What are runway status lights (RWSL)?
Red lights that come on by themselves when a runway is not safe to enter, cross or take off from. Surface surveillance, not the controller, decides when they light, so they back up the clearance rather than replace it.
Runway incursions often involve two people believing different things about where traffic is. Runway status lights remove the guesswork with a signal the pilot cannot misread: if the lights in front of the aircraft are red, the runway is not safe.
1Driven by surveillance, not by the controller
The system takes position data from surface surveillance, such as radar, multilateration or ADS-B, tracks aircraft and vehicles, and works out whether the runway is clear. No controller has to switch the lights, and the controller can supervise the system and turn it off.
2Red lights where the danger is
Runway entrance lights on the taxiway show that the runway ahead is not safe to enter or cross. Take-off hold lights on the runway at the departure position show that it is not safe to take off. Runway intersection lights do the same ahead of an intersecting runway. The next entries cover runway entrance lights and take-off hold lights.
3A second layer, not a replacement
RWSL does not give clearances. It works alongside stop bars, runway guard lights and the controller's instructions. If the red lights are lit, the pilot stops or does not proceed, even with a clearance, and tells air traffic control. Lights that are off carry no meaning: they do not show the runway is clear, and the clearance still comes from air traffic control.
4What sits behind the lights
The chain is surveillance, then logic, then light control, then the lights. The in-pavement lights are red LED fixtures on circuits switched by the logic, so the same selective-switching ideas from the taxiway entries apply, with the control coming from the RWSL system rather than the tower panel.
| Light | Where | Lit when |
|---|---|---|
| Runway entrance lights (REL) | Taxiway centreline from the hold line to the runway edge, plus one light on the runway centreline | It is unsafe to enter or cross the runway |
| Take-off hold lights (THL) | Runway centreline at the departure position | It is unsafe to take off |
| Runway intersection lights (RIL) | Runway centreline ahead of an intersection | Traffic on the intersecting runway makes it unsafe to continue |
Frequently Asked Questions
Q1What are runway status lights?
Runway status lights, or RWSL, are an automatic system of red in-pavement lights that show a pilot when a runway is not safe to enter, cross or depart from. Surface surveillance detects aircraft and vehicles and the system lights the red lights without the controller having to switch them.
Q2How is RWSL different from a stop bar?
A stop bar is switched on and off by the controller as part of a clearance. RWSL is driven automatically by surveillance data about where traffic actually is, and is a second layer of warning on top of the clearance. Many aerodromes use stop bars, some add RWSL, and the two can both be present.
Q3What should a pilot do when the red lights are on?
Stop, or do not proceed onto or along the runway, even if a clearance has been given, and tell air traffic control. The lights show a conflict the system has detected, and the pilot does not resolve it by continuing.
Q4Which lights make up RWSL?
The main types are runway entrance lights on taxiways leading to the runway, take-off hold lights on the runway at the departure position, and runway intersection lights ahead of an intersecting runway. The next entries cover runway entrance lights and take-off hold lights.
Q5Does RWSL replace air traffic control?
No. It does not issue clearances or give instructions. It is a safety net that warns pilots when what the surveillance system sees does not match a safe situation, while the controller still separates traffic and issues clearances.
Q6Is RWSL part of ICAO Annex 14?
RWSL was developed by the FAA and is installed at selected US airports, and similar entrance-light systems are used in some other countries. ICAO Annex 14 standardises stop bars and runway guard lights for similar purposes, so check which of these your project or aerodrome is designed to.
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