Understanding Airport Light Systems

by Hal Davis
WisDOT Bureau of Aeronautics
Published in Midwest Flyer – August/September 2017 issue

Unless you regularly find yourself flying into large airports at night or during inclement weather, chances are there are a number of airport lights you’ve heard of, but never seen in person. For those of us who have the luxury of flying primarily on sunny days, a review of airport lights may then be helpful. This article will cover the various runway lights, taxiway lights, and runway status lights. Watch for the next issue of Midwest Flyer Magazine for a review of the remaining airport lights in what will be the final article in this series on airport navigation aids.

Runway Lights

To start, runway lighting systems are classified as high, medium or low intensity runway lights or HIRL, MIRL, and LIRL respectively. HIRL and MIRL systems have variable intensity controls, typically controlled by air traffic control (ATC) or the pilot using a radio control system. LIRL systems on the other hand usually only have a single intensity setting.

Runway edge lights are designed to help pilots distinguish the edge of the runway. These lights are offset from the physical edge of the runway pavement by 10 feet for runways serving jet aircraft, and 2 feet for other runways. Runway edge lights emit white light; however, on a runway with an instrument approach, runway edge lights emit a yellow light to notify pilots they are approaching the runway end for the last 2,000 feet or last half of the runway, whichever is less.

Runway threshold/end lights are used to identify the runway threshold/end. To do so, a green light is emitted outward from the runway for arriving aircraft, while a red light is emitted toward the runway to indicate the end of the runway for departing aircraft.

White centerline lights are installed at 50-foot intervals along the runway centerline to help pilots with alignment during approach, landing and takeoff. Much like the runway edge lights, centerline lights also change color to warn pilots of the impending runway end. Alternating red and white lights are installed 3,000 feet from the runway end, while all red centerline lights are installed for the last 1,000 feet.

Touchdown zone lights are installed on some precision approach runways to indicate the touchdown zone during landing. These lights consist of two symmetrical columns of light bars emitting white light and spaced on each side of the runway centerline. The two columns of lights extend for one-half the runway length or 3,000 feet, whichever is less.

Taxiway Lights

Much like their runway counterparts, taxiway edge lights are used to outline the edges of taxiways and aprons. Taxiway edge lights are located parallel to the taxiway centerline, not more than 10 feet from the edge of the usable taxiway. In straight sections, taxiway edge lights are typically spaced between 50 and 100 feet depending on the length of the straight section. On curves, taxiway edge lights may be spaced more closely together. As an economical alternative, some airports may use blue reflectors in lieu of or to augment taxiway edge lights.

Taxiway centerline lights emit a steady green light and are logically located along the taxiway centerline. The frequency of the centerline lights depends on the curvature of the taxiway. In those areas where the taxiway centerline leads on/off a runway or into an instrument landing system critical area, the taxiway centerline alternates between green and yellow, beginning from one light beyond the runway holding position marking to the runway centerline.

Safety Focus

Much like airport markings, the advancement in airport lighting has focused on making taxiway/runway intersections more conspicuous to prevent runway incursions. The alternating green and yellow taxiway centerline marking is one such example. Runway guard lights are another.

There are two types of runway guard lights, elevated and in-pavement. Elevated runway guard lights consist of a pair of elevated flashing yellow lights installed on each side of the runway holding position marking. In-pavement runway guard lights consist of a row of alternately illuminated, unidirectional yellow lights located 2 feet prior to the holding side of the runway holding position marking. To ensure the runway guard lights are not obscured by snow, both types of runway guard lights are often installed here in the Midwest. In warmer parts of the country, only one type of runway guard light is typically used.

During periods of low visibility, some airports will implement a surface movement guidance control system. As part of this system, stop bar lights are used to further enhance taxiway/runway intersections. These lights are used by ATC to provide pilots a visual cue for when they are clear to enter or cross an active runway. A stop bar consists of a row of steady red, unidirectional lights, as well as a pair of elevated steady red lights on each end. Stop bar lights are generally located with the runway guard lights. Following clearance to enter the runway, ATC will turn off the stop bar and turn on the taxiway centerline lead-on lights. Pilots should never cross an illuminated stop bar, even if ATC has provided clearance. Instead, notify ATC that the light bar remains illuminated.

Similarly, a clearance bar consists of a row in-pavement, steady burning yellow lights at a low visibility hold point, also typically associated with a surface movement guidance control system. A low visibility hold point consists of a taxiway holding position marking, geographic position marking and a clearance bar. Check out the February/March 2017 issue of Midwest Flyer Magazine if you missed my overview of airport markings, including the taxiway holding position marking and the geographic position marking. Pilots should hold at this location as instructed by ATC.

Runway Status Lights

The most recent airport lighting innovation is the deployment of runway status lights (RWSL). As of 2017, 20 U.S. airports have operational RWSL systems. Included are three airports in the Midwest: Minneapolis-St Paul International Airport, Chicago O’Hare International Airport, and Detroit Metropolitan Wayne County Airport. Through the use of surveillance equipment, runway status lights are able to automatically detect traffic on a particular runway and relay that information to pilots and vehicle operators in real-time via a series of lights. The system is currently comprised of two subsystems – the runway entrance lights (REL) and the takeoff hold lights (THL).

The REL system consists of a series of in-pavement, steady red lights which follow the taxiway centerline and extend from the runway holding position marking to the runway edge. When illuminated, the lights indicate that there is traffic detected on the runway moving at a speed of at least 30 knots or on final approach within 1 mile of the threshold. As the detected traffic approaches the intersection, the lights will extinguish three to four seconds before the aircraft reaches the intersection. This allows ATC the opportunity to expedite clearances based on anticipated separation.

Whereas the REL system is located at runway/taxiway intersections, the THL system is located on the runway itself extending 1,500 feet from the “line up and wait point.” Like the REL, the THL consists of steady red lights in symmetrical pairs with a light on each side of the runway centerline. The THL system illuminates when it senses the presence of another aircraft or ground vehicle on the runway or about to enter the runway.

Should you encounter a RWSL system, it’s important to remember the system is independent of ATC and therefore not a substitute for an ATC clearance to cross, takeoff from or land on a runway. If ATC gives verbal clearance which contradicts the status lights, continue to hold and notify ATC.

Find Out More!

To find out more about airport lights, check out Chapter 2 of the FAA’s Aeronautical Information Manual or Advisory Circular 150/5340-30H.

608-266-3351 www.wisconsindot.gov

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