Runway status lights (RWSL)

Implemented at Paris Charles de Gaulle (CDG) airport

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Runway Status Lights (RWSL) is a fully automatic, advisory safety system designed to reduce the number and severity of runway incursions and thus prevent runway accidents while not interfering with airport operations.

The RWSL increases pilots and vehicle operators' situational awareness by directly providing runway occupancy status through the autonomous illumination of in-pavement lights on runway and taxiways. The concept of operations relies on the ability to warn at least one of the aircraft or vehicles in a conflicting scenario and in some cases, provide additional warnings to everyone involved for increased safety.RWSL is designed to be compatible with existing procedures.


The Runway Status Lights (RWSL) concept uses fused surveillance data processed through complex state and safety logic. This safety information is then automatically conveyed to the pilots and vehicle operators via three types of in-pavement lights on the airport surface:

Runway Status Lights in-pavement lightsRunway Entrance Lights (RELs) are placed at runway/taxiway intersections (one line of lights, close to taxiway centreline, from CAT 1 holding point, located at about 90m from RWY centreline, up to the runway) being visible to pilots and vehicle drivers taxiing toward runways. It warns pilots and vehicle drivers it is unsafe to enter or cross a runway because it is, or soon will be, occupied by high-speed traffic such as an aircraft taking off or landing.

Note: When LVP are in force at CDG airport, the REL behaviour is not changed. Note that RELs are not available between CAT 3 holding points (located at about 150m from RWY centreline) used in that case and CAT1 holding points.

Takeoff Hold Lights (THLs) are placed on runways at departure positions (a double line of lights on a length of about 450m) and are visible to pilots in takeoff position. It warns pilots it is unsafe to takeoff because the runway ahead is occupied by another aircraft or vehicle.

Runway Intersection Lights (RILs) are placed on runways approaching a runway/runway intersection. It warns pilots in a takeoff or landing roll that the intersection ahead is unsafe to enter or cross because a conflict exists.

Note: The Runway Intersection Lights are not applicable to the CDG airport.

The concept operates in four stages:

Surveillance Input: The RWSL system utilises existing airport surveillance technology, in conjunction with advanced data fusion techniques and state logic, to automatically drive the status lights on the airport’s surface. RWSL has knowledge of the location of each aircraft and vehicle on the airport surface and arriving or departing the airport based on information provided by Surface Movement Radars (SMRs), Airport Surveillance Radars, GPS and the time difference of arrival multilateration utilizing interrogation and replies from transponder-equipped aircraft.

Surveillance Data Fusion: RWSL fusion uses all input sources available to create “clean” system tracks that are finally processed through the state machine and light logic. It allows for system tracks to be generated from single or multiple surveillance sources to address the issue of malfunctions or non-existence of sensors or equipment. This is an extremely valuable feature that greatly increases RWSL’s flexibility in providing a safety benefit in variable situations.

RWSL Processor: RWSL safety logic process accepts the fused surveillance, determines the operational state of the track (e.g., stopped, taxiing, landing, or departing), predicts likely future behaviour based on the current state, and determines when and which lights should be illuminated. Location of traffic and their dynamics states drive the decision-making process for light illuminations.

Table REL Status for departure procedure
Aircraft Behaviour Stopped on the runway awaiting takeoff clearance Begins departure roll Transition to high-speed operation (> 25-30 kts) Passes taxiway intersections Rotates and begins climbing
Track state in RWSL Processor STP (Stopped) TAX (Taxi) DEP (Departure) DEP (Departure) AIR (Airborne)
RELs OFF  OFF  ON OFF at no threat locations, ON downfield All locations OFF

Light Control Computer: Using output data provided from the surveillance, fusion, and safety logic processes, the Light Control Computer (LCC) will then communicate with the Field Lighting System (FLS) to activate and de-activate lights installed on and around the equipped runways. Illumination of these lights will indicate to pilot or vehicle operators the status of the runway or runway intersections.

Protocol for Operations

The pilot and vehicle operator compliance with established protocol for viewing illuminated red status lights is imperative for the system to be effective. Users of the system are trained to respond to Runway Status Lights in the following ways:

Runway Entrance Lights (RELs):

  • When RELs are red, the runway is unsafe to enter or cross and pilots /vehicle drivers should stop immediately.
  • When the lights are off, pilots/vehicle operators shall not enter or cross the runway without ATC clearance. 

Note: When LVP are in force at CDG airport, the REL (available from CAT 1 holding points up to the RWY) behaviour is not changed, and the above protocol for operators is the same (If RELs illuminate red, stop immediately and contact ATC). When LVP are in force the CAT 3 holding points are used and stop bars located at these holding points are active. These stop bars should not be mistaken for RELs; stop bars are operated by the tower controller and their switching off should always be associated with an ATC clearance, whereas RELs are fully automated, and are an additional safety measure.

Takeoff Hold Lights (THLs):

  • If lined-up and waiting on the runway and THLs are red, the runway is not safe for takeoff and pilots should remain in position for takeoff.
  • If the takeoff roll has begun and the THLs turn red, pilots should safely stop the aircraft and notify ATC that they have stopped because of red lights.

In all cases, if following the established protocol is not safe, pilots should proceed according to their best judgment of safety (understanding that illuminated RWSL indicates a potential conflict exists) and contact ATC at the earliest opportunity. When RWSL lights contradict air traffic clearances, pilots and vehicle operators are trained to respond first to the status lights since they are intended to serve as a backup safety net when the humans in the loop make errors. Conversely, pilots and vehicle operators are trained that RWSL is an advisory safety system only and that RWSL off NEVER should be perceived as an air traffic control clearance. RWSL only has two states:

  • On – lights are red;
  • Off – lights are switched off.

Note: The RWSL never illuminates green lights to convey safety or clearance.

Training Scenarios

To help pilots and vehicle drivers be prepared to respond correctly to an airport equipped with Runway Status Lights below you can find four scenarios to help train and test your knowledge of the system. Once you are certain that the information in these scenarios is understood you can test yourself via our Self-evaluation tests for Pilots and Vehicle Drivers.

Scenario 1: Multi-Crossing-Take-Off Scenario (Under Nominal Conditions) 

The Scenario 1 shows the basic behavior of the RWSL system on the RWY 27L and its vicinity. This scenario illustrates how the THL and the REL systems work under runway crossing and take-off cases. It illustrates a typical situation that you will most likely see on CDG RWYs. Download Scenario 1.

Scenario 2: Multiple Line-Ups Take-Off & Crossing Scenario (Under Nominal Conditions)

The Scenario 2 shows the behaviour of the RWSL system on the RWY 27L and its vicinity. This scenario illustrates how the THL and the REL systems work under runway multiple line-ups take-off and crossing. Download Scenario 2.

Scenario 3: Landing-Crossing-Take-Off Scenario (Under Nominal Conditions)

The Scenario 3 shows the behaviour of the RWSL system on the RWY 27L and its vicinity. This scenario illustrates how the THL and the REL systems work in an approach case, runway crossing and take-off line-up case. It illustrates the use of the inner RWY in mixed mode (departures and arrivals on the inner RWY), and with a vehicle. The scenario has been planned with one landing case (demonstrating the REL system), one RWY crossing case, once one airplane has been already lined up on the RWY (demonstrating the THL system) and then, with one take-off case (demonstrating the REL system). Download Scenario 3.

Scenario 4: Aborted Departure Scenario (Under Nominal Conditions)

The Scenario 4 shows the behaviour of the RWSL system on the RWY 27L and its vicinity. This scenario demonstrates how the THL and the REL systems work under an aborted departure case. The scenario has been planned with one take-off case when one airplane is crossing the runway forcing to abort the take-off procedure (demonstrating the THL system). Two airplanes are involved in the sequence of events (one airplane taking-off and another crossing the RWY 27L). Download Scenario 4.

Test yourself

Was the information above clear? If yes test yourself using our Self-Evaluation Tests.


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Runway Status Lights Support team

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