INJURED WAS OPERATING A 21 SC SHUTTLE CAR ON THE #2 SECTION MMU 003-0 WHILE TRAVELING 1A THE #5 ENTRY 40' INBY THE INTERSECTION, A RIB OF APPROX. SIZE 29""L X 36""W X 1"" TO 6"" THICK FELL INTO TH E OPERATOR'S COMPARTMENT, STRIKING HIM ON THE RIGHT LEG, RESULTING IN POSSIBLE KNEE DAMAGE.