During cruise flight, the crewmembers heard a “bang” from the rear of the cabin, and the airplane momentarily shuddered and bounced. After landing uneventfully at their destination airport, the crewmembers observed that the number two right engine cowling had separated and struck the right inboard elevator. Inspection of the airplane revealed that the engine cowling upper forward hinge required three bolts. However, only one bolt remained attached, and there was no evidence that the other two bolts tore away during flight. Fretting was observed in the area of the bolt holes, consistent with an oscillation over a period of time, due to the absence of two bolts. The airliner maintenance manual required a general inspection of the area, but did not specifically mention cowling hinge bolts. In addition, the hinges were located in an area that was not easily accessible to maintenance personnel. Probable Cause: The failure of the number two upper forward right engine cowling attachment, which resulted in substantial damage to the right inboard elevator. A factor was the inadequate procedures in the airliner maintenance manual.