Pilots were forced to abort a takeoff and evacuate passengers from a burning American Airlines flight on a runway at Chicago O'Hare International Airport after the airliner experienced what a federal official said was a rare and serious type of engine failure. Flames and heavy black smoke poured from the side of the Boeing 767 jet as it sat on the runway Friday after the aborted takeoff. Officials said the incident left 21 people injured. Footage from the scene showed passengers coming down emergency slides and hurrying across grass next to the runway as emergency vehicles surrounded the plane. The right wing was drooping toward the ground and appeared to have partially melted.