1. Introduction

It is well-known fact that most of the smaller aircraft can be manually controlled since the loads on the flight control surfaces are less and well within the handling capability of pilot. However, to reduce the pilot’s workload, hydraulic boosters are used in such aircraft with lower operating pressures. Redundancy of hydraulic system is not an essential consideration in such applications since the aircraft can be fully controlled by the pilot in the event of hydraulic power failures. However, many modern-day aircraft with large take-off weights and propelled by jet engines result in control loads which go beyond pilot’s handling capability and positively calls for a powered flight control system. Further, this powered flight control system gets more complicated due to the interfacing of automatic flight control system (AFCS). Factors like reliability and high response characteristics with high operating load holding features required for the flight control operation have inadvertently resulted in the use of hydraulic systems in modern-day aircraft. These systems are used in conjunction with electrical interfaces like electrically operated direction control valves and sensors for precision actuation of the flight control surface. Hence, the system in whole is referred to as electro-hydraulic actuation systems.