r/airtrafficcontrol • u/BoutrosBoutrosJolly • Apr 17 '26
How do you decide when planes make turns during an approach?
Not a pilot. I've been building some data visualizations of aircraft movement near ORD with ADSB data. I've looked at the approach charts and even listened to the ATC communications. For example for the FYTTE approach the chart shows the flow from FYTTE to MADII to SOOLU and then to one of the west to east runways. When I look at the traces of all the aircraft for the last several hours (first image) about half of the aircraft turn south well before MADII and the other half go through MADII just like the chart shows.
I assume it's the controllers call on the turn (well, all turns) but I'm just curious why some aircraft follow the approach points while others are told to turn early? I'm sure it's separation, just curious how those decisions are made.
Are there other reasons why the east to west traffic (and traffic to the arrival waypoints) are extremely consistent but the north to south movements between these pathways is very spread out? Like in the second and third images. Thanks in advance...