What is selective switching of taxiway lights?
Not every light needs to be on at once. Selective switching lets the control system light only the sections of a route an aircraft should follow, and hold it at a lit stop bar, while the rest of the taxiway stays dark.
Selective switching means that individual sections of taxiway lights, and individual stop bars, can be switched on and off separately rather than as one whole taxiway. It is what turns a taxiway lighting system from a map of lit lines into a way of showing one aircraft its own route.
1Light the route, not the network
At a busy junction, many lit green lines can hide which one an aircraft is meant to follow. With selective switching, only the cleared route is lit, so the pilot follows one visible line. This is the idea behind the "follow the greens" method used with surface movement guidance and control systems.
2Lit up to the stop bar
The route is lit ahead of the aircraft, up to the next stop bar or holding position. The sections behind it can go dark. The section beyond a lit stop bar stays off until the controller switches the bar off, then it lights. This is the interlock covered in the Stop bar control entry, applied along the whole route.
3Each section needs its own switch
For a section to switch on its own, it must be separately controllable. In the field that can mean a separate circuit and regulator per section, a switching unit that isolates a section of a circuit, or individually addressable lights. More independent sections give finer guidance but more circuits, equipment and control to maintain, so the split is a design decision.
4One system holds the logic
The airfield lighting control and monitoring system turns a route into a set of section commands, switches them in step with the aircraft, and reports each section's state back to the controller. The aerodrome decides the default state if control is lost, for example all sections lit.
| Section | State | Why |
|---|---|---|
| Behind the aircraft | Off | Already passed; avoids a second green line |
| Ahead on the route | Lit green | The cleared path to follow |
| Up to a lit stop bar | Lit green, then bar red | Route stops at the holding position |
| Beyond a lit stop bar | Off | No green path past the bar until it is cleared |
| Branches not in use | Off | Removes competing lines at junctions |
Frequently Asked Questions
Q1What is selective switching of taxiway lights?
It is the ability to switch individual sections of taxiway lights, and individual stop bars, on or off separately instead of lighting a whole taxiway at once. The control system then lights only the sections a given route needs.
Q2What does "follow the greens" mean?
It is the guidance method where the centreline lights ahead of the aircraft are lit along its cleared route, up to the next stop bar or the runway holding position, and the sections behind it or off the route are switched off. The pilot simply follows the lit green line.
Q3Why not simply light every taxiway all the time?
Lighting everything is simple and many aerodromes do it. At complex junctions, though, a network of lit green lines can hide which one is the cleared route. Selective switching removes that ambiguity and can also reduce energy use and lamp hours.
Q4How is selective switching done on a series circuit?
Each section that must switch independently needs its own way to be switched. That can be a separate circuit and regulator per section, a switching unit that isolates or shorts out a section of a circuit, or individually addressable lights. The right choice depends on the layout, the cost and the control system, so it is set in the design.
Q5How does it work with stop bars?
Stop bars are selectively switched too, and each is interlocked with the centreline lights beyond it. The route is lit up to a lit stop bar and stays dark beyond it until the controller switches the bar off and lights the next section.
Q6Who decides which sections are lit?
The controller, through the control system. At aerodromes with an advanced surface movement guidance and control system, the route can be selected once and the system switches the sections in turn, but the controller stays responsible for the clearance.
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