What is the Runway Status Light (RWSL) system and what does it do?

2-1-6. Runway Status Light (RWSL) System
a. Introduction. RWSL is a fully automated system that provides runway status information to pilots and
surface vehicle operators to clearly indicate when it is unsafe to enter, cross, takeoff from, or land on a runway.
The RWSL system processes information from surveillance systems and activates Runway Entrance Lights
(REL) and Takeoff Hold Lights (THL), in accordance with the position and velocity of the detected surface traffic
and approach traffic. REL and THL are in-pavement light fixtures that are directly visible to pilots and surface
vehicle operators. RWSL is an independent safety enhancement that does not substitute for or convey an ATC
clearance. Clearance to enter, cross, takeoff from, land on, or operate on a runway must still be received from
ATC. Although ATC has limited control over the system, personnel do not directly use and may not be able to
view light fixture activations and deactivations during the conduct of daily ATC operations.
b. Runway Entrance Lights (REL): The REL system is composed of flush mounted, in-pavement,
unidirectional light fixtures that are parallel to and focused along the taxiway centerline and directed toward the
pilot at the hold line. An array of REL lights include the first light at the hold line followed by a series of evenly
spaced lights to the runway edge; one additional light at the runway centerline is in line with the last two lights
before the runway edge (see FIG 2-1-9 and FIG 2-1-10). When activated, the red lights indicate that there is
high speed traffic on the runway or there is an aircraft on final approach within the activation area.
1. REL Operating Characteristics - Departing Aircraft: When a departing aircraft reaches a site adaptable
speed of approximately 30 knots, all taxiway intersections with REL arrays along the runway ahead of the aircraft
will illuminate (see FIG 2-1-9). As the aircraft approaches an REL equipped taxiway intersection, the lights at
that intersection extinguish approximately 3 to 4 seconds before the aircraft reaches it. This allows controllers
to apply "anticipated separation" to permit ATC to move traffic more expeditiously without compromising
safety. After the aircraft is declared "airborne" by the system, all REL lights associated with this runway will
extinguish.
2. REL Operating Characteristics - Arriving Aircraft: When an aircraft on final approach is approximately
1 mile from the runway threshold, all sets of taxiway REL light arrays that intersect the runway illuminate. The
distance is adjustable and can be configured for specific operations at particular airports. Lights extinguish at
each equipped taxiway intersection appr oximately 3 to 4 seconds  before the aircraft reaches it to apply
anticipated separation until the aircraft has slowed to approximately 80 knots (site adjustable parameter). Below
80 knots, all arrays that are not within 30 seconds of the aircraft's forward path are extinguished. Once the arriving
aircraft slows to approximately 34 knots (site adjustable parameter), it is declared to be in a taxi state, and all
lights extinguish.
3. What a pilot would observe: A pilot at or approaching the hold line to a runway will observe RELs
illuminate and extinguish in reaction to an aircraft or vehicle operating on the runway, or an arriving aircraft
operating less than 1 mile from the runway threshold.
4. When a pilot observes the red lights of the REL, that pilot will stop at the hold line or remain stopped.
The pilot will then contact ATC for resolution if the clearance is in conflict with the lights. Should pilots note
illuminated lights under circumstances when remaining clear of the runway is impractical for safety reasons (for
example, aircraft is already on the runway), the crew should proceed according to their best judgment while
understanding the illuminated lights indicate the runway is unsafe to enter or cross. Contact ATC at the earliest
possible opportunity.
Airport Lighting Aids 2-1-7

 AIM 2/20/25
FIG 2-1-9
Runway Status Light System
c. Takeoff Hold Lights (THL) : The THL system is composed of flush mounted, in-pavement, unidirectional
light fixtures in a double longitudinal row aligned either side of the runway centerline lighting. Fixtures are
focused toward the arrival end of the runway at the "line up and wait" point. THLs extend for 1,500 feet in front
of the holding aircraft starting at a point 375 feet from the departure threshold (see FIG 2-1-11). Illuminated red
lights provide a signal, to an aircraft in position for takeoff or rolling, that it is unsafe to takeoff because the
runway is occupied or about to be occupied by another aircraft or ground vehicle. Two aircraft, or a surface
vehicle and an aircraft, are required for the lights to illuminate. The departing aircraft must be in position for
takeoff or beginning takeoff roll. Another aircraft or a surface vehicle must be on or about to cross the runway.
1. THL Operating Characteristics - Departing Aircraft: THLs will illuminate for an aircraft in position for
departure or departing when there is another aircraft or vehicle on the runway or about to enter the runway (see
FIG 2-1-9.) Once that aircraft or vehicle exits the runway, the THLs extinguish. A pilot may notice lights
extinguish prior to the downfield aircraft or vehicle being completely clear of the runway but still moving. Like
RELs, THLs have an "anticipated separation" feature.
NOTE-
When the THLs extinguish, this is not clearance to begin a takeoff roll. All takeoff clearances will be issued by ATC.
2. What a pilot would observe: A pilot in position to depart from a runway, or has begun takeoff roll, will
observe THLs illuminate in reaction to an aircraft or vehicle on the runway or entering or crossing it. Lights will
extinguish when the runway is clear. A pilot may observe several cycles of illumination and extinguishing
depending on the amount of crossing traffic.
3. When a pilot observes the red light of the THLs, the pilot should safely stop if it's feasible or remain
stopped. The pilot must contact ATC for resolution if any clearance is in conflict with the lights. Should pilots
note illuminated lights while in takeoff roll and under circumstances when stopping is impractical for safety
2-1-8 Airport Lighting Aids

2/20/25 AIM
reasons, the crew should proceed according to their best judgment while understanding the illuminated lights
indicate that continuing the takeoff is unsafe. Contact ATC at the earliest possible opportunity.
d. Pilot Actions:
1. When operating at airports with RWSL, pilots will operate with the transponder/ADS -B "On" when
departing the gate or parking area until it is shut down upon arrival at the gate or parking area. This ensures
interaction with the FAA surveillance systems such as ASDE-X/Airport Surface Surveillance Capability (ASSC)
which provide information to the RWSL system.
2. Pilots must always inform the ATCT when they have stopped due to an RWSL indication that is in conflict
with ATC instructions. Pilots must request clarification of the taxi or takeoff clearance.
3. Never cross over illuminated red lights. Under normal circumstances, RWSL will confirm the pilot's taxi
or takeoff clearance previously issued by ATC. If RWSL indicates that it is unsafe to takeoff from, land on, cross,
or enter a runway, immediately notify ATC of the conflict and re-confirm the clearance.
4. Do not proceed when lights have extinguished without an ATC clearance. RWSL verifies an ATC
clearance; it does not substitute for an ATC clearance.
e. ATC Control of RWSL System:
1. Controllers can set in-pavement lights to one of five (5) brightness levels to assure maximum conspicuity
under all visibility and lighting conditions. REL and THL subsystems may be independently set.
2. System lights can be disabled should RWSL operations impact the efficient movement of air traffic or
contribute, in the opinion of the assigned ATC Manager, to unsafe operations. REL and THL light fixtures may
be disabled separately. Whenever the system or a component is disabled, a NOTAM must be issued, and the
Automatic Terminal Information System (ATIS) must be updated.