Per reader request; we’ll jump straight to the question. “Who changes—or how does the system change—the IFR flight plans I file?” When filing IFR, you have the option to file the exact route you want. However, in doing so, you might or might not get it. A route in the ATC system depends on a lot of things: type of aircraft, altitudes, equipment, and what you file. All these variables are fed into a grand equation that ends up producing your final route, which might not be what you want at all.
Now to clarify, we’re talking about normally used routes that are meant to be flown with minimal re-routing due to weather, active MOAs, etc. Obviously with those items in the equation, you often get a re-route, but it will most likely be given to you en-route and not by Clearance before departure. But wait. How on earth does this “system” figure this all out? The original question has two parts, so the answer will as well.
