NASA and the U.S. Army have designed, developed, and tested a Computer Aiding for Low-Altitude Helicopter Flight guidance system. This system provides guidance to the pilot for near-terrain covert helicopter operations. The guidance is presented to the pilot through symbology on a helmet mounted display. This system has demonstrated the feasibility of a pilot-centered concept of terrain flight guidance that preserves pilot flexibility and authority. The system was developed using extensive piloted simulation and then implemented in a UH-60 Blackhawk helicopter for flight development and evaluation. A close correlation between simulation and actual flight was found; however, in flight overall pilot workload increased and performance decreased. This paper presents a description of the basic system design, simulation, and flight evaluations. Following the description will be a discussion of differences reported by the evaluation pilots between simulation and flight that affected their workload and performance. These differences included background lighting, aircraft vibration, and aircraft response to turbulence. The net effect of these differences was to significantly increase pilot workload and decrease performance by degrading the pilot's ability to use the helmet mounted display symbology.