Some airspace symbols on the sectional chart are not defined on the chart’s legend; that’s not so helpful, but maybe this is.
Look at the chart symbol of the Pittsfield Penstone (PPQ) airport — Figure 1. The airport symbol and data block of information both look normal, but what is that thin magenta circle around the airport telling us? The chart’s legend has no thin magenta circle like the one around Pittsfield Penstone, so what is going on in there?
PROCESS OF ELIMINATION
Let’s first look at what it is not. Understanding the subtle differences of each marking will make quick recognition that much easier. The particular magenta line in question does not indicate:
Mode C: Magenta circles do ring large Class B airports that require aircraft flying inside the ring to have a transponder that reports the aircraft’s altitude, BUT true Mode C lines are labeled with the words ‘Mode C.’ The ring around PPQ is not so labeled and PPQ is not a Class B airport.
Class C: The magenta ring around PPQ is not the symbol for Class C airspace. At airports designated as Class C, the chart shows two circles — one inside the other. These lines designate the horizontal diameter of the Class C area — the inner circle’s Class C airspace touches the surface. Class C rings are magenta, like the ring here at PPQ, but Class C lines are thicker, and if Class C airspace were present here there would be another outside ring and the inside ring would have a symbol indicating that the Class C went down to the surface. That symbol also has the MSL altitude of the top of the Class C above a line and the letters ‘SFC’ for surface, below the line. The PPQ airport has none of these, so the magenta ring can’t be a Class C ring.
Class D: The Class D symbol is a blue dashed line around the airport, so PPQ can’t be Class D.
Class E: In the case where controlled, Class E airspace, reaches the ground, a magenta dashed ring is shown around the chart. The ring at PPQ is not dashed — it is a solid line — so it can’t be Class E.
Class G: The symbol used for the edge of an area where Class G starts at the surface and stops at 700 feet above the surface is a magenta shading… not a solid line. So this ring around PPQ is not telling where the boundary between Class G and Class E is located.
SOLVING THE MYSTERY
We now know what the airspace at PPQ is not, but how do we figure out just what it actually is. To do that, it will help to have a little wider view of the area surrounding PPQ — see Figure 2, below.
This is the same area as before, but now we can see that the PPQ airport and the surrounding area also contains the symbol around it of a Military Operations Area (MOA). The MOA symbol looks like a ‘comb,’ with the straight line indicating the outside boundary of the MOA and the ‘teeth‘ of the comb pointing into the MOA. The chart indicates that the Pruitt A MOA overlaps the PPQ airport area.
Looking at the chart, as it appears, we only see the MOA’s boundary and shape — we don’t know the vertical boundary of the MOA. In other words, we don’t know the altitude the MOA begins and how high up it goes? To answer these questions you must look at the ‘flap‘ of the chart — there will be a table that lists all MOAs on a particular chart somewhere on the chart itself (usually behind the chart legend). Figure 3, below, is the MOA information for this chart.
You can see that the Pruitt A MOA starts very low — at 500 feet above the ground — and extends up to 6,000 feet above sea level. Because the Pruitt A MOA starts so close to the ground, this MOA would ordinarily swallow up the traffic pattern at the Pittsfield Penstone airport, BUT that is where the magenta solid line around PPQ comes in.
Look again at the second figure from the top and find the note about the line that is printed north of the PPQ airport symbol. The note says: ‘Pruitt A MOA excludes airspace 1500 feet AGL and below.’ A magenta dotted line ending with an arrow connects this note to the magenta ring. Mystery solved!
Unlike most line markings, this magenta ring is not a symbol that tells us what is inside the circle — it is a symbol telling us what is outside the circle. Ordinarily the MOA — and its associated traffic — would come across the PPQ airport at 500 AGL, but to prevent a potential conflict between the MOA and the traffic pattern at PPQ an inner ‘core‘ of the MOA has been cut out. Now we could fly a normal 1,000-foot-AGL traffic pattern at PPQ, and we would still be 500 feet below the base of the MOA… at least when we’re inside the magenta ring area. Figure 4, below, is a vertical cross-section diagram of what the airspace looks like.
BOTTOM LINE: If you’re confused, try to keep an open mind; sometimes airspace symbols tell us what is not there, instead of what is.