Every airport has more runways than it uses at any moment. Which one is "in use" is one of the most consequential decisions in air traffic control, and it is almost never posted anywhere a flight tracker can read. Here is how controllers choose it, why it changes, and how ATC Guy works it out from the traffic itself.
An aeroplane's wings care about airspeed, not ground speed. Landing into a headwind lowers the speed over the ground, shortens the landing roll and makes the approach more stable; a tailwind does the opposite. Most operators limit tailwind to 10 knots, many to 5, and a wet runway lowers that further. So the first question is simply: which runway points most nearly into the wind? Runway numbers are magnetic headings divided by ten — runway 27 points roughly 270°, west — so a wind from 250° favours 27 and rules out 09.
At big airports the wind is only the starting point. Heathrow alternates its landing runway at 15:00 every day to share noise between west-London suburbs. Zurich flips between northern, eastern and southern concepts by time of day under a treaty with Germany. Amsterdam picks from six runways using a preference table that ranks combinations by how many people they overfly. Many airports also apply a "preferential runway" rule: use the quiet option until the tailwind component exceeds a threshold, then switch.
Where runways are parallel, controllers choose configurations that maximise movements: segregated mode (one runway lands, the other departs) at Heathrow and Dubai, triple simultaneous approaches at Atlanta when weather demands instrument arrivals, mixed mode when demand is lower. Crossing-runway airports like JFK choose pairs that let departures roll while arrivals are still on final, timed to the gaps.
A runway change is a small production. The supervisor picks a moment with a gap in arrivals, departures already lined up are re-sequenced, and inbound aeroplanes are re-routed to a different final — sometimes with a hold. It happens a few times a day at most airports, and it is why a tracker that "assumes" the runway from the wind is wrong a surprising fraction of the time.
ATC Guy does not look at the wind at all. It watches the traffic. Every aeroplane that is low, slow and close to the airport is a vote: its track is compared with each runway heading, and the runway that collects three or more votes wins. That handles the simple case — but parallel runways share a heading, so track alone cannot separate 34L from 35R at Istanbul or 27L from 27R at Heathrow.
For that, the scope clusters the final-approach traffic laterally: it measures how far each aeroplane sits across-track from the field centre, groups them, and treats each cluster 0.8 NM or more apart as a separate landing stream. Each stream is then anchored to the runway whose centreline it actually follows. When Istanbul is landing on 34L and 35R at once, two clusters appear and both corridors light up; when Heathrow switches at 15:00, the cluster moves to the other runway within a few arrivals. The measurement is locked on the first good sample of a session so the corridors do not wander under the traffic you are already vectoring.
Because you are landing real aeroplanes onto the runway they are really using. The approach corridor you line them up on is the one the real crews are flying; the runway assignment on the data block matches the world. It is the difference between a flight tracker with a game skin and a radar scope you can trust.
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