For example, aircraft arrive at an intermediate-range airport that has two runways, one for landing and the other for takeo¤. Aircraft that cannot land upon arrival in the air space of the facility must circle within speci›ed “holding stacks” until a runway becomes available. If the number of “holding” planes exceeds a certain quota, the air tra¬c controller must clear the takeo¤ runway and allow circling planes to land. €e runways are connected to the passenger terminal via taxiways. A plane that cannot take o¤ immediately upon reaching the end of the taxiway waits on a holding apron. €e airport authority is currently conducting a study with the objective of improving the landing and takeo¤ services at the facility.