“The main feature of the circular runway is that it will become possible to let an aircraft operate always at landing and take-off with headwind. Whatever its strength and direction, the Endless Runway becomes independent of the wind. When allowing limited crosswind, airspace users can shorten the global trajectory of the flights through optimized departure and arrival routes.
The circle of the runway, whose diameter is set to 3 kilometers, is large enough to provide sufficient room for infrastructure preferably inside the circle, even for a hub airport. This makes the airport compact, while allowing current-day aircraft to use the circle without significant structural modifications.” (http://www.endlessrunway-project.eu/) March 16, 2018