Confused yet? I know. It's just not a world that is well known. I'll explain some parts of it further, during the week. But for now, I want to get to a really simple part of the air traffic control world. It's the part I always want you to think about first, whenever air traffic control comes up as a topic. Especially when someone tries to convince you--the taxpayer--that we need to buy a new, expensive piece of equipment. Hold this thought in your mind and I'll continue explaining (after the jump). It's the runways. It's always the runways.

If you're just the average airline passenger that wants to know why your plane is late, the key to understanding the system is the runways. If you want to be the next Administrator of the Federal Aviation Administration (FAA), you need to understand that the key to the National Airspace System (NAS) is the runways. No matter how complicated all this gets (and it is complicated) everything comes back to the runways.

Step outside and look up into the sky. How many airplanes do you see? How many airplanes do you think we could fit up there? The sky is vast and mostly empty. It is not uncommon to have 5,000 airplanes in the skies over America at any one time. The sky could easily hold a hundred times more. The problem is that they want to get back on the ground. And that takes a runway.

America is blessed with a large number of runways. We have 5,194 paved runways. That would be great if we used them all because every airplane in the sky at any one moment would have its own runway. But, as you know, that isn't the way it works. As every air traffic controller in Atlanta Center knows, they all want to land at Atlanta. At 5 p.m. on Friday.

Okay, maybe it just seemed that way. But let me use that as a mental exercise I think you will find useful. Atlanta Hartsfield-Jackson Airport has five runways. Unfortunately, you can only use three of them to land on simultaneously. (That's the rule. This is hard enough without explaining the rules right now.)

Five thousand airplanes divided by three runways equals 1,666 airplanes per runway. Let's see how fast we can get them on the ground.

The rules say you can only have one airplane on the runway at a time. On average, it takes about 60 seconds for an airliner to land, slow down and get off the runway. The math is as simple as it is inescapable. You can land about 60 airplanes an hour per runway. With Atlanta's three (arrival) runways, that would be 180 airplanes an hour. So, for 5,000 airplanes, it would take 27.7 hours to get them all on the ground. If everything went perfectly.

Too bad there are only 24 hours in a day. And that things never go perfectly.

It's a good thing the Atlanta airport only runs about 2,724 operations (landings and takeoffs) a day. (According to my calculations anyway. 994,364 per year divided by 365 days.) With 24 hours in the day that's 113 operations per hour. Of course, Atlanta's runways look a lot like Atlanta's interstates after midnight--mostly empty--so if you cut out the hours between midnight and 6 a.m. you're left with 151 operations per hour. That sounds a lot like the way this chart on FlightAware looks.