The Class G/E Boundary

While conducting a Biennial Flight Review, I once asked a man to get his chart out and point out some uncontrolled airspace — how would you have done? He said he could not point out any uncontrolled airspace, because he only had a Charlotte Sectional with him. I asked why that would be a problem and he said, ‘Because there is no uncontrolled airspace east of the Mississippi River.’ That BFR lasted longer than anticipated…

The VFR flight rules are different depending on if you are operating in Class G or Class E airspace — so how do you figure out where you are?

The truth is that uncontrolled airspace is just about everywhere, but mostly it is down low. In most cases the ground itself is the base of uncontrolled airspace, which has been designated Class G — it usually starts at the surface and goes up to 1,200 feet above the surface. At 1,200 feet there is an invisible boundary line where Class G — uncontrolled airspace, meets Class E — controlled airspace. Below that boundary line, in Class G airspace, 1 mile and clear of clouds is the VFR minimum, but above that line in Class E airspace (where more IFR traffic is likely to be) the VFR minimums raise to 3 miles and 500 below, 1,000 above, and 2,000 horizontal to clouds.

Now look at the French Lick airport — surrounding this airport is a magenta shaded line. This particular shaded line is round on the east side but flat on the west side. The magenta shading indicates that anywhere inside this shaded area the Class G — uncontrolled airspace — does not extend up as high as it normally does. Inside the magenta shaded area the Class G only rises to 700 feet AGL. The boundary between Class G and Class E is lower in the area around the French Lick airport.Look at the chart section of Figure 1 and find the Paoli Airport. Paoli Airport has an elevation of 817 feet above sea level. From the surface at Paoli and extending up to 1,200 feet above Paoli is Class G (uncontrolled) airspace. This means that you and I could practice our takeoffs and landings all day long at Paoli even if the visibility was as low as 1 mile.

Why: French Lick has an IFR approach and the lower Class E allows IFR pilots to descend through the clouds while remaining in controlled airspace. Hopefully, the IFR pilot will descend below the clouds and when this happens there will be less collision threat because VFR pilots flying in controlled airspace will allow more space between them and the clouds. But VFR pilots flying in Class G have less room to operate under the Class E at French Lick.

If you and I wanted to practice our takeoffs and landings here with only one-mile visibility, we would have to fly a very short downwind leg. We could not climb higher than 700 feet above the surface unless the visibility improved to 3 miles and most downwind legs are 1,000 feet above the surface.

Look now at figure 2. This is a drawing I made of the same area, but unlike a sectional chart that looks down on the area, this diagram looks at the surface and airspace above from the side. You can see the Class G airspace is taller over Paoli and shorter over French Lick. Also notice that the edge of the Magenta Shading is actually a stair-step. When traveling from Paoli to French Lick, the overhead Class E drops down to 700 feet from 1,200 feet. The boundary between Class E and Class G also is a mirror image of the surface itself. Paoli is higher than French Lick, having an elevation of 817. The terrain slopes down to French Lick, with an elevation of 792 — so the Class E/G boundary slopes down, too. The boundary is always either 700 or 1,200 feet above the surface depending on if you are within the shaded area or not.


Look at the sectional chart and the side-view drawing and test your airspace skills with these questions:

    1. An airplane is flying directly above the Paoli Airport. The airplane’s altimeter read 2,000 feet. Is this airplane flying in controlled (Class E) airspace, or is the airplane flying in uncontrolled (Class G) airspace?

To determine the boundary between Class G and Class E airspace you must first know the elevation of the ground. In this case the ground is 817 feet above sea level. Next, you must determine if the Class G/E boundary is located at 700 or 1,200 feet AGL. In this case, Paoli does not have any magenta shading; therefore the boundary is 1,200 feet higher than the airport’s surface. The airport is at 817 feet MSL. The boundary between Class E and G would be 2,017 feet MSL (817 + 1200 = 2017). The airplane’s altimeter reads only MSL altitudes and it indicates 2,000. So this airplane is in Class G airspace… but only under Class E by 17 feet.

    2. An airplane is flying directly above the French Lick Airport. The airplane’s altimeter is reading 1,500 feet. Is this airplane flying in controlled (Class E) airspace, or is the airplane flying in uncontrolled (Class G) airspace?

French Lick has a field elevation of 792 and is inside a magenta shaded area. That means that the boundary between Class G and Class E airspace is 700 feet AGL and the Class E boundary would be located at 1,492 feet MSL (792 + 700 = 1492). The airplane is lower than in the one in the first question, but nevertheless it is in Class E. An airplane at 1,500 feet would be in Class E, but just eight feet lower at 1,492 is where Class G begins.

    3. An airplane is flying in the daytime over the Paoli Airport with a visibility of 2 miles due to haze conditions. The airplane’s altimeter is reading 2,500 feet. Is this airplane operating legally for VFR?

No, this airplane is not flying legal VFR, but more importantly, it’s greatly increasing the danger of a mid-air collision. It would be possible to have an IFR aircraft climbing, descending, or crossing the area at 2,500 feet. With only 2 miles visibility, the ability to see and avoid is greatly reduced. VFR at that altitude over Paoli requires visibility of at least 3 miles.

    4. You have an airplane that needs radio work and you want to fly VFR from Paoli to French Lick to get the work done. The visibility on the day of your flight is only 2 miles. Can you still make this trip flying VFR or must you wait for another day?

A VFR flight from Paoli to French Lick with only 2 miles visibility is legal… so long as you fly where only one-mile visibility is allowed. Look back at the side-view of airspace in figure 2. As long as the entire flight was conducted in Class G airspace it would be possible. The pilot would have to stay within 1,200 feet of the ground (under 2,017 MSL) departing Paoli and then duck under to within 700 feet of the ground when crossing the magenta shading.

Important: The fact that it is possible to fly VFR between these airports with such poor visibility points out an important principle that all pilots should understand: Some things that are legal, are neither safe or smart! I would recommend against a low altitude, low visibility flight unless you are extremely familiar with the route and any obstructions along the way.

Editor’s Note: Since 1995, Paul A. Craig has conducted innovative seminars on the airspace system at college and universities, Pilot Proficiency programs, and Flight Instructor Refresher Clinics across the country.