Traffic at Chicago's two busiest airports was slowly returning to more normal levels Monday, three days after a fire at a regional air traffic control center disrupted much of the nation's air-travel system. The Federal Aviation Administration has temporarily assigned other controllers to oversee the airspace normally covered by the crippled suburban facility, but full service may not be restored for two weeks. A closer look at some key questions surrounding the many delays and cancellations:
Q: How many flights have been canceled?
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A: Since Friday morning, more than 3,700 flights have been canceled at O'Hare Airport, the nation's second-largest airport by passengers, and Midway Airport, the nation's 21st largest.
Q: Which airlines have been hardest hit?
A: United Airlines and its regional partners canceled 1,579 flights, according to flight tracking site FlightAware.com. American Airlines and its regional carriers canceled 1,415 flights, Southwest Airlines 601 flights and Delta Air Lines 46 flights.
Q: How did airlines decide which flights to scrap?
A: The vast majority of flights canceled by American, Delta and United were on their regional partner airlines. Those jets, which fly to smaller cities, typically carry 50 or 75 passengers, compared with the 150 or 200 on larger planes. Yet they still require the same amount of work for controllers.
Q: What started all these problems?
A: A contractor at the FAA's center in the Chicago suburb of Aurora allegedly set a fire Friday in the basement telecommunications room and then attempted to commit suicide by slashing his throat. Employees had to evacuate the center. Fire, smoke and water damaged vital telecommunications equipment.
The contractor, a 36-year-old man, appeared in federal court Monday on charges of destruction of aircraft facilities, a felony punishable by up to 20 years in prison.
Q: So did this just affect Chicago?
A: No. As planes fly across the country at high altitudes, they are handed off from one regional control center to another. There are 21 such facilities. The one in Aurora covers 91,000 square miles of airspace over five states and hundreds of airports.
Q: What did the FAA do to keep planes flying?
A: Responsibility for any air traffic flying above 18,000 feet in that region has been split up among the four neighboring air traffic control centers in Minneapolis, Kansas City, Cleveland and Indianapolis. Further, the FAA is redirecting transcontinental flights around Chicago's airspace.
Q: What about flights to the Chicago area?
A: Traffic below 18,000 feet is being handled by 19 regional facilities, which hand off approaching flights to the individual airport control towers for clearance to land.
Q: If the FAA was able to create such a backup system, why were so many flights canceled?
A: First, it took some time to get the plan in place. Next, there was a problem sharing data between the various control centers. Since the fire took out telecommunications, there was no easy way to exchange flight data between Chicago and the four neighboring centers. The FAA was typing in flight-plan information for each plane and printing it out on a strip. To safely manage that process, the number of planes in the sky had to be reduced. The government has since been able to automate much of the process, significantly improving the number of planes that can be handled.
Q: What are delays like now?
A: The FAA said Monday that the takeoff and landing rate at O'Hare was at 80 percent of the rate last week at the same time. Midway's rate was at more than 90 percent of the prior week's rate. Luckily for the airlines and passengers, the area has experienced mostly calm weather, with no severe thunderstorms that could further slow flights.
Q: When are operations expected to get back to normal?
A: The FAA says it will have the Aurora center open on Oct. 13.
Q: Why will it take two weeks to fix the problem?
A: First, crews are cleaning the area damaged by the fire, making sure the building is safe. Other crews are reconfiguring space on a different floor to house new equipment. The work involves replacing 20 of 29 racks of fiber-optic cables and other telecommunications equipment and re-establishing more than 600 circuits, "so it's a very, very large job," FAA Administrator Michael P. Huerta said.
Q: In light of the fire, will the FAA change how it operates and secures these facilities?
A: Huerta said the agency might need to treat some buildings, like the center in Aurora, differently because they are more critical to the national air traffic system than others. Over the next 30 days, the FAA will conduct a review of its contingency plans for unexpected events like Friday's fire, as well as security at its facilities.
Associated Press Writer Joan Lowy in Washington, D.C., contributed to this report.
Scott Mayerowitz can be reached at http://twitter.com/GlobeTrotScott.