–30 flashes per minute.
• Heliport and water aerodrome beacons: A heliport beacon shows a green–yellow–white sequence at 30–45 flashes per minute; a water aerodrome shows alternating white and yellow; a military airfield shows two quick white flashes between green flashes.

Where a structure carries several flashing lights they are synchronised to flash together, so the outline and height of the obstacle read as one object rather than as unrelated lights. Intermediate levels matter as much as the top light: ICAO caps the spacing between levels at 105 m where medium-intensity Type A lights are used, and at 52 m where Type B lights alternate with low-intensity Type B.

Obstruction lights are mounted at the top of the structure and, on taller masts and towers, at intermediate levels as well, so the outline and height of the hazard remains visible from any direction rather than just marked at its highest point; on structures using a layered intensity scheme, the level of each light is set by the applicable obstacle-marking standard rather than convenience of access. Fixtures are fixed directly to the structure's own steelwork or a dedicated mounting bracket, with power and control cabling run inside or alongside the structure back to a control unit that may be independently powered — sometimes with a solar panel and battery at remote, hard-to-wire sites — rather than tied into the airfield's own AGL circuits. The airport beacon itself is installed at the highest practical, unobstructed point on the airfield, typically the control tower roof or a dedicated mast built for the purpose, mounted so its rotating or flashing beam has a clear 360° sweep with nothing on the structure itself blocking the light in any direction. Because these systems are frequently on independent power and control, their installation includes commissioning the local control unit's day/night switching (typically a photocell) separately from the main AGL circuit switching used for runway and taxiway lighting.

Obstruction lights use incandescent or halogen lamps, xenon discharge tubes for high-intensity strobes, or — now the default for new and retrofit work — high-brightness LEDs. The flash pattern is produced by a control unit that either switches the lamp supply or fires a high-voltage trigger pulse into a discharge tube. Colour comes from a red filter on a white source, or directly from monochromatic red emitters in an LED unit, which is why LED units avoid the filter-fade and filter-cracking failures that older filtered fixtures suffer.

Key functional aspects include:

• Colour & intensity classification: ICAO and the FAA both define low, medium and high intensity classes, each with its own type variants, photometric minima and beam spread requirements. Class selection follows structure height and the finding of any aeronautical study, not the installer’s preference.
• Flash rate: Medium-intensity units flash at 20–60 fpm under ICAO (FAA L-864 red is 20–40 fpm, L-865 white is 40 fpm); high-intensity units at 40–60 fpm. All flashing lights on one structure, and normally across a group of structures, flash in unison.
• Day / twilight / night switching: Switching is three-state, not two. A photocell facing the northern (polar) sky drives the step change: full intensity by day, a reduced twilight step, and a much lower night step — 200,000 / 20,000 / 2,000 cd for a high-intensity Type A unit. Dual units also change colour, white by day to red by night. Getting the twilight step wrong is a common commissioning miss.
• Power supply: Normally a dedicated mains supply at the structure rather than the airfield series circuit; solar LED units with battery storage are used where cabling a remote mast is impractical. A series-circuit feed, where used, goes through an isolation transformer and a circuit adapter.
• Beacon rotation: A rotating beacon carries a two-sided optical assembly — for a civil land aerodrome, one clear lens and one green lens 180° apart — turning at constant speed so an observer sees alternating white and green flashes at 24–30 per minute. Strobe and LED beacons produce the same apparent flash rate and colour sequence electronically, with no moving parts.

An obstruction light or beacon's control logic is local and self-contained rather than networked back to ALCMS in most installations. A control unit generates the required flash pattern — steady-burning for low-intensity red, or a timed flash sequence for medium/high-intensity units, firing a high-voltage pulse for xenon strobes — and where multiple units share a structure, a synchronisation signal keeps their flashes aligned so aircraft see a single coordinated pattern rather than random flickering. A photoelectric cell, sighted at the northern (polar) sky so it is not fooled by direct sun or by nearby lighting, feeds ambient light level back into the control unit, which steps the fixture through its day, twilight and night levels — and, on dual units, from white to red — without operator intervention. Published switching thresholds exist and should be verified rather than assumed: a single L-810 type unit, for instance, turns on when ambient light falls to no less than 35 foot-candles and off when it rises to no more than 60, and many modern units include a self-monitoring circuit that detects lamp or LED failure and reports it — via a dry contact, a wireless link, or a visible local indicator — to maintenance or the ALCMS where such an interface exists.

Obstruction beacon fixtures are built around a lamp or LED source housed within a weatherproof, typically die-cast aluminium or polycarbonate enclosure, with a hardened glass or polycarbonate dome or lens sealed to the housing to achieve an IP65/IP66 rating against rain, dust, and UV exposure at the beacon's typically elevated, exposed mounting location. High-intensity strobe-type beacons use a xenon flash tube or high-output LED array driven by a capacitor discharge or solid-state driver circuit housed in a separate weatherproof enclosure at or near the beacon head, connected by heavy-gauge cable rated for the beacon's peak power demand. Rotating beacon designs incorporate a motor-driven optical assembly within a sealed housing — for a civil land aerodrome, one clear and one green lens 180° apart — with a low-friction bearing supporting the rotating element and slip rings carrying current to the rotating light source. The optical system is designed to put its useful beam roughly 1–10° above the horizon, though the flash remains visible well above and below that band. The mounting bracket or pole attaching the beacon to the structure it marks, whether a chimney, mast, or building rooftop, is corrosion-resistant steel or aluminium engineered to withstand the wind loading and vibration typical of an elevated structure-top installation, with cabling routed through weatherproof conduit down to the power supply and any photocell or timer control gear at the base.