A methodology for optimal trajectory planning useful in the nap-of-the-earth guidance of helicopters is presented. This approach employs an adjoint-control transformation along with a one-dimensional search scheme for generating the optimal trajectories. In addition to being useful for helicopter nap-of-the-earth guidance, the trajectory planning solution is of interest in several other contexts such as robotic vehicle guidance and terrain following guidance for cruise missiles and aircraft. A distinguishing feature of the present research is that the terrain constraint and the threat envelopes are incorporated in the equations of motion. Second order necessary conditions are examined.