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I am trying to find out in layman's terms how does ATC determine the direction a flight has to take after takeoff, or when should it start descending for the destination airport. I understand it has to do with the flight plan but Googling revealed lot of technical stuff that I couldn't understand. In my limited experience of listening to LiveATC and YouTube video, ATC never inquired about the destination - it felt like they already knew.

Is destination information always available on the screen to ATC or does the flight crew request changes based on their flight plan? How does it work when you cross international boundaries? Is the same flight plan available to ATC in all the countries the flight will be flying through?

For example, a flight taking off from SFO could be northbound towards Seattle or southbound to LA or eastbound. ATC has to make room for the proper direction of the flight and hence needs to know the destination.

To complicate matters, I have seen flights taking different routes while the destination is same e.g. one SFO-FRA flight hugged the west coast (magnificent views of the coastline) up to Canada before making a right turn, whereas other times - but with the same flight number - it kind of crosses the continental United States diagonally. So I feel it's not just about the destination, there are other factors involved.

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Under instrument flight rules (which all scheduled passenger flights operate under), the pilot will file a flight plan with the local air traffic control authority (in the US, that is the FAA). This flight plan describes in fairly specific terms where the pilot intends to fly, by going from point to point. You can see flight plans on a site like https://flightaware.com.

These flight plans are entered (electronically, in most cases, but sometimes typed in) into the air traffic control system. So each controller who handles the flight knows where it is supposed to go next.

The details of the start and end of the flight isn't determined by the pilot, but by ATC when they set up the departure or arrival. These are done using instructions called SID (Standard Instrument Departure) and STAR (Standard Terminal Arrival Route). Depending on the traffic patterns around each airport, ATC may assign a particular SID or STAR. These instructions are like "fly at this altitude until this point, turn right, descend to this other altitude, fly until you get to this other point", and so on.

For international flights, the flight plans are transmitted from one country to another using AFTN (Aeronautical Fixed Telecommunications Network). So each country that the flight flies over knows where it is supposed to be going.

Other factors which can influence the actual route a flight takes can include:

  • Weather
  • Other air traffic
  • Military operations
  • International agreements (or more specifically, disagreements)
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For example a flight taking off from SFO could be north bound towards Seattle or South bound to LA or east bound. ATC has to make the room for the proper direction of the flight and hence need to know the destination.

All airplanes wanting to fly IFR have to put in a IFR flight request. This will necessarily have the origin and destination as well as the approximate departure time. But the request will also have a path between (by specifying intermediate waypoints and/or airways) and a cruising altitude.

Before takeoff, ATC will then clear the airplane for a particular path (which may be somewhat different from that initially filed). (If you listen to "clearance delivery" frequencies, you can hear these being used). Assuming the pilot accepts the clearance, ATC and pilots now agree on the path that will be flown.

Once in the air (assuming no emergencies) pilots will need to request if they want any changes to the flight plan (perhaps to detour around weather, or to find a more comfortable cruising altitude).

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VFR flight (non-airlines): Often times, we just tell ATC, no flight plan required; flight following can be requested while enroute. For example, I have flown from my home airport west of the Boston Class B southeasterly down to Cape Cod (Nantucket Island in this case). I took off, contacted Boston Approach while climbing, and requested flight following. They ID'd me via radar/ADB-S, gave me a squawk code, and noted the altitude I was climbing to. Several miles later, we were handed off to a different sector, then a 3rd, maybe even a fourth. As I changed frequencies for each, I checked in, told them the altitude I was at, and that I was landing Nantucket. Eventually I was handed off to Nantucket tower about 10 miles out.

Other times, we have depart from a towered airport. Before taking the runway, you contact the tower and let them know your desired route of flight and altitude after takeoff. Tower will tell you the direction to turn after takeoff to work you into any other traffic in the area if needed, otherwise straight out, or left or right turns away from the airport are approved, and any restrictions needed to keep you away from other aircraft if needed.

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For IFR, the desired route is in the flight plan, which is almost always filed ahead of time. When an aircraft is ready for departure, ATC assigns them a transponder code (squawk) that is linked to their call sign, which is in turn linked to their flight plan. As they move from one controller to the next, that all gets passed along. Occasionally the squawk will change as they move (there are only 4096 codes, so the code you get here may already be in use over there), but the new one is then linked to the callsign and therefore your flight plan.

For VFR, flight following is (basically) when you get a squawk from ATC without a flight plan, so you're responsible for your own navigation. That still gets you the benefit of traffic advisories, but more importantly, if you run into trouble, they already know who and where you are so they can help you a lot faster than some random callsign they know nothing about.

(You can file a VFR flight plan, but ATC won't use it; it's just a hint where to look for wreckage if you don't arrive at the destination, which doesn't seem all that useful to the pilot. Prepare one for your own planning on long trips, sure, but instead of filing it just get flight following.)

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