What are take-off hold lights (THL)?
Red lights on the runway at the departure position that come on by themselves when it is not safe to take off. They tell a pilot who is in position to hold, even with a take-off clearance in hand.
One of the most hazardous moments on a runway is an aircraft lined up to depart while something else is still on the runway ahead. Take-off hold lights put the warning right in front of the pilot: if the lights ahead are red, it is not safe to take off.
1Two red rows on the runway, ahead of the aircraft
The lights are in-pavement fixtures in two rows either side of the runway centreline, along the first stretch ahead of the departure position. They are on the runway, so they are in the pilot's view at the moment the take-off decision is made.
2Lit by what is actually on the runway
They are driven by surface surveillance, not by a controller's switch. When the logic sees an aircraft in position or starting its roll, and another aircraft or vehicle is on or about to enter the runway ahead, it lights the hold lights. When the runway ahead is clear they go out.
3What the pilot does
Red means do not take off, even with a take-off clearance, and then tell air traffic control. A clearance is an instruction from the controller; the lights show what the system sees on the runway, and the pilot does not resolve a conflict by going.
4What sits behind the lights
The chain is surveillance, then logic, then light control, then the lights. The fixtures are red LED in-pavement lights on circuits that the system switches, with control coming from the RWSL system rather than the tower panel. They work as a pair with the entrance lights on the taxiways.
| Light | Position | Warns | Lit when |
|---|---|---|---|
| Take-off hold lights (THL) | On the runway, either side of the centreline at the departure position | The aircraft in position | It is not safe to take off |
| Runway entrance lights (REL) | On the taxiway, holding position to runway edge | Traffic about to enter or cross | It is not safe to enter or cross the runway |
| Stop bar | Across the taxiway at the holding position | Traffic holding short | The controller has not cleared it to proceed |
Frequently Asked Questions
Q1What are take-off hold lights?
Take-off hold lights, or THLs, are red in-pavement lights on the runway at the departure position. They light automatically when the runway ahead is not safe for take-off, for example because another aircraft or vehicle is on it or about to cross. They are one part of the runway status lights system.
Q2Where are take-off hold lights installed?
On the runway, in two rows of red lights either side of the centreline, ahead of an aircraft that is in position for departure. The exact layout follows the approved design for each aerodrome.
Q3When do take-off hold lights come on?
When the surveillance and logic behind the system see an aircraft in position or starting its roll, and traffic on or about to enter the runway ahead makes take-off unsafe. When the runway ahead is clear they stay off.
Q4What should a pilot do when the hold lights are red?
Do not take off, even if a take-off clearance has been given, and tell air traffic control. If the lights come on during the take-off roll, the crew follows their operating procedures for that situation. The lights show a conflict the system has detected.
Q5How are take-off hold lights different from runway entrance lights?
Runway entrance lights are on the taxiway and warn that it is not safe to enter or cross the runway. Take-off hold lights are on the runway and warn that it is not safe to take off. Both are red, both are switched by the runway status logic, and both are part of the same system.
Q6Do take-off hold lights replace the take-off clearance?
No. They do not give clearances or instructions. They are a safety net on top of the controller's clearance, warning the pilot when what the system sees makes take-off unsafe.
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