How much of the airspace over a major city is actually free to use? On a scope the empty space is everywhere, yet little of it is free: the room to vector an arrival lives along the routes, and it looks generous until you count the traffic already using it. Two cities make the point: the one everyone pictures as the busiest, and a quieter one that should have room to spare.
The most crowded approach airspace there is. Reveal each airport's arrivals in turn and watch the free airspace disappear. The space was never free; it was another airport's all along.
Surely a quieter sky has real room? It looks far emptier, and it keeps a little more. But the same force takes it. Here it is not other airports but the overflights crossing the same airspace.
Popocatépetl is an active volcano just under 18,000 feet, and it erupts often enough that ash is a constant hazard. Aircraft avoid it because it is active, at every altitude and not only over the summit, since volcanic ash will flame out a jet engine. The empty space over the volcano is the one place on the scope where empty plainly means unusable. It is also the check on the method: a detector that flags a known no-fly volcano is finding real holes, not gaps in the data.
Every scope here looks straight down, flattening the sky into a plane. Turn one on its side, slice it by altitude, and the same airspace is not flat at all:
Fewer aircraft fly at night and the scope goes quiet. But you have seen that empty is not the same as free. So when the traffic thins, does the negative space become available, or only quieter?
What this isTwo metros, arrivals into each on their published routes; Mexico City is shown in its predominant 05 landing configuration. The amber is airspace clear of traffic; the headline counts the part within vectoring reach of a route, clear of the traffic the data can see. New York reveals the three airports' arrivals in turn; Mexico City reveals traffic by class, arrivals, then departures, then the overflights crossing it. The room is what stays clear of the traffic observed over a few days below roughly flight level 300, and it does not subtract terrain or the minimum vectoring altitudes. Each of those would compress it further, not open it up. Public inputs only: the published arrival routes and observed aircraft positions.
The pointThe vectoring room looks generous, then gives out once you count what already uses the same airspace. The quieter sky keeps a little more room, but the same force takes it. The volcano is the lone exception, empty for a physical reason no traffic count can return.
DataAircraft positions from adsb.lol globe_history, licensed ODbL 1.0.