# How are "preferred routes" assigned if there is more demand than capacity on a route?

There might be multiple routes, belonging to the “Preferred Routes” class, connecting the same pair of airports.

However some of them might be more appealing for non-urgent circumstances such as less turbulence, more favorable winds, etc. Therefore there will be a route (as long as I understand) that is the desired one by many airlines at the same time.

Let’s suppose we have airlines A, B, and C requesting a flight on route x, but the capacity is one per route, or stated another way, only one of the three airlines can take the route and the other two must choose another route.

Which is the protocol followed to decide if the route x is assigned to A, B, or C?

• Why wouldn't they be able to fly one after another? Or at different levels? Jan 15, 2017 at 0:28
• It's hard to imagine, for the comments posted above, that a route can be so saturated that it isn't available. Separation is just a few miles horizontally and add low as 500 feet vertically. The real problem is sequencing into or out of airports. Jan 15, 2017 at 0:47
• Understood, you are right, I can reformulate my doubt in the following way: which one would have preference to take off first and why? Referring to the saturation, I don't know how in practice is implemented but in the mathematical models there is what is called the "cell occupancy limit" that roughly limits the total number of flights allowed in a certain region, which could cause that after the first flight the cell occupancy reaches its threshold and the model doesn't let more flights to traverse the area. Jan 15, 2017 at 1:55
• Dispatchers file flight plans in the national airspace system. Each flight plan is given a "slot" for departure (and for arrival). The flight needs to depart within this slot or it may be put in a waiting list for another slot. They can also file "suboptimal" flight plans to take less desirable routes for better departure slots. Jan 15, 2017 at 3:16
• Related: FAA traffic flow management for the US and ETFMS for the EU. Article: Air traffic flow management problem with enroute capacties. And Can airlines schedule more flight arrivals than the airport capacity?
– mins
Jan 15, 2017 at 15:25

Air traffic services (at least in the US) are handled on a first come first serve basis. That's the overarching protocol. Beyond that, it's situation dependent.

In the air, controllers give pilots what they want if possible. In practice the situation is quite fluid. Maybe all three can't go on the same path now, but in a minute there will be enough clear airspace to make it happen. Or maybe they go on a route very close to but not exactly what they requested.

If (for some reason like a prominent jet stream) many airplanes are filing the same (or very similar) flight plans so that certain airspace will be overwhelmed, there is an entire division of the FAA dedicated to deciding where and when aircraft need to be rerouted. Here's some info @mins linked to in a comment.

Also, your question seems to assume competing airlines. For all ATC cares, A, B, and C could all be flights of the same airline.

Preferred routes as well as the Tower Enroute Control (TEC) routes are established to facilitate flight operations between major hubs which alleviates congestion for controllers, providing expedient sequencing for departures and arrivals as well as travel along airways. This has to do with a number of factors, including terrain, routing navaids and waypoints, airport and airspace configurations and existing arrival and departure procedures. They are also chosen for specific types of aircraft e.g. Turbojets or turboprops and various onboard navigation equipment e.g. RNAV, GNSS, etc.

You can, at least technically, file any kind of flight plan you like, but if it doesn't use these preferred routes, controllers will likely amend your clearance, routing you along a preferred route of their choosing based on anticipated traffic congestion at both departure and arrival times. If an urgent situation arises, a pilot can declare this and ATC will work with them to resolve the situation.