How does follow-the-greens work?
The lit window that moves with the aircraft: only the green lights ahead on the assigned route are on, lights behind go out, other branches stay dark, and a red stop bar always wins.
Follow-the-greens is the guidance function of an A-SMGCS in operation. For each aircraft, the system lights only the taxiway centreline segments ahead on the assigned route. As the aircraft moves, the lit window rolls forward, the lights behind go out, and branches that are not on the route stay dark.
A lit stop bar always wins. The lights beyond it stay off and the aircraft holds until the bar is switched off. Stop bars on routes the aircraft is not using stay on, so a wrong turn still meets a red bar.
If the pilot sees no green lights ahead, the pilot stops and tells ATC. The system also needs a safe fallback, with ATC taking over the switching, if surveillance, routing or the link to the ALCMS fails.
| Phase | What the lights do | What the pilot does |
|---|---|---|
| Taxi clearance | The first segments of the assigned route come on | Confirms the route with ATC and follows the green lights |
| Straight section | The lit window moves with the aircraft; lights behind go out | Follows the lit centreline |
| Junction | Only the branch on the route is lit; other branches stay dark | Takes the lit branch and never an unlit one |
| Crossing or hold point | A stop bar comes on; lights beyond it go off | Stops at the bar and waits for it to go out |
| Route change by ATC | New segments come on and old ones go out | Gets a new clearance before moving on |
| No green ahead | The lights are missing, wrong or conflicting | Stops and tells ATC |
| Design parameter | What it sets | Design consideration |
|---|---|---|
| Look-ahead length | How much of the route ahead is lit | Long enough to see the next lights and to stop, for the lowest planned visibility |
| Segment length | How finely the route can be switched | Short enough to switch cleanly, with ends at junctions and stop bars |
| Switch-off delay behind | When the lights behind go out | Short delay so lights do not go out under or beside the aircraft |
| Update speed | How fast the lights follow the aircraft | Position data, link and switching must keep up with taxi speed |
| Fallback | What happens on a failure | Defined safe state, ATC manual control, stop bars not left off |
Frequently Asked Questions
Q1What is follow-the-greens?
It is a way of guiding an aircraft on the ground by lighting only the green taxiway centreline lights along its assigned route, a short distance ahead of it. The pilot follows the green lights to the destination. The lights behind go out, and branches that are not on the route stay dark. It is the guidance function of an A-SMGCS, and it is most useful in low visibility.
Q2Who or what decides which segments are lit?
The A-SMGCS does. It knows the aircraft position from surveillance and the route from the routing function, and it sends switching commands to the airfield lighting control and monitoring system. ATC supervises, can change the route or take over the switching by hand, and decides when the stop bars are switched off.
Q3Why light only part of the route?
A short lit section gives the pilot one clear thing to follow, and a dark branch tells the pilot that it is not the way. It also lets the system keep a conflicting taxiway or a hold clearly marked. The look-ahead has to be long enough for the pilot to see the next lights and to stop in time, so it is set for the lowest visibility the aerodrome plans to use.
Q4How do stop bars work with it?
A lit stop bar overrides the green lights. The lights beyond it stay off, and the aircraft holds until ATC, or the system under ATC control, switches the bar off. Stop bars on other routes stay on, so a wrong turn into a runway is still blocked. Crossing traffic can also light a stop bar short of the crossing.
Q5What should a pilot do if the green lights ahead go out?
Stop, and tell ATC. A missing or conflicting indication is treated as a reason to hold, not to guess. Aerodrome procedures describe the exact phraseology and what ATC does next, and the pilot should not move on a route that is not lit.
Q6What must the airfield lighting system support?
The centreline must be switchable in short segments, which needs separately controlled circuits or individually controlled lamps, depending on the design. The control system needs a fast and reliable interface to the A-SMGCS and must report the real state of each segment and stop bar back, so that a lamp or a link failure is noticed. See the selective switching entry for the circuit side.
Q7What happens on failure, and is it safe to work on?
The aerodrome defines a safe fallback, normally with ATC taking over manual switching and the SMGCS procedures applying, and with stop bars not left off by a lost link. Changes to segments, look-ahead, delays or the interface alter what pilots see in the worst conditions, so they need ATC and authority agreement, a permit-to-work and a documented test.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.