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The Logic Behind Class B Airspace

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Class B airspace covers some of the busiest commercial airports in the world, with 37 airports in the United States designated as Class B. Most of the traffic in this airspace is on an IFR flight plan, but you'll find VFR traffic inside, too. So what is the logic behind Class B airspace?

What Is Class B Airspace?

Class B airspace protects some of the busiest commercial airports in the world. You'll find a constant flow of airliners and regional jets arriving and departing, and, no matter what the weather, Class B airspace is always busy.

Add to the mix corporate jets, cargo operations, and personal aircraft on both VFR (Visual Flight Rules) and IFR (Instrument Flight Rules) flight plans, and you've got a busy mess. To accommodate all of these flights, Class B airspace has some of the strictest equipment and communication requirements of any airspace. But, it also has some of the most lax weather minimums. Why? Read on and find out...

Andres Nieto Porras

It's Highly Controlled

Air Traffic Control makes Class B airspace possible by constantly monitoring and separating each flight in the airspace; that's also why it has some of the most relaxed weather minimums, because ATC always has eyes on you. Approach and departure control transitions aircraft into and out of the airspace, and tower controllers sequence them in for landing and takeoff.

ATC controls everything you do in Class B airspace. As you're learning about the airspace and its requirements, keep in mind that they're in place so that you and ATC remain in constant communication. ATC is always aware of where you're at and what you're doing.

Sam Chui

How To Find It

Class B airspace surrounds the largest airports in the United States. Denver International (KDEN), Los Angeles International (KLAX), Chicago O'Hare (KORD), and Atlanta Hartsfield (KATL) are all examples of airports in Class B airspace. Identifying Class B airspace on a VFR sectional map is pretty easy. There are two markings you need to know to identify Class B airspace:

  • Horizontal boundary markings
  • Vertical boundary markings

The horizontal boundaries of Class B airspace are marked with a thick blue line. Class B airspace typically has lots of different sections, so expect to see lots of thick blue lines that make up the horizontal limits of the airspace. The different sections of Class B airspace often form a perfect circle, but in some cases, the horizontal boundaries of Class B can be all kinds of shapes, due to mountainous terrain, neighboring airports, and other airspace.

In this example, the west side of Denver's Class B airspace is 'cut-off' because of its close proximity to the Rocky Mountains.


Vertical boundaries of Class B airspace are easy to identify as well. There are two sets of bold blue numbers, separated by a blue horizontal line. The top number represents the ceiling of Class B airspace in hundreds of feet MSL. For example, if the top number is "120," it means the ceiling of Class B for that section is 12,000 feet MSL. The altitudes are inclusive, so if you're flying in that section at 12,000 feet MSL, you're in Class B. The bottom number represents, you guessed it, the floor of Class B airspace in hundreds of feet MSL. For example, if the bottom number is "080," it means the bottom of Class B airspace for that section is 8,000 feet MSL. So, if you're flying at 8,000 MSL in that section, you're in Class B.

When a section of Class B airspace extends to the surface, the bottom number is replaced with the letters "SFC", for "surface."


But how high does it go? Class B airspace typically extends up to 10,000 feet MSL, however, it can vary. In this example, Denver's Class B goes up to 12,000 feet MSL because it's a high-altitude airport. (Arrival into Denver KDEN shown below)

Swayne Martin

The Mode-C Veil

Air Traffic Control closely monitors everything that happens inside - and around - Class B airspace. They need to have accurate altitude information on each aircraft in the area to make sure they remain separated from the Class B traffic. Because of that, all aircraft need to use a Mode-C altitude reporting transponder inside and around Class B airspace.

Class B airspace is surrounded by what's known as a "Mode-C veil." The Mode-C veil is marked by a thick magenta ring, with the words "30 NM MODE C" next to it. What does it mean? The Mode-C veil is NOT part of Class B airspace, however, any flight within the veil requires you to use a Mode-C transponder.

But how high does it go? The Mode-C veils extend vertically from the surface to 10,000 feet MSL. It stops at 10,000 MSL because any flight above that altitude requires a Mode-C transponder, regardless of where you are.

So what happens if your airplane doesn't have a Mode-C transponder? Are you banned from flying within 30 mile Mode-C veil? Not necessarily - aircraft not originally certificated with an engine driven electrical system, or aircraft that have not subsequently been certified with a system installed can fly within the Mode C veil, provided the aircraft remains outside Class A, B or C airspace; and below the altitude of the ceiling of a Class B or Class C airspace area designated for an airport or 10,000 feet MSL, whichever is lower.

Jim Raeder

Why The Funny Shape?

Lots of people say Class B airspace looks like an upside-down wedding cake. While it's not always made up of perfect circles, it is typically narrow at the surface and wide at the top. Take the Denver Class B airspace on the screen for example. There are lots of different sections at different altitudes, but for the most part, it's narrow at the bottom and wide at the top.


As planes arrive and depart a Class B airport, they need to be kept at a safe distance from other aircraft in the area. Many of those planes are lower, slower, and in different airspace around the Class B. The cake shape allows arriving and departing aircraft to remain in the Class B airspace as they transition to what's called the 'enroute structure', or the big and relatively open airspace at higher altitudes. At the same time, lower and slower airplanes can continue operating safely at smaller airports outside (but near) Class B airspace.

Swayne Martin

Easy Enough, Right?

When you think about it, Class B isn't too difficult to understand. It's simply there to adequately handle a large influx of VFR and IFR traffic arriving into one area. If you understand its shape and purpose, it'll be much easier to plan your flight through Class B.

Looking For More Info?

Want to learn more about airspace? Try our National Airspace System online course. With tons of quizzes and simple explanations, it's an easy way to get ready for your next checkride or flight review.

Whether you're starting your airline career or looking to make the move from another job, check out what ExpressJet has to offer here.

And when you're within 6 months of earning your flight time, apply to ExpressJet and get ready for the right seat of a jet.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at, and follow his flying adventures on his YouTube Channel.

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