# 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? – David Richerby Jan 15 '17 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. – Ron Beyer Jan 15 '17 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. – A. Frenzy Jan 15 '17 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. – Ron Beyer Jan 15 '17 at 3:16