This year, if you're flying up or down the East Coast around Thanksgiving, you might be passing through airspace that's usually reserved for the U.S. military.

But as you're dozing off in that airspace during your flight home - groggy from that third helping of turkey - you might start to wonder: Why can't I fly in this part of the sky all year round?

President Bush announced Thursday that this Thanksgiving season, from 4 p.m. EST Wednesday through Sunday, the Pentagon will allow commercial airliners to use two corridors that cut through airspace that's usually restricted to the military. It's part of a plan aimed at easing flight delays around the holiday.

Why does reserved military airspace exist in the first place? And when it comes to flight delays, will the opening up of the flight paths be a Thanksgiving treat - or a turkey?

Here are answers to those questions and others.

Q: Why does the military need its own airspace?

A: The military needs the space to conduct exercises - anything from flight maneuvers to simulated combat to dropping practice bombs - where they want to keep their distance from civilian flights for safety reasons. (Some of the swaths of airspace cover tens of thousands of square miles.)

Fred Pease, executive director of the Department of Defense's Policy Board on Federal Aviation, offered the example of fighter planes conducting combat practice.

"You wouldn't want that activity to by carried out - because it's very dynamic - close to an airliner that's trying to travel from point A to point B. So you segregate that activity."

Q: How much use does the military get out of its reserved air space?

A: That varies quite a bit, based on geography and what's going on at nearby military bases. If a given base is conducting an elaborate exercise, a large area of reserved airspace will get busy for a stretch of time.

Q: Can civilian flights use military-reserved flight space when no military exercises are taking place?

A: Sometimes, yes. So the administration's plan may not be as revolutionary as it may sound.

While it's unusual for military flight paths to be made available for civilian use days in advance - as is happening this Thanksgiving - the military and the Federal Aviation Administration will often agree to let this happen on a case-by-case basis.

Military officials "don't just keep everybody out for no reason," said Cass Howell, chairman of the aeronautical science program at Embry-Riddle Aeronautical University in Daytona Beach, Fla. "If the airplanes from the military are not there, then the FAA would typically ask and they would get permission."

Q: Where are the two air corridors that are being made available to commercial flights?

A: They're just off the East Coast, extending from an area near Long Island in New York to a spot in the Atlantic east of the Florida-Georgia border. From that point, the corridors meet up with flight paths already available to civilian aircraft. One of the paths goes north, the other goes south. In both corridors, commercial airliners are allowed to fly at altitudes of 24,000 feet and higher.

Q: What is the administration hoping to accomplish?

A: It's all about reducing congestion in the crowded skies around New York - which is to blame for 75 percent of the nation's air traffic delays.

The chief goal is to "get people out of the New York area quicker, especially if we have (bad) weather up and down the East Coast," said Nancy Kalinowski, systems operations vice president at the FAA.

Q: Why is the airspace around places like New York, Atlanta, Chicago and Los Angeles so congested?

A: Howell, of Embry-Riddle, attributes this to the airlines' "hub and spoke" system, where large numbers of flights converge on a handful of airports, where passengers are then redistributed on connecting flights.

"Whenever you have that kind of system, where people are going to all go through one of those chokepoints, at some point, you reach capacity. And some of these places reached capacity years ago."

Q: Will the availability of the two corridors of military airspace help solve this problem?

A: It might help somewhat. But congestion in the skies is one of many issues causing delays - a relatively minor one, some say.

"It's not an airspace issue," Howell said. "It's a lack-of-concrete issue, in my view. We just don't have enough airports for the activities that we need to do in terms of air commerce."

David A. Castelveter, vice president for communications at the Air Transport Association - a trade group representing major airlines - says the use of the military airspace is "a very good first step that will help us should we run into any severe weather over the holiday." But he adds, "There is no one solution to reducing delays at JFK (Airport) or in the congested New York airspace."

Doug Church, spokesman for the National Air Traffic Controllers Association - a union representing controllers - doesn't think opening the military airspace will help reduce delays at all.

"The reason is we just don't have enough staffing," he said, saying there are 7.5 percent fewer fully trained veteran controllers on staff now than this time last year.

"Any time you open up a new route of airspace … it takes an extra position to be able to handle that," he said, adding that large air traffic centers are "working short-staffed as it is, over the course of the summer having to combine positions, having to combine sectors of airspace, which slows things down because you just can't run more traffic with fewer controllers. It's not safe."

- Associated Press writer Michael J. Sniffen in Washington contributed to this report.