DALLAS (AP) — A key computer system used to run many daily operations at American Airlines failed Tuesday, forcing the nation's third-largest carrier to ground all flights across the United States for several hours and stranding thousands of frustrated passengers at airports and on planes. Flights already in the air were allowed to continue to their destinations, but planes on the ground from coast to coast could not take off. And travelers could do little to get back in the air until the computer system was restored.