Airport Lighting Explained: What All Those Lights Mean

Approach Lighting Systems: Guiding the Descent

Approach lighting systems (ALS) are arrays of high-intensity lights installed on the ground before the runway threshold, extending up to 900 meters (3,000 feet) ahead of the touchdown zone on precision approach runways. Their purpose is crucial: to provide pilots transitioning from instrument flight (heads-down, following cockpit displays) to visual flight (looking outside for the runway) with an immediately identifiable visual reference aligned with and centered on the runway. In low visibility conditions — approaching minimums — the approach lights are often the first visual cue that confirms the runway environment is actually where the instruments said it would be.

The most common approach lighting configuration at major US airports is the ALSF-2 (Approach Light System with Sequenced Flashers), which includes a centerline of lights extending 900 meters ahead of the threshold with crossbars at 150 and 300 meters, plus sequenced flashers that create the appearance of a ball of light "running" toward the runway at 2 flashes per second (called the "rabbit" by pilots). This dynamic element is remarkably effective at capturing attention in low-visibility environments, drawing the eye down the approach path. European airports typically use ICAO-standard CALVERT systems with a similar philosophy but different crossbar geometry.

The PAPI (Precision Approach Path Indicator) provides glidepath guidance on visual and most instrument approaches. Four lights are installed in a row to the left of the runway threshold. Each light has a color-change mechanism calibrated so that it appears white when seen from above the glidepath angle and red when seen from below. The standard PAPI is set to a 3-degree glidepath. The combinations of red and white lights tell the pilot their position: four white means high; three white and one red means slightly high; two white and two red means on the glidepath; one white and three red means slightly low; four red means dangerously low. VASI (Visual Approach Slope Indicator), an older design using only two boxes of lights, follows the same principle but with less precision.

Runway Lighting: The Threshold, Edges, and Touchdown Zone

Runway threshold lights are a row of green lights across the width of the runway at its beginning (threshold), marking where the landing zone starts. Paired with the green threshold lights at one end are red lights visible from the opposite direction, warning departing aircraft that they are approaching the end of the runway. This dual-function design — green on the threshold for arrivals, red on the same fixtures seen from behind for departures — elegantly conveys critical safety information with minimal infrastructure.

Runway edge lights define the boundaries of the paved surface. White edge lights run the full length of the runway on both sides, spaced every 60 meters (200 feet). In the last 600 meters before the runway end, edge lights transition from white to yellow, providing a warning that the runway is ending. The runway end lights — red lights perpendicular to the runway at its far end — are the final visual warning before the paved surface terminates. During Category II and III low-visibility operations, a complete set of centerline lights running down the middle of the runway is activated: these are white from the threshold, then alternating red-and-white in the last 300 meters, and all red in the last 90 meters — a color-coded countdown to the runway end that is critical when pilots may not be able to see far ahead.

Touchdown Zone lights (TDZs) are rows of transverse white lights extending 900 meters from the runway threshold on each side of the centerline. These indicate where aircraft should aim to touch down — the optimal zone that provides maximum runway remaining for braking while being well beyond the threshold. Runway centerline lights (on precision approach runways) provide immediate guidance along the centerline during the landing rollout and any low-visibility takeoff, a period when jet blast and spray can further reduce visibility and precise directional control is essential.

Taxiway Lighting: Navigating the Ground Network

Once an aircraft is on the ground, a entirely different set of lights guides it through the taxiway network. Taxiway edge lights are blue — a distinctive color that differentiates taxiways from runways instantly. Blue is used because it is not found naturally in the airport environment at night and is visually distinctive from the white runway lighting. Taxiway centerline lights are green, providing a follow-the-green guidance system that allows pilots to navigate complex taxiway junctions by simply following the green centerline lights toward their assigned gate area.

Stop bars are one of the most safety-critical lighting elements on the airport surface. Rows of red in-pavement lights across taxiway intersections with runways, stop bars can be illuminated or extinguished by the ground controller. When a stop bar is illuminated (red lights visible to the approaching aircraft), the pilot must stop and await explicit clearance to cross. When the controller clears the aircraft to cross or enter the runway, the stop bars extinguish and the centerline lights ahead illuminate, creating an unambiguous green-means-go, red-means-stop visual signal. During low-visibility operations, stop bars are the primary technical safeguard against runway incursions — situations where an aircraft enters an active runway without clearance.

Runway guard lights (formerly called wig-wags) are pairs of alternating amber flashing lights installed on either side of a taxiway just before a runway intersection. Unlike stop bars (which require controller action to activate), runway guard lights are always on at active runway intersections, providing a persistent warning that a runway is ahead. The combination of runway guard lights at the taxiway edge and stop bars at the hold short line creates two independent layers of visual warning before any runway crossing.

Obstruction Lighting and Approach Slope Indicators

Aircraft must navigate not only the airport surface but also the three-dimensional obstacle environment around an airport. Tall structures — antenna towers, buildings, wind turbines, cranes — within the vicinity of airports and along approach/departure paths must be lit to make them visible to pilots at night. Low-intensity obstacles below 45 meters (150 feet) use steady red lights. Medium-intensity obstacles use flashing red or white lights. Very tall structures above 150 meters use high-intensity white strobe lights that are visible many miles away in all weather conditions.

The Instrument Landing System (ILS) glide slope transmitter sits beside the runway about 300 meters beyond the threshold, its antenna aligned precisely to project the 3-degree glide path. While the ILS signal itself is invisible, the glide slope ground station is surrounded by a critical area where vehicles and aircraft must not enter during ILS operations, to prevent reflections that could corrupt the signal and give pilots incorrect glidepath information. Yellow zigzag markings on the taxiway pavement delineate the ILS critical area hold points.

Displaced threshold markings deserve specific mention. When a runway's threshold is displaced — moved down the runway from its physical beginning due to obstacles, pavement strength limitations, or noise considerations — the area before the threshold is marked with large white arrows pointing toward the actual threshold, and the non-threshold pavement is marked with chevrons (V-shaped markings open toward the runway end). This area may be used for taxiing and takeoff roll but not for landing. Understanding displaced thresholds is important for anyone learning to read airport diagrams, as a displaced threshold changes the effective landing distance available on that runway even though the physical pavement begins earlier.