The mistake during a test caused the shutdown of the Chicago-based airline's main flight-operations computer and a backup system, Chief Operating Officer Pete McDonald said in a telephone message to employees today.

"Our operations are virtually back to normal," Chief Executive Officer Glenn Tilton said on the call, adding that the airline had "only a handful of delays remaining this morning."

United relies on the operations computer for everything needed to dispatch flights, including managing crew scheduling and the required measurement of planes' weight and balance before takeoff. The breakdown delayed hundreds of flights and forced cancellation of dozens of others.