by Jeff Casto

Created on: September 18, 2009

Say "sectional" to a pilot and the first thought isn't a piece of furniture. Instead, they will immediately think of the sectional aeronautical chart used in aviation. One special group of shaded lines and circles on the sectional is especially important because each of these indicates a designated controlled airspace.

In the United States there are six distinct classes of airspace, A, B, C, D, E, and G, assigned by the Federal Aviation Administration (FAA). There is no Class F airspace in the U.S. Each airspace has its own set of rules and restrictions. These regulations are designed to reduce the chance of a mid-air collision between aircraft.

To a new aviation student, learning how to recognize the different airspaces on the sectional and the rules associated with each can seem overwhelming, but, like most complicated tasks, once you break the airspaces down into individual pieces, understanding the structure becomes easier.

Class A

The most logical starting point for studying U.S. Airspace is Class A, not because it is first alphabetically, but because it sits on top of all the other airspaces. Starting at 18,000 feet above sea level, or "MSL" (Mean Sea Level), and extending up to 60,000 feet MSL, Class A airspace is generally the domain of larger, pressurized aircraft, although there are exceptions. All flights in Class A airspace are IFR, or Instrument Flight Rules, and pilots must be instrument rated. IFR allows pilots to fly in clouds or other reduced visibility situations. In addition, the FAA requires pilots to file a flight plan before every flight in this airspace.

Pilots who are not instrument rated, or choose not to fly IFR, fly VFR, or Visual Flight Rules, which means just what it sounds like-flying by recognizing landmarks and features on the ground. When flying VFR, pilots are responsible for avoiding other aircraft by using the "see and be seen" method. Using "see and be seen" in class A airspace would not be practical. Imagine two airliners flying toward each other at 500 miles per hour. Their closing rate would be 1000 mph. By the time the pilots saw each other, it might be too late to avoid a collision. Thus, the requirement to always file a flight plan and fly IFR in Class A airspace.

Every aircraft in Class A airspace must have a working mode C transponder on board. This device provides Air Traffic Control (ATC) with information about the plane's position. With this information, ATC can assist in maintaining aircraft separation.

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