Say Again #19:
ATC 302 — The Hub

  • E-Mail this Article
  • View Printable Article
  • Text size:

    • A
    • A
    • A

AVweb's Don Brown continues his 300-level series of classes by discussing the interaction of DPs and STARs and how they affect traffic flow into multiple, nearby airports. When you know some of what ATC has to consider when working heavy traffic, you can use them as an asset in your cockpit rather than an inconvenience.

Say Again?

There are few things that have influenced the National Airspace System (NAS) as much as the implementation of the "hub and spoke" system the major airlines developed decades ago. Depending on which side your bread is buttered, it's the best thing that ever happened or it's the worst thing to ever happen. There's not much arguing that it's been successful for a few major airlines. But as I'm fond of pointing out to folks, the major airlines aren't the only users of the NAS.

The concept of the hub is alive (if not particularly well) right in my backyard at Atlanta Hartsfield International (ATL). Since I was a kid, there's been a saying in the South: "When you die and go to heaven, you'll probably have to change planes in Atlanta." As anyone who flies in the South will tell you, that saying is all too true. Obviously, the concept has worked for Delta and for Atlanta. I don't think it worked so well for the folks that used to work at Eastern though.

Before I spark another debate on deregulation -- the dubious benefits of corporate raiders and junk bonds -- let me see if I can steer you toward thinking about the effect hubs have had on ATC. I'll try to stick to how that translates into the effect that it has on you, the pilot.

Growing Pains

I have the unique perspective of watching the hub at Charlotte, N.C. (CLT) grow right in front of a radar scope. Piedmont Airlines (anybody remember them?) was well on their way toward making CLT a hub by the time I showed up at Atlanta Center (ZTL) in 1982. But the FAA wasn't.

Both CLT and ATL are in the Southern Region of the FAA. That's significant because, in any organization, there's always the proverbial 500-pound gorilla. In the FAA's Southern Region, that would be ATL. In 1981, CLT was just a little baby gorilla. When it comes time to divvy up the bananas (i.e., money), ATL gets the lion's share (to mix my metaphors) of the budget for the FAA's Southern Region.

So, when I started training at ZTL, CLT had a hub operation without having an airspace structure to go along with it. We controllers old enough to remember it called it the "ram and jam" days. Every aircraft inbound to CLT was going direct and every aircraft outbound was on course to the first fix. It was crazy.

Circle the Wagons

One of the most vivid mental images I have regarding ATC was from these days. I sat down at an empty scope and "quick looked" the Center sectors around CLT. The "quick look" function allows you to bring up all the data blocks displayed on other scopes. As I'm looking at all the data blocks, I run the "vector lengths" out to their maximum. The "vector length" is just a straight line drawn on the radar scope by the computer. It projects where an aircraft will be in a certain length of time (one, two, four or eight minutes) predicated on the aircraft's recent history of heading and speed. The image I saw was a wagon wheel. The "rim" was about 20 data blocks in a circle around CLT, the vector lengths were the "spokes" and they all touched the "hub" of the CLT airport.

That's probably not the image the airlines want you to have in your head when they refer to the "hub and spoke" system but that's the image I've always carried around with me. It's vividness is surpassed only by another image I acquired during that same period.

ATC Math

Keeping in mind the "wagon wheel" image of aircraft inbound to CLT, I want you to think about where we put the departures. You have 20 aircraft all due at the airport in eight minutes, descending, coming from all points of the compass. CLT rolls a departure off the runway. Where do you put him? You're going to have to do a little thinking here so put your thinking cap on.

If you have 20 airplanes spread around 360 degrees of the compass, you have an average of 18 degrees separation between each airplane. That's fine when they're 80 miles out but as they converge on a common point (the airport) the distance between them gets tighter and tighter. Standard separation is five miles so when you run out of distance (lateral) you have to use altitude (vertical.) Standard vertical separation, as you all know, is 1,000 feet.

As some of you have no doubt already noticed, you can't put 20 arrivals on the runway in eight minutes either. Adding the two factors together, you now have a snarl of airplanes -- at different altitudes, different speeds, different headings -- all converging on the same spot. Sounds a lot like "Free Flight" doesn't it? Let's not get sidetracked on that point either.

Turnin' and Burnin'

Back to that other mental image I was telling you about. As we at the Center were handing off this mess to CLT Approach, they would be launching the departures that were leaving the gates so the arrivals would have some place to park. That's the way a hub-and-spoke system works, see. In that every point on the compass was already occupied by an arrival, the only "out" that CLT had was altitude. If I had a nickel for every time I heard "Give me higher I'll miss him" come over the landlines between CLT and ZTL I could have retired before I ever finished training.

CLT Approach is twisting and turning the arrivals trying to get them spaced out and lined up for the runway and trying to miss this mess with all the departures they were flinging out. As soon as they topped their arrivals they'd turn the departures "on course" and switch them over to us at the Center. Sounds confusing doesn't it? That's because it is.

So one day I'm plugging into the sector during one of these free-flight-for-alls and I hear the R-side say, "Airliner123 stop your turn, turn back left heading 360." That got my attention. I didn't hear the reply (like I said I was just plugging in) but the next transmission was something like "Airliner123 turn north and expedite the turn." It was almost like a being at a movie as the camera keeps zooming in tighter and tighter. You've got four or five controllers huddled around a radar scope but your field of view keeps getting tighter and tighter and suddenly all you can see is two targets and their altitude readouts. I can't even see the call signs in the data blocks. All I can see is two targets about to merge with one altitude readout showing 107 (10,700) climbing and the other one showing 113 (11,300) descending.

If you ever wanted to make a movie out of this, the soundtrack should be to the sound of breathing and heartbeats. Every time the computer updates the position and altitude of the targets, as the targets blink forward, you get another heartbeat. Right until that last update. Then all breathing stops and everyone's heart skips a beat, waiting to see if the targets are going to come apart.


You would think that cured us of the "ram and jam" theory at CLT. It didn't. We had to go through it again a short time later. The time frame is a little hazy (it was 20 years ago, after all) but in a few weeks we duplicated that scene in virtually the same spot. Even the most hardheaded among us couldn't write that off as just bad luck and we started using arrival and departure gates.

Now class ... are there any questions as to why we have structured airspace around airline hubs? I didn't think so. Let's move on.

Airspace Modeling

It never ceases to amaze me how hung up pilots get about Class B airspace. It's like all the airplanes and all the boogey men are safely contained inside the walls of the Class B. Neither of the above incidents occurred in Class B airspace. If that doesn't clue you in maybe this little tidbit of knowledge will. I work with CLT every day and I can't tell you exactly where CLT's Class B airspace begins much less what it looks like. It's not even depicted on our radar scopes at the Center. It's all but irrelevant to me as a Center controller. I realize that doesn't make it irrelevant to you as a pilot. But the reason for having Class B airspace and the influence a hub has on airspace extends far (very far) outside the Class B.

There are a couple of mental models I'm going to use to describe how the airspace is structured around a hub. The first one is a pie. For you non-Southerners out there, pie are (not) square. Pies are round. Cut your pie into eight equal slices. Pick a slice. Any slice. That's a departure sector. The slices on either side of that departure sector are arrival sectors. Get it? You have four individual arrival sectors separated by four individual departure sectors.

For those of you who are remembering to think in three dimensions you'll realize that the pieces of pie are actually more like funnels. I'd explain that there ain't no such thing as "funnel pie" but I think I lost the Yankees with the "Pi-R-squared" joke. Anyway, the departure sectors "funnel" the departures out from the airport and the arrival sectors "funnel" the arrivals in.

Like any model, this one isn't perfect. The ATL hub actually does have four arrival and four departure sectors, for a total of eight Center sectors dedicated to working the airliners into and out of Hartsfield Airport. CLT, on the other hand, only has six sectors dedicated to this task and one of those is in Jacksonville Center (ZJX). That means that the Center sector on the southwest side of CLT is doing double duty. They're working the west departures and the arrivals from the southwest. Don't let it all confuse you, just realize that the model may not exactly fit the airspace where you fly.

More Models

STARS into KCLT Where were we? Oh yes, the pie/funnel model. Now that we have that model firmly in place let's move on to the other model. Once you get about 40 miles away from the airport the "funnels" start branching out. For instance, if you'll look at the SHINE STAR you'll see a "branch" comes over HMV on the northwest side of CLT and another comes over VXV to the west of CLT. If you could see all these "branches" running into and out of a hub it would look much like the circulatory system running into and out of the heart. The arteries/DPs carry airplanes away from the hub/heart and the veins/STARs carry them back in.

Of course, that model isn't perfect either. But if you're like me and can think of the NAS as a huge, multi-hearted monster with an incredible ability to funnel airplanes/blood in just to make it come back out again you can make it work for you. Otherwise you'll have to come up with your own models.

STAR Gazing

Just to make you even crazier, I'm going to explain how a hub (CLT) influences airspace by detailing the STAR into Greensboro, N.C. (GSO). You see, the STAR into GSO really doesn't have much to do with running airplanes into GSO. It has more to do with running airplanes into and out of CLT.

We'll start with the BROOK arrival at SPA (Spartanburg, S.C.). SPA is 49 miles west of CLT (on a 263 bearing) and 120 miles southwest of GSO. If you weren't paying attention, you might think it would be okay to file SPA J14 GSO. And if you could land at FL180 (Jet Routes are for FL180 and above) you might get away with it. If you looked in the low-altitude stratum for a Victor airway you wouldn't find one between SPA and GSO. There used to be one (V20) but it got moved over SUG and BZM. That's a major hint.

SPA is a major en route point, with the ATL east departures and the CLT west departures both using it as a navigational point. If you're an arrival to GSO over SPA, sector 32 (SPA HI) will descend you to cross MURKY (SPA035030) at FL240. Sector 32 will hand you off to Sector 44 (SHINE). Would anyone in the class care to guess what the SHINE sector does? Very good. That sector runs the SHINE arrival into CLT. The SHINE sector will descend you to FL190. You won't find that altitude on the BROOK arrival but it is in our SOPs (Standard Operating Procedures). By the time you are reaching FL190 and approaching the KHAOS intersection you will have been handed off and switched to the MOPED sector.

STAR Crossed

BROOK2 STAR Would anyone care to venture what the MOPED sector does? Well, yes, they do run the BROOK arrival into GSO. But the sector was designed to run the CLT departures. You're now in the piece of "pie" that constitutes the north departure sector for CLT.

At the risk of making your eyes glaze over, let's look at the BROOK arrival again. This time though let's use CLT as our reference point. You start at SPA (CLT263049) then go to MURKY (CLT286038) and then to KHAOS (CLT332038.) You're now at FL190. If you'll look out the right side of the aircraft you'll see the CLT turboprop departures climbing to FL180 on the CLT347 radial, better-known as the HUGO FIVE departure. As soon as you cross that stream of departures you will be descended to 11,000. Quickly. That's because the controllers are now trying to get you under the CLT jet departures climbing out on the CLT 011 radial, better known as the HORNET ONE departure.

You're now approaching the BROOK intersection. BROOK is 51 miles west of GSO on the 282 radial. More important, it's 59 miles north of CLT on the CLT 002 radial. The jets coming out of CLT on the 011 radial (if they are climbing properly) should now be above your altitude (if you are descending properly). "Properly" has absolutely nothing to do with the Flight Management System, the company's desire to save fuel or any other reason except to separate airplanes. Safe (the arrival is specifically designed to separate airplanes), Orderly (it's well-charted and the procedures are defined) and Expeditious (we can move a large number of aircraft though the sector quickly).

More Pie Anyone?

One last leg and we're done. From BROOK you proceed direct to GSO via the 282 radial. You'll cross TRAKS (35 west of GSO) at 11,000. Again, that has more to do with CLT than it does with GSO. That restriction keeps you below the piece of "pie" just east of TRAKS that is running the MAJIC arrivals into CLT from the northeast. Simple huh?

OK, I know it's not simple. Nobody ever said it was. As a matter of fact, controllers keep telling folks it takes about five years to train a controller. We don't really believe people "get it" but this is one of the many reasons why. Controllers have all this in their heads. We don't have it all memorized (a lot of it actually is) but we know what it looks like and how it works. We know how it is supposed to work and we know how to "make it work" if it doesn't work the way it's supposed to. Like when we get somebody that thinks a slow descent rate to save fuel is more important than descending quickly to save their airplane.

Your Horoscope

I know there are many general aviation guys out there who wonder what any of this has to do with them. If you've stayed with me this far, I guess I should tell you. As most of you already know, you're not going to get within 50 miles of a hub airport when you're below 10,000. If you do, you will be nailed down to a very specific route or you're going to be vectored all over the sky. I believe many people associate that with Class B airspace.

While the effect may lessen 30 miles away from the airport when you leave Class B, it doesn't go away. It lasts until you're 40 miles away and leaving the Approach Control's airspace. Not just the Class B, but their airspace. It's not the same thing. When you reach the 50-mile ring around the hub, you're mixing with all the traffic that is being routed around the hub.

By the time you reach the 60-mile mark, if you're at 10,000 or below, you're pretty much clear of it. If you're above 10,000 though the effect will last further and further the higher your altitude. Imagine a Twin Cessna at 14,000, 60 miles north of CLT, westbound, in the description I gave you of the BROOK arrival. At BROOK (the CLT002059) the airplane inbound GSO is descending from FL190 to 11,000. But he's not the main show. We're just trying to squeeze him in between the two CLT departure streams. I hope you can see how the addition of one more aircraft can greatly increase the complexity of the situation.

Does this mean that you can't fly through these places? Certainly not. I'm sure many of you have. But the next time that you do, I hope you'll have a greater appreciation of what's going on around you. I hope you'll also have a better understanding when a controller needs to move you for traffic.

The Cosmos

Once last thing and I'll let you go. The actual situation is vastly more complex. I didn't bother adding in the other "branch" of the BROOK arrival. I didn't mention the arrival into GSP that's just south of the BROOK arrival. Nor the GSO departures, nor the CLT satellite departures. And if you think this is starting to sound complex, imagine what the airspace around New York, Chicago or Los Angeles looks like.

Have a safe flight.

Don Brown
Facility Safety Representative
National Air Traffic Controllers Association
Atlanta ARTCC

Editor's Note: We believe Don's series of "classes" should be used in the same way that familiarization (fam) flights are used to show controllers what it's like in the air. There will be parts that are confusing to some readers, but as Don indicates, this is just a small piece of why ATC is so complex. Don doesn't plan to offer a "400-series" level of classes, I mean columns, in the future, but we're going to keep challenging you with this material. Let us know how we're doing!