During taxing, the aircraft can be treated as a three wheeled vehicle. This work deals with the dynamic behavior of this three-wheeled vehicle during the steering mode. Conventionally the hydraulic power steering systems incorporate either hydraulic or electrohydraulic servo-systems. Nevertheless, Part 1 of this paper deals with the dynamic behavior of a steering system incorporating three directional control valves of poppet type in one housing. A nonlinear mathematical model of the directional control valve assembly was developed and used to simulate the valves dynamic behavior. The validity of the simulation program was evaluated in the steady state by comparing simulation results with results of experimental work conducted on the studied valves. The construction of the mechanical parts is explained and the kinematics of the steering system was analyzed and presented. The transient response of the steering system incorporating the directional control valve assembly was calculated, presented and discussed.