Hundreds of flights have been cancelled across the US as airlines begin complying with the Federal Aviation Administration's unprecedented order to scale down routes nationwide because of the government shutdown.

While the agency order left some passengers scrambling to figure out backup plans, most were relieved to find their planes still on schedule as airlines slowly phased in reductions at the nation's busiest airports.

The 40 airports selected for the slowdown span more than two dozen states and include hubs such as Atlanta, Dallas, Denver, Los Angeles and Charlotte, North Carolina.

More than 800 flights were called off nationwide — four times the number cancelled Thursday, according to FlightAware, a website that tracks flight disruptions.

Airports in Chicago, Atlanta, Denver, Da

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