Here's how Class D airspace works, plus the weather and communication requirements you need to fly in it.

Why Class D Exists

Class D airports have an Air Traffic Control Tower to coordinate airport operations. However, unlike Class B and C airports, they don't require a mandatory approach and departure control (though many do have approach/departure). Don't underestimate how busy Class D airports can get. Some of the busiest general aviation airports in the world, like Van Nuys and Denver Centennial, are Class D.

You can find almost any kind of traffic inside Class D airspace. Weekend fliers, airlines, corporate traffic, and cargo operators all share the airspace at many Class D airports. In many large cities, buisness jet operators avoid larger Class B airports and instead utilize more conveniently located Class D airports.

It's Not Always A Perfect Circle

The horizontal boundaries of Class D airspace are marked with a dashed blue line. Class D airspace only has one circular ring, but it often has shapes that jut out of it. Check it out...

What's the extension for? Class D extensions are designed to protect IFR aircraft on arrival and departure. The extension may cover an instrument approach, or it may cover a departure path that extends along rising terrain.

The airspace shape can also be modified because of other airspace in the surrounding area as well. For example, if a Class D airport is next to Class B airspace and the two touch, the Class D might be cut off where it meets Class B. Here's an example:

Vertical Boundaries

Generally, Class D airspace extends from the surface to 2,500 feet above the airport field elevation. The vertical boundaries are marked with a bold blue number, surrounded by a bold blue dashed square.

The number represents the ceiling of Class D airspace in hundreds of feel MSL. In this example, the altitude is "29," or 2,900' MSL. Altitudes for Class D airspace are inclusive, so in this example, if you were flying at 2,900' MSL, you'd be in Class D airspace.

If a Class D airspace lies under a higher class of airspace, its published ceiling is actually part of the higher class airspace. In this case, you'll see a "-" before the ceiling number. Centennial's Class D airspace extends to, but does NOT include, 8000' MSL. This is because Denver International's Class B airspace starts at 8000' MSL. If you were flying at 8000' MSL over Centennial, you'd be in Denver International's Class B airspace.

Class D Weather Minimums

Class D minimum weather requirements exist so that you can see and avoid other aircraft. Since not all Class D control towers have radar scopes, ATC wants you to stay far enough away from the clouds so you can see and avoid other airplanes, especially jets flying faster approaches.

An easy way to remember VFR weather minimums for Class D airspace is the phrase "3 Cessna 152s". Each number in the phrase stands for one of the distances:

• 3 Statute Miles of visibility
• 1000' above clouds
• 500' below clouds
• 2000' horizontal from clouds

Since Class D is controlled airspace all the way to the surface, you can't fly VFR when the ceiling (a broken or overcast cloud layer) is less than 1000' AGL (FAR 91.155 (c)), or when the visibility is less than 3 SM. When weather conditions are worse than 1000' and 3 SM, IFR aircraft could be flying instrument approach procedures, and you wouldn't have the ability to see-and-avoid them as they break out to land.

Since Class D airspace is controlled to the surface, you can request a Special VFR (SVFR) clearance when weather conditions are below the standard minimums. Under Special VFR, you need to remain clear of clouds and maintain a flight visibility of at least 1 SM. If you're taking off or landing, the reported ground visibility must be at least 1 SM.

You also need to hold a private pilot certificate, and you can only operate SVFR between sunrise and sunset. To operate SVFR after sunset, you must be instrument rated, and your aircraft must be equipped for instrument flight.

To request a Special VFR clearance, contact the tower. If traffic permits, they can clear you into the airspace under Special VFR. However, if aircraft are using instrument approach procedures for the airport, you most likely won't be cleared.

Radio Procedures And Required Equipment

All aircraft operating in Class D airspace need to establish two-way radio communication with ATC prior to entering the airspace. What's that mean? It means you need to:

1) Call ATC on the radio before you're in Class D and tell them:

• Your position
• Altitude
• Your destination/intentions

2) Get a response back from ATC that includes your callsign (tail number)

• Once you hear your tail number, you've established two-way radio communication, and you can enter Class D airspace
• Note: if the controller responds with "(Aircraft tail number) standby", you have established two-way radio communication, and you can enter Class D airspace.
• If you don't hear your tail number, you CAN NOT enter the airspace
• If the controller is busy, they can ask you to stay out of Class D airspace until they're ready.

While a Mode-C transponder and/or ADSB-out are helpful to the tower, it isn't required to be installed on your aircraft to enter class D airspace.

Speed Restrictions

In any airspace, aircraft can't exceed 250 knots when they're below 10,000' MSL. (This is the light blue area in the graphic.)

However, when you're within 4 NM of the primary Class D airport and at or below 2,500' AGL (above the ground), you can't exceed 200 knots. (This is the dark blue area in the graphic.) What's a primary airport? It's the main airport inside Class D airspace.

Fly With A Plan

While a Class D airport might not seem as intimidating as one found inside Class C or B airspace, you shouldn't let your guard down. Traffic patterns can get full of light aircraft quickly. Mix in a jet or two, or an airline departure, and you've got yourself a busy day of flying. If you think it's hard for you, imagine the tower controller trying to keep track of everyone!

Want to learn more about airspace? Check out our National Airspace Course.