To those on the ground, airspace is invisibly boundless. All that changes in an airplane. To make flying safe for passengers and people on the ground, the sky is subdivided into unseen slices delineated by altitudes and distances from specific points on the grounds, like airports. In each of five categories of airspace, pilots and aircraft must meet specific training, equipment, and procedural requirements, from weather and visibility minimums to getting clearance from air traffic control before crossing one of these invisible dividing lines.
Naturally, the more airplanes and airports there are in a given area the more complex the airspace, like DFW. It anchors an angular funnel of Class B controlled airspace that’s roughly 10 miles across where it touches the ground and 60 miles across at its top at 11,000 feet. In this rigidly controlled funnel airliners make their way to and from DFW. It sounds simple until you consider that DFW is not the only airport inside this 60-mile wide circle. Addison is highlighted yellow on this diagram that shows only a few of the 119 airports in this space.
And it is a busy cylinder of airspace. The FAA’s Air Traffic Activity System tracks and reports the number of operations (a takeoff or landing) at airports with control towers. In 2009, the 39 tower controlled airports in Texas handled 4,417,245 operations. Among the top 24 busiest airports in the state, 11 are covered by the Class B airspace over Dallas/Fort Worth, and they account for 37 percent of the state’s operations, or 1,635,443 takeoffs and landings.
Number 1 is, naturally, DFW, with 639,782 operations. Fort Worth Spinks (FWS) is Number 24 with 66,275. (Number 39, if you’re interested, is Beaumont/Port Arthur’s Southeast Regional Airport (BPT), 26,708.) Addison (ADS), with 104,833 operations in 2009, was 11th overall and fourth in DFW airspace, just ahead of Fort Worth Meacham (FTW), 103,694, and just after Dallas Love (DAL), 172,962, and Denton Municipal (DTO), 142,104.
The point is that not all of these 1.635 million operations are circling down in the funnel toward their destinations. A good many of them are flying under the lower levels, at least for a while. The Class B layer over Addison starts at 3,000 feet and goes to 11,000 feet. Nearly all business jets leave Addison on an instrument flight plan. Before takeoff, air traffic control clears them through and out of the Class B airspace on a specific departure route, and they reverse the process when coming to Addison.
When the weather is good, most of the other general aviation traffic, from people flying their own planes on business or for pleasure and newcomers learning to fly, stay out of Class B by flying lower than 3,000 feet, just as the pilots do at the other airports under DFW’s airspace. So when you look up at the sound of an airplane flying over, see no other airplanes in the immediate vicinity, and wonder why he or she is so low, remember that invisible layer of Class B airspace that starts at 3,000 feet, and that this single airplane is just one of nearly 2 million making its way to or from greater Dallas every year.