Modern flight control systems are analyzed and certified to meet the requirements of the Federal Aviation Administration (FAA) and the Joint Aviation Association (JAA). However, discrepancies in the performance of the hydro-mechanical flight control systems may appear during the life of the aircraft, and analyses must be performed to track and solve these anomalies.  A number of airplanes of different types and manufacturers have exhibited small and uncommanded yaw shudders or "kicks". Concordia University and Bombardier Aerospace collaborated to understand the yaw activity and to track the root causes of lateral phenomena through numerical simulation.  The objective of this thesis is to study more closely the hydro-mechanical power control unit, which seems to be in most cases the suspected part of the rudder control system for yaw activity. The performance of the power control unit was evaluated to track any discrepancies in this system. Three hydraulically independent servo-actuators actuate the rudder control surface. Backlash, deadband or friction is present in the three servoactuators assembly, and each actuator may not take the same load when the aircraft is in flight. Therefore, a load sharing analysis between these actuators is performed in this thesis. The flow forces acting on the servovalve are also presented; and the pressure relief valves that regulate the pressure in the power control unit manifold are also modeled and simulated in detail. Conclusions and recommendations will be made based on the simulation findings and results.