Just before Thanksgiving, President Bush announced a number of measures intended to mitigate commercial airline delays over the Thanksgiving and Christmas holidays. One of the measures included opening up military airspace over the East Coast to ease congestion. Seeing as the skies above are pretty wide open, is there such a thing as a midair "traffic jam?"
Yes. Passenger jets and corporate jets flying long distances must fly on one of just 12 routes running north-south along the East Coast, or others that crisscross the nation. The same commercial airways have been administered by the Federal Aviation Administration for decades, and it's a long, slow process to map out a new one. (Because of environmental regulations, this can take up to 20 years.) Still, the established routes do have enough capacity to handle the thousands of flights each day in the United States, but airports don't always have enough runways to land them all—and that creates backups.
Only a certain number of planes can fly the same route at the same time. Because of the limited range of radar signals and imprecise altimeters, the FAA requires that all passenger jets, corporate jets, and personal planes maintain strict separation standards to mitigate the risk of midair collisions. Planes flying at the same altitude must be either three miles or five miles apart, depending on the type of radar system used. (The newer equipment allows for closer flying.) Two planes can also fly one on top of another, so long as they maintain a 1,000-foot vertical separation.
A sector of airspace reaches its maximum capacity when there's so much traffic that planes are close to violating the separation standards. If a given route becomes too crowded—or if bad weather makes portions of it unsafe for flight—air-traffic controllers will deny other planes access and send them onto another airway. Another option is to transfer planes onto open military airspace over the ocean. If the military knows that a particular sector of its airspace will be unused, it can offer the real estate to the FAA for commercial flights.
It doesn't matter how many planes a route can hold if there aren't enough runways to land them. If an airport fills up on the ground, then incoming planes must be placed into holding patterns. That means that other planes seeking to enter the airspace around the airport might need to be redirected so as not to violate the separation standards. In crowded and busy airspace, such as that above New York City or Chicago, these delays could have repercussions far down the line.
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Explainer thanks John Hansman Jr. of the MIT International Center for Air Transportation, Paul Takemoto of the Federal Aviation Administration, and Brian Turmail of the U.S. Department of Transportation.