We had a saying, when things went wrong and someone asked—“how are you separating those two?” “The big sky theory,” was the inevitable response.
When you look at a scope, which may be 18-20" across, and which displays airspace as much as 200 miles or more in diameter, the targets take up an appreciable amount of space. An airplane that isn't as much as 200' long in real life (which is most of them) may look like 2000' or more in scale on the scope. Add to that the five miles of separation we advertise to provide and every target is really more than ten miles across insofar as our thinking has to be.
In actuality, if you were to look at a busy session on a radar scope (noting how congested it appeared) and then run outside to see what it looks like in the sky above (which I used to be able to do, living pretty close to an ORD arrival fix), you would be hard pressed to be able to see even as many as a half dozen airplanes, and they'd be quite far apart. The truth is, in order for two airplanes to trade paint, they have to be a whole lot closer—which is hard to do, because it truly is a big sky.
Last updated: 14 April 2009