Class A Airspace:
It covers the entire nation, so all we have to remember is that its lower limit is 18,000 MSL.

The term "controlled airspace," by the way, doesn't mean that somebody or some agency is up there controlling traffic, but controllers have a pretty good idea of what is up there. Use of our national air traffic control (ATC) service is mandatory in Class A airspace that begins at 18,000 feet MSL and extends upward to 60,000 feet MSL. Altitudes at 18,000 feet MSL and above, in Class A airspace, are commonly referred to in thousands of feet as "Fight Levels," abbreviated FL. For example, flight level two zero zero, or FL 200 = 20,000 feet MSL, FL 600 = 60,000 feet MSL, etc.

The United States, of course, does not own or control airspace outside its territorial limits, but our government has extended Class A airspace out to twelve miles from the coast of the contiguous 48 states along with Alaska and Hawaii.

Class B Airspace:
Class B airspace surrounds the nation's busiest airports and usually goes as high as 10,000 feet MSL, in some cases even higher. The uppermost level of Class B airspace may extend horizontally with radius of up to 15 nautical miles around the airport tower. There is, however, no universal set of Class B dimensions since the flow of traffic, geography and other considerations determine the exact architecture of each Class B area. A sectional or a VFR Terminal Area Chart is very helpful in understanding the design and lateral dimensions of each Class B airspace.

On your sectional chart, horizontal Class B airspace limits are outlined in concentric solid blue circular lines that may be indented or extended in certain places due to geography or air traffic routes. The top and bottom of each layer of airspace, as you can see, are given in what looks like a fraction; for example, 90/40.

That means that this particular layer of airspace lies between 9,000 and 4,000 feet MSL. (SFC stands for surface) A two dimensional sectional map can only show you the horizontal outlines of different layers, you have to use the fractions to visualize the vertical dimensions. If it helps, close your eyes and try to "see" the architecture of the different layers. If you try to think in pictures rather than in words, that may help. Imagine an "upside down wedding cake." with at least three layers.

Check your sectional and you will find that the surface area of a city lying in Class B airspace is colored yellow as are all cities on the sectional. Obviously, this is what the FAA would call "congested" area.

The inner circle of Class B airspace extends all the way to the surface. The outskirts of cities like Detroit, Cleveland and Minneapolis are surrounded by many delightful strips where small planes and ultralights fly, and their pilots know pretty well what to avoid in terms of location and altitude. Pilots who fly near major Class B airports may prefer to use a VFR Terminal Area Chart since these depict a smaller area with much more detail using a scale of 1:250,000. If you fly from a field that is under or near Class B airspace, you should study your sectional or Terminal Area Chart to memorize landmarks that might help you recognize the locations and altitudes of various sections of Class B space.

Intruding into Class B airspace in an extremely serious matter. You may endanger the lives of many others since a fully loaded passenger jet carries hundreds of people. While a violation of Class D or Surface Area Class E space (discussed below) might bring only a strong lecture, a violation of Class B or Class C will almost certainly bring strong penalties.

Class C Airspace:

On your sectional, horizontal Class C limits are shown by solid magenta lines. It is similar in layout to Class B but in magenta not blue.

On your sectional, horizontal Class C limits are shown by solid magenta lines. It is similar in layout to Class B but in magenta not blue.

Altitude fractions show the upper and lower vertical limits as they do with Class B. As with the shelves of Class B airspace, you may fly under or over shelves of Class C airspace, but never into the airspace itself.

If "B" stands for "Big Cities," let "C" simply stand for Cities. The cities under class C airspace are our mid-sized cities. The towers at these fields are equipped with radar -- something that smaller controlled fields (Class D airspace) do not have. Ultralights may not fly in Class C airspace without special permission from ATC. Although the city involved may not be as big as New York or Washington, D.C., it will usually have heavy jet and commercial traffic coming and going.

Class D Airspace:
Small city airports with control towers are usually designated as Class D airspace. Associate "D" with "Diminutive" or "Dime-sized" cities if you like, and do not go there without permission either. Commercial and other IFR traffic may be flying into these fields, and there is often much general aviation activity and pilot training. Look over your sectional and find several Class D fields. The field silhouette itself (not the city) will be pictured in blue -- as all controlled fields are -- with a dashed blue circle around it. By now you have probably noticed that the runways of any airport are drawn to show their direction in terms of the compass, and that runway length is also given. There is more information to look for as well, but back to Class D airspace.

The ceiling of Class D airspace generally extends upward to 2,500 feet AGL over the airport surface but the exact upper limit is shown with a number inside a dashed box outline. The example below has a "54" meaning the upper limit is 5,400' msl. Some Class D fields have little extensions, what look like cogs on a wheel. The entire airspace may look like a key hole with one or more extensions out from the five-mile circle. These extensions of Class D airspace accommodate approaching and departing IFR flights that use standard approach/departure routes.?

It is customary for general aviation traffic that is just passing over or near a Class D field, but not planning to land there, to call the tower (the frequency is on the sectional). Such a courtesy call lets ATC know there is traffic out there in the general vicinity. Remember, a Class D airspace tower may have no radar and may, except for the controllers' eyes, be blind. If you have an aircraft frequency radio and understand radio communication procedures, it may be wise to inform a nearby Class D tower where you are and where you plan to go.

Class E Airspace:
Controlled airspace is officially defined by exclusion, which often does not tell you much. By that reasoning, Class E airspace is controlled airspace that is not Class A, B, C or D or G (explained below) airspace. Not too helpful, but you can be sure that there is a lot of Class E airspace, so much that one could think of it as "E" for Elemental or Everywhere airspace, the airspace out of which all other types are carved. It is the filler that fills in under Class A, and between Classes B,C and D. Its volume is vast. If we ignore the upper cover of Class A airspace, it is safe to say that there is a lot more E than all the other kinds combined.

Class E almost always has one of four lower limits: surface, 700' AGL, 1200' AGL or 14,500' MSL. Most of the country has a Class E lower limit of 1200' AGL.?

Remote areas of the country sometimes have Class E that has a lower limit of 14,500' msl.

Class G airspace:
Class G airspace is a mantle of low lying airspace beginning at the surface. Class G is airspace that is completely uncontrolled. This low lying blanket of uncontrolled airspace only ends when it meets Class B, C, D or E airspace.

Think of Class G as "ground" airspace. It covers almost the entire country. In very remote areas it has an upper limit at 14,500' msl. However the vast majority of area of the country it has an upper limit that follows the contour of the ground. The top of Class G usually is where the floor of the overriding Class E is. Like a coat of paint following the contour of the land. Sometimes the depth of Class G is 700', some times it's 1200', and where Class B, C, D or E extend to the surface there is no surface Class G.

Special Use Airspace:
Military Operations Area (MOA) -- These large areas of the country are shown on your sectional as enclosed by a line of magenta hash marks with a sharp outer edge. Military operations such as training exercises come and go. Permission to fly in an MOA is not required, but a pilot may determine the hours of any current activity by calling the nearest Flight Service Station (FSS). You are at your own risk an responsible for any crash if you choose to fly into a MOA.