No. 1 London Heathrow
London Heathrow airport handled 14.4 million passengers traveling to and from the U.S. in 2013, representing a growth of 2.1 percent. Reuters

[This is a breaking story. Check back for updates.]

The London airspace has reopened after a power outage at a control center forced Heathrow Airport to divert flights trying to land and to delay those still waiting to take off. The airspace closed because of a power outage, Heathrow Airport reported on its Twitter feed Friday. It was not immediately clear what caused the power outage or how many flights were affected.

London airspace was originally supposed to be closed until 7 p.m. GMT (2 p.m. on the U.S. East Coast). Huge delays were predicted at all of the London airports, CBS reported. But about an hour after the initial announcement, the Associated Press reported that the computer glitch causing air delays over London had been resolved.

Pictures of flight radar Friday showed a cluster of planes over Heathrow Airport, the third busiest in the world by passenger traffic after Atlanta and Beijing.
Although Heathrow Airport said London airspace was closed because of a power outage, others on Twitter said it was because of a computer failure. NATS confirmed that a technical problem was reported at Swanwick air traffic control center. The computer failture could have been caused by anything from a software malfunction to hacking.
All flights in and out of Heathrow have been delayed. Planes full of passangers are currently sitting at their gates.
Passengers sitting on the tarmac at Heathrow complained about the delays on their Twitter accounts, while others said they were being provided with refreshments.
Heathrow Airport is normally one of the most crowded in the world. According to a recently published book, a plane lands, on average, every 45 seconds at the airport. Seven out of 10 flight arrivals to Heathrow come from the east over London and the remaining 30 percent come from the west over Windsor.
According to the book, Heathrow has 65 air traffic controllers who undergo three years of training each.
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