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This question already has an answer here:

Please give a basic explanation of how commercial aviation traffic is routed for readers without any foreknowledge:

  • Can commercial aircraft only follow airways like cars on highways ?
  • Or are airlines free to choose a straight line between airports (excluding probhibited and restricted airspace) ?
  • Or is it a mix of both ? If so:
    • When must airliners follow airways ?
    • And when are they permitted to leave them (absent emergency) ?
  • What differences (if any) are there between traffic over sea and over land ?
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marked as duplicate by Zach Lipton, Pondlife, fooot, Ralph J, Jan Hudec Dec 20 '18 at 18:01

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    $\begingroup$ My experience is a bit dated having retired in 1999, but even back then, once we got going, it was common practice to "request direct xxxxxx" where xxxxx was a fix you knew from previous experience was likely to be approved. Approval was usually granted subject to traffic. If it was the middle of the night (flying freight) and the en route controller wasn't working a lot of traffic and sectors had been combined, you could usually expect the request to be approved. $\endgroup$ – Terry Dec 20 '18 at 1:34
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Commercial airliners generally fly IFR as such they do what ATC tells them to do. They are not free to chose their own path. The airway system in the USA and elsewhere on the globe stems from the pre-GPS era when VOR's and other ground based nav aids were the predominant method of navigation. Airways tend to be either between VOR's or between points you can triangulate with a VOR. The system was built around this idea of traffic routing and in the GPS days it has simply stuck. As the FAA shuts down VOR's slowly some of the airways remain and seem a bit arbitrary but GPS allows us to follow them easily.

The FAA has a brief on it here.

Broadly speaking aircraft can request a deviation for weather or other inflight issues like turbulence but aside from that courses are followed as they are assigned.

The FAA has discussed routing directly but until the nextGen system is in place its likely not going to be a widespread reality.

What differences (if any) are there between traffic over sea and over land?

At least over the Atlantic there is the North Atlantic track which is a set of predefined routes. Since there is no ATC over the ocean aircraft do a certain amount of self regulating.

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IFR aircraft (which will include nearly all commercial flights) must follow whatever clearance they are given. The clearance will generally match the flight plan they filed except for some minor vectoring at each end, though it may be amended in the air by ATC, e.g. to deal with weather or conflicting traffic.

So, it really comes down to the flight plan they asked for, which may not be "direct" along the great circle route due to trying to catch better winds, avoiding known weather, avoiding flying through certain airspace or staying within a certain distance of land. If there are convenient airways that approximate the route desired (and there often are), those can be used and are more likely to be approved, but they can also request their own routes and see what they get.

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