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This report describes the operational evaluation of an enroute system terrain-avoidance function in conjunction with the National Airspace System Enroute A3d2.1 system. Tests were conducted at the National Aviation Facilities Experimental Center, Atlantic City, New Jersey, in a low-altitude environment with simulated digital target data. Tests were designed to evaluate the performance of the terrain-avoidance function with respect to detection capability and adequacy of warning provided. Results indicate that although the terrain-avoidance function performed adequately for most enroute situations, false or late alerts could occur, due to the lag between the terrain-avoidance vector line and true aircraft heading. In most instances, the displayed information could be immediately and correctly interpreted by air traffic controllers and relayed to the pilots in adequate time for a safe response. Several functional changes made to the terrain-avoidance program to prevent problems encountered during testing were not subjected to detailed verification, thereby indicating a need for further testing of the function. Although these problems were encountered, the desirability of having a terrain-avoidance function as part of the enroute National Airspace System was not derogated.