Airspace Overlap

Often when airports are located close together, their overlying airspace can conflict, and the resultant airspace overlap can be more than a little confusing.

Many medium and large size cities in the United States built an airport for their community about 50 or more years ago. As time passed, and the city grew, it absorbed the airport that had originally been built on what, at an earlier place in time, had been the city’s outskirts. A big city demands big airplanes, but the original airport was not built to handle jumbo jets, and now can not lengthen its runways to handle larger airplanes, because of its new status as ‘land-locked.’ So the city builds a new airport, farther out where there is yet room for longer runways. In most cases, the original airport will remain, to become the general aviation airport of the city. This scenario has been repeated in the city of Dallas, Chicago, Las Vegas, Orlando, Cincinnati, Atlanta, Washington D.C., and many others.

The original airport and the new airport in any given situation will, by necessity, be several land miles apart, but the airspace associated with each airport is wider ranging and often finds conflict with the other. These conflicts bring on interesting and sometimes very confusing airspace congestion.

Pilots must be able to imagine the airspace in three dimensions simply by looking at the symbols on a flat chart.

Look at the example of Louisville, Kentucky, above. The largest airport is the Louisville International/Standiford Airport (SDF). Standiford has multiple long runways — some as long as 10,000 feet — and has Class C airspace at and around the airport. The chart tells us more about SDF…

  • The inner circle of the Class C airspace begins at the surface and tops out at 4,500 feet MSL. (We know this from the chart symbol printed just to the left of the airport symbol — the symbol has the number 45 above a line and the letters SFC below the line.)
  • The airport elevation at SDF you can see from the airport information is 501 feet MSL, so the Class C airspace itself is approximately 4,000 feet tall.
  • The outer ring of the Class C airspace at SDF is divided into two parts.
    1. The southern half of the outer circle has a base at 1,700 feet MSL and a top again at 4,500.
    2. But the northern half of the outer circle has a higher bottom — probably because the majority of the actual city lies under this part of the circle. This part of the circle has a Class C base of 2,200 feet and a top of 4,500 feet MSL.

    All this looks normal except for one thing

  • The inner circle at SDF has a large jutting corner intruding into the circle.

Rather than being completely round, the northeastern boundary has a pizza-slice, cut out of it. This unusual shape has been carved into the Class C airspace to accommodate the airspace of another airport — the Bowman Field Airport (LOU). Bowman has Class D airspace, because it has a part-time control tower. Now, most Class D airspace starts at the surface and extends up into the air to 2,500 feet. But here at Bowman, the sky is too crowded for that — the overlapping Class C airspace from SDF starts at 2,200 feet.

Whenever two types of airspace conflict, the one with the letter closer to the front of the alphabet wins. In other words, wherever a Class C attempts to infringe on a Class B, the airspace remains Class B. At Louisville, we have a Class D and Class C conflict — so Class C trumps. You can tell this by looking at the chart symbols on the chart. (The blue dashed line that signifies a Class D is very hard to see with all the other chart clutter, but it’s there.)

The blue dashed line overlaps the inner Class C line in the ‘pizza-slice‘ cut out. Then, in that same area cut out by the pizza slice is the number [–22]. The number [–22] is contained in a four-cornered bracket. The [–22] indicates that the Class D airspace extends up to — but does not include — 2,200 feet MSL. Why? Because the Class C overhang begins at exactly 2,200 feet MSL. The Class C airspace from SDF completely overlaps the Class D airspace at LOU, making the LOU Class D a short and stubby shape.

To see what the airspace at Bowman really looks like, see the figure below. Pilots must be able to visualize that airspace so that they can avoid being in the wrong place at the wrong time.

BOTTOM LINE: It is not enough know where you are above the ground. A pilot must also know what airspace exists at what altitudes over the ground.