Say Again? #15:
ATC 301 – Airspace Stratums

You're a pilot, you can think in three dimensions. But can you do it as well as a controller? In this month's Say Again, AVweb's Don Brown shows us that, with a little knowledge of the various levels of airspace and how controllers use them, you can make your flight easier for both yourself and your controllers.


Hello again, class. This series of 300-level classes will be dealing with the structure of airspace. This particular article will be about airspace stratums. While most everyone is comfortable thinking in terms of lateral boundaries depicted by almost any chart, air traffic control must be thought of in three dimensions.

Unfortunately, we are not all familiar with the same airspace. But as I mentally walk you though the structure of my airspace, I hope I can guide you though understanding the structure of the airspace around your local airport. I believe it will lead you to a better understanding of how the ATC system operates.

One, Two, Three

First, what I’d like you to do is pull out the approach plates for your local airport. All of them. For those of you who are based at an airport without an instrument approach procedure, use the airport you would use as an IFR destination or alternate. I’ll use trusty old Hickory, N.C., (HKY) airport. (All the HKY approach charts are available here in a 194 Kb zip file.) That means I’ll have the VOR/DME RWY 24, ILS RWY 24, NDB RWY 24, and GPS RWY 24 approach plates. Now (mentally) lay them all on top of each other (choosing a common point of reference) and notice their relationship to each other. I’ll use BZM VOR (Barret’s Mountain, N.C.) as my common point of reference. Remember, this is about stratums, so take note of the altitudes for the various approaches and how they relate to each other. Make sure to include the missed approach procedures in your review.

You should now have a three-dimensional mental model of your local airport and its associated approaches. Now pick the two or three closest airports and add in their approach plates. Again, you’ll need a common point of reference. You’ll understand why I chose a VOR instead of the airport or NDB. For my airspace, I now have BZM VOR in the middle, HKY slightly to the southwest, Morganton (MRN) further west, North Wilkesboro (UKF) to the northeast, and Statesville (SVH) to the southeast.

It should be relatively easy for you to do this two dimensionally, as if looking at a map. What I’m trying to get you to do is look at it in three dimensions. It’s like the difference between looking at a map of the solar system and viewing one of those mechanical models of the solar system they always had in science class. Put a few airplanes in your mental model and set them in motion. If you can do this, you will notice not only where the approach courses overlap, but also where the altitudes overlap. It might help if you think of yourself standing at your common point of reference (BZM VOR for me), and mentally looking “up” at the airplanes flying by.

The Low Road

Enroute Chart Sample (48 Kb)

Low-Altitude En Route Chart – Vicinity of BZM VOR (click for larger version)

Now it’s time to add in the low-altitude airways. (I hear a collective groan from the class.) Come on guys. Stick with me. I promise it will be worth the effort.

Pull out your low-altitude en route or sectional chart. Instead of looking at the airways in terms of the VORs, I want you to first look at your approach plates and note the intersections depicted there. For example: The approach plate for the ILS RWY 1 at UKF has MULBE, BURCH, and JOTTA intersections depicted, in addition to the BZM VOR. If I flip over to the LOC RWY 10 at SVH, I’ll notice that it has a transition off of JOTTA intersection too. Take the approach plates from each of your airports and notice the intersections they have in common. Locate those intersections on your sectional. I know, it’ll take a few minutes. I’ll wait.

Everyone ready? Okay, lets fill in the airways. In my example, I notice that JOTTA is at the intersection of V37 and V20. V37 is the airway from PSK (Pulaski, Va.) to CLT (Charlotte, N.C.). V20 is the airway from BZM to SBV (South Boston, Va.). Looking at V20, I notice that SANFI intersection is eight miles southwest of JOTTA on V20. SANFI is an initial approach fix (IAF) for the VOR/DME RWY 24, ILS RWY 24, and GPS RWY 24 approaches to HKY.

All right class, fill in your airspace model with all your airways and include all the intersections used for all the approaches in your selected approach plates. Oh, stop your whining. Controller trainees have to do this from memory. They have to put in the VOR radials and the DME for each intersection, too. I’m not asking the impossible here. If I wanted to be ugly about it, you’d have to draw the IRs (military IFR routes) and the STARs too.

One more thing while you’re drawing in the airways: Take note of the MEAs on the airways. While you could legally allow someone to fly on V20 between JOTTA and BZM at the MEA of 3,500, it’s going to play havoc with your sequencing into HKY, because V20 (at SANFI) is part of every approach you have into HKY. Even if you’re vectoring for the ILS at 3,400 (the initial approach altitude) and not even thinking about airways, it doesn’t change the fact that V20 crosses the ILS final approach course (FAC) into HKY.

Moving Mental Metal

Getting the flick yet? Start flying your mental airplanes around your airspace and see what you notice. Can you picture it all in your mind? Good. See how many airplanes you can picture at the same time. Have a few en route, a couple taking off, and three on approach (don’t forget the missed approach). Are you feeling a little mentally taxed yet? Or just mental? Imagine having to talk to them all too. “Hey Center, can we get direct Newark?” Sorry. I just had to throw that in.

Now we come to the “meat” of this article. Imagine if you had to work all this airspace from the ground up (which they do in places like Alaska.) It would tend to limit the number of airplanes you could work at one time, wouldn’t it? That’s one reason airspace is divided into sectors not only geographically but by altitude. It also tends to group the type of operations. Let me explain.

The mental exercise that I’ve been running you through has been dealing (for the most part) with approaches. Depending on the terrain at your location, most of us have been working below 5,000 feet. So naturally, your first thought would be that the lowest altitude stratum tops out around 5,000. And you’d be wrong.

The Chop Shop

The most common dividing line between sector stratums in the low altitudes is 10,000 feet. In the example of the airspace that I’ve been using, the WILKES sector “owns” 10,000 feet and below. The airspace/facilities bordering this sector, GSP, AVL, and TRI Approach, all own 10,000 and below. Then you start running into the exceptions. To the south, CLT owns 14,000 and below. To the east, GSO Approach owns 12,000 and below. To the north, the SPRING sector owns from FL230 all the way down to the ground. To the northeast, ROA Approach owns one portion of airspace at 6,000 and below, while in the eastern half of their airspace they own 10,000 and below.

I don’t want to bog you down with the specific reasons for each exception (at least in this article), so let’s just stick to the norm for the moment. One of the reasons for the dividing line being at 10,000 is to provide a buffer between the zones of transition. In other words, we don’t want you (the pilot) to have to transition from the approach/departure environment (typically 5,000 and below) and switch frequencies (enter a new sector) at the same time. For a departure, 10,000 feet of altitude is usually enough to get you vectored clear of all the other airport traffic and turn you on course, i.e., transition into the en route environment. There are other reasons, of course, but I think that one will get you thinking in the right direction.

Movin’ On Up

As we increase altitude, the next most common sector stratum we run into is 11,000 to FL230. To be more precise, I should say “Above 10,000 feet up to and including FL230.” Can you guess why I need to be more precise? I’ll tell you at the end of the article.

Once again, you’ll notice that the transition zone from the low-altitude airspace (below FL180) to the high-altitude airspace (FL180 and above) is placed firmly within the sector stratum. The next most common sector stratum is FL240 to FL330. And last but not least, we have FL350 and above. You’ve probably had enough already, so I won’t require you to draw the jet routes and see how they fit in with the victor airways. If you spend time in the flight levels, though, you might want to do that.

Practical Practice

I see that Mike (in the back row, of course) is starting to yawn, so we’ll move into the practical aspects of all this. Once again, you’re going to “fly” in my airspace. Follow along with me and then try a few flights through your airspace. For the first flight, pretend that you’re departing HKY and going to Dekalb-Peachtree in Atlanta, Ga. (PDK). We’ll pretend you’re flying a Baron, requesting 12,000. Just to add a little realism, we’ll say you filed HKY direct PDK.

You call HKY Tower for your clearance, and you hear,

“Baron 34567, Hickory, Full Route Clearance, advise when ready to copy.”

Geez, you haven’t even cranked up yet and you’ve gotten on the controller’s bad side.

“We’re ready, Hickory, go ahead.”

“Baron 34567 cleared to Peachtree-Dekalb direct Sugarloaf Victor 222 LOGEN direct…”

You copy your clearance, taxi out, and call Hickory Tower, ready to go. Fly runway heading, altitude assignment is 5,000, and off you go on runway 24. Shortly after you’re airborne, you’ll hear HKY Tower say,

“Baron 567 contact Atlanta Center,” and you switch over.

“Atlanta Center, Baron 34567 is leaving one thousand eight hundred climbing to five thousand.”

“Baron 34567, Atlanta Center, radar contact, climb and maintain 8,000.”

“Leaving two thousand climbing to eight thousand, Baron 567.”

Changes in Altitude

OK, let’s look at what’s going on here altitude-wise. The initial 5,000 altitude assignment was probably just an altitude to get you off the ground. The Center might have had some traffic at 6,000 but it definitely wasn’t there when you checked in because he climbed you to 8,000 right off the bat. (Author’s note: Don’t try this particular logic with an Approach Control. It doesn’t apply.) So what’s so special about 8,000? Because you know your airspace, you know this particular controller owns up to 10,000. Why didn’t he climb you to 10,000?

Traffic of course. If you really knew this airspace you’d know that the SHINE STAR runs into CLT just west of the HKY airport. OK, maybe I should have made you draw the STARs too. Anyway, when CLT is landing south, the turboprops cross SHINE at 9,000. Aircraft coming out of HKY heading southwest tend to get tied up with the CLT arrivals on a regular basis. We’ll know for certain in just a minute.

“Baron 567, traffic, two o’clock, eight miles, southeast bound, a Dash eight descending niner thousand, expect higher in one five miles.”

“Baron 567, Roger.”

“Commuter 3223 traffic, ten o’clock, seven miles, southwest bound, climbing to eight thousand, a Baron.”

“Looking, 3223.” (No, I don’t think it’s cool using half of your callsign. I’m just trying to be realistic.)

“Baron 567 cleared direct Sugarloaf.” (Thought I’d forgotten, didn’t you?)

“Direct Sugarloaf, Baron 567.”

After you cross the SHINE arrivals (there’s usually more than one), you should hear,

“Baron 567 climb and maintain one zero thousand.”

“Baron 567, leaving eight thousand climbing to one zero thousand.”

In a minute you’ll hear,

“Baron 567, contact Atlanta Center 132.62.”

You (making sure you use your callsign when you say goodbye) switch over and check in,

“Atlanta Center, Baron 34567 leaving niner thousand two hundred climbing to one zero thousand.”

The next controller answers with,

“Baron 567 climb and maintain one two thousand.”

You acknowledge the clearance to 12,000.

Just as you’re leaving 9,500 you break out of the overcast and decide you might as well stay at 10,000.

“Atlanta Center, Baron 567, requesting one zero thousand.”

“Baron 567 roger, stand by.” Stand by? What’s up with that?

Changes in Attitude

Remember what this article is all about: The controller you are talking to now doesn’t “own” 10,000. So he can’t instantaneously approve your using that altitude. He has to call back to the sector you just left and get it approved. You see, when a controller takes a handoff on an aircraft that wants to climb or descend into his stratum, by accepting the handoff, he is saying to the controller that is giving the handoff, “I will transition this airplane out of your stratum and into mine.” If he’s not going to do that, he must call the sector and inform him when he will transition the aircraft into his airspace.

To further complicate matters, you are now getting close to AVL (Asheville, N.C.) Approach’s airspace. One of three things can happen:

  1. The controller you are talking to (who owns “above 10,000” to FL230) will call back to the previous controller and advise him that you are now requesting 10,000. That controller (who owns 10,000 and below) can say, “radar contact,” and you’ll be switched back to him.
  2. Or the previous controller will say “10,000 approved. Will you put him on AVL?” (In other words, he’s asking your current controller to hand you off to AVL Approach.)
  3. The third thing that could happen would be that the previous controller says, “Unable 10,000,” and we start working on “Plan B.” That could mean that you have to climb to 12,000 or descend back down to another altitude.

What will most likely happen is that 10,000 will be approved. The controller will make the necessary changes to your flight plan, and the next thing you know, you’ll hear,

“Baron 34567, amend altitude, maintain one zero thousand.”

A couple of minutes later you’ll hear,

“Baron 34567, contact Asheville Approach 124.65.”

Those pilots who aren’t reading this article will be blissfully ignorant that they’ve caused three times as much work for the controller by asking for an amended altitude in a bad spot.

I hope that doesn’t sound like a complaint, by the way. I figure pilots have enough to worry about trying to fly without trying to figure out how our airspace is structured. Ask for what you need, and let us worry about the details of getting it. If it takes a few minutes to work out, though, now you might understand a little more about what’s taking so long.

In the Opposite Direction

Would you like to try another? OK, this time you get to be a Gulfstream pilot going to Cincinnati. (Try not to get too excited.) Seeing as we’re pretending, we’ll use the tail number N1 (it saves on typing – look up that tail number if you don’t recognize it), and we know you’ll file the appropriate route. (Sometimes I crack myself up.)

Speaking of the route, here it is: HKY – HMV.SWEED6 – CVG. Requesting FL390. Just to be different, you’ll take off on runway 06. The initial altitude is 5,000. Runway heading. Let ‘er rip.

“Atlanta Center, November one, leaving two thousand two hundred climbing to five thousand.”

“November one, Atlanta Center, radar contact, leaving three thousand four hundred turn right heading zero eight zero.”

A right turn? Cincinnati is the other way. Big duh. Okay, I’ll explain it to you. First, there’s someone at 6,000 flying southwestward over BZM on V222. Gulfstreams burn a lot of fuel at 5,000. I know. They’ve told me many times. In addition, there’s the traffic to the north, descending to 4,300, direct PUMMP, for the ILS RWY01 into Wilkesboro. Assuming I’m successful in getting the guy descending to 4,300 under the 6,000 traffic, the last thing I want him to do is meet a Gulfstream climbing to 5,000. 5,000 versus 4,300 does not equal 1,000 feet. But there’s more.

Borderline Decisions

The border of the sectors above me lie between HKY and BZM. In that I’m ahead of the ballgame for once, I’ve looked at both sectors. The one to the west is working a dozen inbounds to CLT and trying to get them 20 miles in trail. He’s got his hands full. The sector to the east is working two airplanes, waiting for the CLT departure push to start. So, I can turn you in your departing aircraft into an arrival sector with 20 airplanes, or I can run you east for 10 miles into a departure sector, spread out the workload, and probably get you an unrestricted climb.

“November one, climb and maintain one zero thousand, leaving seven thousand turn left heading three six zero.”

After you acknowledge that clearance, you’ll get,

“November one, contact Atlanta Center 134.55.”

You’ll acknowledge that, switch over and check in. The next controller says,

“November one, climb and maintain one seven thousand.”

You acknowledge that and wonder why you haven’t been given on course.

“November one, traffic, eleven o’clock eight miles, northwest bound at flight level one eight zero, a Dash eight.”

There’s your answer. That would be a turboprop on the departure transition out of CLT. That transition parallels the border between the arrival sector and the departure sector. So the turboprop is tracking about a 340 heading and you’re on a 360 heading. That gives the controller 20 degrees of divergence so that, very soon, there will be five miles between you both, and the controller can keep climbing you.

I Want to Take You Higher

It wasn’t by chance that the controller who owned 10,000 and below put you on the 360 heading. He sits right next to the controller you are talking with now, in the MOPED sector, and they have coordinated the plan. Would you like to listen in? OK, but first we have to climb you.

“November one, climb and maintain flight level two three zero, fly heading three four zero.”

Notice the altitude assigned, please. FL230 is this sector’s highest altitude. As soon as the MOPED sector controller hears you acknowledge the clearance, he gets on the phone and dials the sector above him, who answers,

PSK Controller: Override Pulaski

MOPED Controller: Moped, North of BZM, November 1, assigned heading 340 to stay out of the SHINE sector. Your control for turns.

The MOPED controller really doesn’t have to explain any further. The PSK controller feeds the CLT arrivals to the SHINE controller (who is sitting right next to him), and he knows how busy the SHINE controller is. Of course, you (sitting in the cockpit) don’t have any idea that all this is happening. You just know that it looks like you’re on the great circle route to CVG. But if you’ll look at the bright side, you’re passing through FL190 and you haven’t leveled off since you left the runway. That, and you haven’t hit anybody either.

Frequent Dialers

Shortly after you leave FL190, the MOPED controller switches you to the PSK controller. (Remember, the Dash 8 is at FL180 off to your left and the MOPED controller released you for turns. He’s not going to switch you before you clear the Dash 8.) You check in on 132.97, and the PSK controller climbs you to FL290. Remembering that I told you the next stratum commonly owns FL240 through FL330 you’re probably think you’ve got traffic at FL310 now. Well, you might, but the PSK controller only owns FL240 through FL290. Sorry, there are always exceptions.

A couple of minutes pass and you hear,

“November one, climb and maintain flight level three one zero, cleared direct PACKO SWEED6.”

Yes, I know I just told you the PSK controller only owns up to FL290. He also owns a telephone:

SALEM Controller: Override Salem.

PSK Controller: Pulaski, southeast of HMV 20 miles, November 1, heading 340, request higher altitude.

SALEM Controller: FL310

PSK Controller: Direct PACKO your approval.

SALEM Controller: Approved. FL310 will be his final. Put him on Indy please.

You never hear these phone calls of course. And you will probably never notice the two CLT arrivals leaving FL290 descending to FL240 sliding underneath you on their way to HMV.SHINE5.CLT.

Oops. Almost forgot.

“November one, flight level three one zero will be your final altitude.”

That’s something else most pilots don’t know. Indianapolis Center, if they need it, will put out a restriction that says the CVG arrivals will cross the common Center boundary at or below FL310 during the busy times. That applies to everyone. Even if you have the FAA Administrator on board.

Knowing Just Enough…

I see that we’re out of time. I hope this session has given you a new glimpse into how your airspace system works. I encourage you to visit your local ATC facility and see the specifics of its operation. As you look to apply this knowledge, please remember how complex this system is. There are hundreds (if not thousands) of rules and regulations that might apply in any particular situation.

Remember I told you I needed to be more precise about describing which sector owns what altitudes? Think of a VFR at 10,500. Who works him? It would be the sector that owns “above 10,000, up to and including FL230.” See what I mean? The more you find out about the system, the more you realize how little you know about it. Use your knowledge wisely.

Have a safe flight!

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