Airspace Misperception

I flew for years with the metal image of the “up-side-down wedding cake” as the shape of Class B and Class C airspace — and the perception I had in my head was completely wrong!

Aviation writers are all well-meaning, but unfortunately have passed on a mis-perception about the size and shape of some airspace types. I have been just as guilty. Figure 1 and figure 2 (below) are taken from FAA materials. Figure 1 is from a quick reference card and figure 2 is from the Aeronautical Information Manual. Both depict Class C airspace. Both give the impression that the shape of a Class C airspace is like a water tower or a mushroom. I saw these diagrams and these images became my own as I learned airspace and as I flew in the airspace system. Now I believe that both images are completely incorrect.

To get an idea of how a Class C really looks and how you should see it in your mind’s eye, take a look at the typical Class C airspace at Austin, Texas, only slightly modified by yours truly.

» View Sectional Example

I have placed a nautical mile scale across the Class C airspace to help give us the proper perspective. Class C airspace sometimes comes in non-standard shapes, but this one is right by the book.

The Class C actually has three rings, but only two are shown on the chart.

The Outer Area is the farthest outer ring — and it is not shown on the chart. The Outer Area has a minimum radius of 20 nautical miles (40 all the way across) and extends as high as the primary airport’s radar and radio coverage. It is in this area that incoming pilots are to establish radio communications prior to entering the Class C Outer Ring.

The Outer Ring is 10 nautical miles in radius (20 miles across, as shown). Pilots must have established radio communications prior to entering and their aircraft must be equipped with a functioning altitude reporting device (Mode C transponder) while operating inside this ring. The Outer Ring does not touch the ground. Instead it exists as a “shelf” that overhangs the surface underneath.

The Inner Ring is 5 nautical miles in radius (10 miles across) and makes up the central core of the airspace. The Inner Ring’s area touches the ground and includes in its middle the Class C’s primary airport.

Altitudes: The standard height of a Class C airspace is 4,000 feet above the surface. The airport elevation is usually rounded off to the nearest 100 feet and then 4,000 feet is added to that number. Austin has an airport elevation of 542 (shown on the chart). That rounds down to 500. When we then add 4,000 we are left with 4,500 and the resultant “45” takes its place inside the north side of the Inner Ring on the chart. Under that number is a line and, beneath that, the letters SFC. The entire collection designates the top of the Class C to be 4,500 feet above Sea Level and indicates the airspace extends down to the surface (SFC) in the Inner Ring.

Please look back at figures 1 and 2. Both illustrate that the Class C is as tall as it is wide. If these shapes were proportional that would make a Class C both twenty miles across and 20 miles tall. The top of the Class C would therefore be approximately 120,000 feet MSL! The actual height of the Class C is less than one mile. (4,000 feet or about 2/3 of a mile) So the images of the Class C shapes that are in most of the books and that we first learned from are incorrect by more than a factor of 20 — a gross caricature of reality.

Below is my attempt to re-size the Class C more correctly. It is much less tall and much wider than more traditional (read: less accurate) illustrations.

BOTTOM LINE: We fly in three dimensions so we must have the correct three-dimensional perception of what surrounds us. When we fly we should be able to “see” the airspace as it truly is. Maybe it’s time we started teaching it the way it truly is