In this thesis, we describe a method for controlling the cycle-averaged velocity direction of a fixed-wing aircraft in an unpowered, helical descent. While the aircraft propulsion system is disabled, either intentionally or due to a failure, the aerodynamic control surfaces (aileron, elevator, and rudder) are assumed to be functional. Our approach involves two steps: (i) establishing a stable, steady, helical motion for which the control surfaces are not fully deflected and (ii) modulating the aircraft control surfaces about their nominal positions to ``slant'' the helical flight path in a desired direction relative to the atmosphere, whether to attain a desired impact location, to counter a steady wind, or both. The effectiveness of the control law was evaluated in numerical simulations of a general transport model (GTM). / Master of Science / When an unmanned aircraft is near an authorized airspace (a region of space where the aircraft is not authorized to fly) and experiences a failure such as loss of communication with the control tower,or failure of the GPS or propulsion system, then the aircraft is generally put into an aerodynamic flight termination. In this flight termination method, the aircraft propulsion system is switched off and the control surfaces (aileron, elevator and rudder) are fixed to induce a spin in the aircraft causing it to descend in a helical fashion. However, in the presence of external gusts the aircraft might drift into the unauthorized airspace; or once the aircraft is put into spin, one may want to be able to change the impact location to a safer place. To the best of our knowledge, there exist no control strategies to alter the impact location of the aircraft once it is put into spin and while is continues to spin. In this thesis we describe a method to do so. The aircraft impact location is altered by controlling the cycle-averaged velocity direction of a fixed-wing aircraft in an unpowered, helical descent. While the aircraft propulsion system is disabled, either intentionally or due to a failure, the aerodynamic control surfaces (aileron, elevator, and rudder) are assumed to be functional. Our approach involves two steps: (i) establishing a stable, steady, helical motion for which the control surfaces are not fully deflected and (ii) modulating the aircraft control surfaces about their nominal positions to “slant” the helical flight path in a desired direction relative to the atmosphere, whether to attain a desired impact location, to counter a steady wind, or both. The effectiveness of the control law was evaluated in numerical simulations of a general transport model (GTM).