How does the ALCMS control the taxiway?
Switching taxiway lights by segment to show a route, and stop bars to hold or clear an aircraft — with the true state of each part reported back.
Unlike a runway, which is usually lit as a whole, a taxiway network is often controlled in segments. The ALCMS-to-taxiway link lets the operator light only the part of the taxiway an aircraft needs, so the centreline lights show a route, while the rest stays off. Where stop bars are fitted at holding positions, the ALCMS switches them on to hold an aircraft and off to clear it.
The stop bar and the route lights beyond it are controlled as a pair: while the stop bar is on, the centreline lights beyond it are kept off (ICAO Annex 14 calls for this over at least 90 m), and they are lit only when the stop bar is switched off to clear the aircraft. The ALCMS therefore reports the actual state of each segment and each stop bar instead of assuming a command worked. A stop bar or route light that is not in the expected state is a safety issue, which is why the status feedback matters so much on taxiways.
- Select the route. The operator chooses which taxiway segments are to be lit.
- Hold or clear. Stop bars are switched on to hold and off to clear, per the controller’s instruction.
- Light the way. The CCRs of the chosen segments set their current and the lights come on.
- Confirm. Each segment and stop bar reports its state back to the ALCMS.
Frequently Asked Questions
Q1What is the ALCMS-to-taxiway link?
The control and monitoring path from the ALCMS to taxiway lighting. Commands go to the taxiway circuits, and status comes back for each segment and, where fitted, each stop bar.
Q2Why are taxiway circuits split into segments?
So the ALCMS can light only the part of the taxiway an aircraft needs. Switching segments separately lets a route be shown and the rest of the taxiway left off.
Q3What does the ALCMS do with stop bars?
Where stop bars are fitted, it switches them on to hold an aircraft at a holding position and off to let it proceed, according to the controller's instruction and airport procedures.
Q4How are stop bars and the lights beyond them related?
They are controlled as a pair. While the stop bar is on, the centreline lights beyond it are kept off. The stop bar is switched off to clear an aircraft and the lights beyond it are lit to lead it on, so the control system and its status must confirm both parts.
Q5Why does status matter more here?
A stop bar or route light that is not in the state the controller expects is a safety issue. The ALCMS therefore reports the actual state of each segment and stop bar rather than assuming the command worked.
Q6Does every airport control taxiways this way?
No. The level of selective switching, whether stop bars are fitted and how they are operated depend on the airport's layout, visibility procedures and design.
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