WASHINGTON — The Federal Aviation Administration will begin reducing at 40 busy U.S. airports on Friday, potentially affecting thousands of flights.

The are among the busiest in the U.S. — including Atlanta, Denver, Dallas, Orlando, Miami, and San Francisco. In some of the biggest cities — such as New York, Houston and Chicago — multiple airports will be affected.

U.S. airlines began canceling flights nationwide Thursday due to the FAA's order. Hundreds of flights scheduled for Friday were already cut at some of the 40 busiest airports, and the number of cancellations climbed throughout Thursday afternoon.

At least 456 flights have already been canceled on Friday, . That’s more than four times higher than the number of flights cancelled Thursday.

Airlines will phase in the cuts at the

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