Airspace Classifications
Here is a profile view of the various classes of airspace.
Classes A, B, C, D and E are controlled airspace where air traffic control (ATC) service is provided. Class F airspace is not used anywhere, and Class G airspace is uncontrolled. This does not mean no-rules, just that ATC services are not available.
Class A airspace is not shown on your sectional because it covers the entire nation. All we have to remember is that it starts at 18,000 MSL. If you have a UAV that will get to this altitude then are probably in the wrong place.  These are military and airline altitudes.
Class B airspace is from the surface to, normally 10,000 feet MSL surrounding the busiest airports, like JFK Airport in New York, or Los Angeles International in California.  The configurations of Class B areas generally resemble an upside-down wedding cake but none of the Class B airspaces in the US are round. They are all configured for the instrument approaches and departures for that airport.
San Diego is the most un-round of all of the Class B airports in the USA due to the proximity of Miramar Naval Air Station, nearby class C and class D airports, the Pacific ADIZ (Air Defense Identification Zone) and of course, Mexico.
Cleveland’s Class B configuration is the least convoluted.
An ATC clearance and a Mode C transponder are required for all aircraft to operate in the class B area. You really do not want to be flying your drone in a Class-B area because this is where the large commercial aircraft are taking off and landing.
Class C airspace is airspace from the surface to 4,000 feet above smaller airports that have a control tower. The Class C areas usually consist of a surface area with a five NM radius, an outer ten NM circle that extends typically from 1,200 feet to 4,000 feet above the airport. It may be possible to get permission from the control tower to operate a small UAV within their airspace. By the way, the speed limit in Class C airspace is 230 miles per hour. Not that you would ever get a speeding ticket in Class C airspace, but there could be airliners flying at 200 MPH that by the time you see them, it’s too late to avoid them.
Class D airspace extends from the surface to typically 2,500 feet above airports that have a control tower. The configurations of each Class D airspace area are normally up to ten statute miles in diameter and sometimes have extensions to accommodate instrument landing procedures. The main difference from Class C and Class D airspace is that the class D airport has no radar.
Class E Airspace extends upward from usually the surface to the overlying or adjacent A, B, C or D airspace.  If we ignore the Class A airspace which is everything above 18,000 ft, then there is a lot more class E airspace than all the other kinds combined. Class E surface airspace is normally 5 statute miles from an airport without a tower but with an instrument approach procedure.
There is no Class F airspace anywhere in the U.S..  Its description is confusing, but best defined as class G airspace with ATC advisories.
Class G is airspace that is completely uncontrolled. This does not mean that there are no rules in Class G airspace; it means that ATC services are not available. You can fly your UAV in any class G airspace. This low lying blanket of uncontrolled airspace ends when it meets Class B, C, D or E airspace.  There are small, nontowered airports in Class G airspace
Important - Do not confuse Uncontrolled airspace (airspace where Air Traffic Control services are not available) with unregulated.  There is no unregulated airspace in the U.S. 
Controlled Airspace
Controlled airspace is a generic term that covers the different classifications of airspace and defined dimensions within which air traffic control  (ATC) service is provided in accordance with the airspace classification. Controlled airspace consists of:
• Class A
• Class B
• Class C
• Class D
• Class E
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