I've just uploaded some new videos to the official X-Plane YouTube video channel. These videos are screencast tutorials for airport authors to help them understand the ATC Airport Flow feature in X-Plane v10.
ATC Airport Flows are essentially a set of rules that control how the runways are used for airport operations. An airport like Chicago's O'Hare (KORD) for example has 7 runways (14 different takeoff/landing directions)! ATC does not simply put aircraft wherever they feel like in the moment or there'd be a massive aluminum traffic jam. They have a set of rules that control which runway(s) are in active and inactive at any given time. These rules are typically based on two main criteria: weather conditions and time of day.
In the real world, at major airports, traffic studies are done to decide which runway combinations are most efficient for traffic flow, safety and workload and those combinations of runways become active when the conditions are just right.
Once the controller deems certain runways active/inactive for the current conditions, there are yet more rules to determine which types of aircraft use each of those runways. For example, if a small Pilatus is flying into KORD, I strongly doubt they're going to block up their major runways for arrivals for a small single engine turbo prop. They're likely to put him on a smaller accessory runway. Also consider some airports which only use certain runways for arrivals while other runways are only used for departures. This is often done for noise abatement or obstacle avoidance. These types of rules can be included in the ATC Airport Flow.
Our goal was to give authors enough granularity to closely mimic the way real airports are run so that when X-Plane's ATC is in action, it's towers can make similar decisions to the real controllers.
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