Reliability Modeling Approach and Hydraulic Actuators Designed Hinge-Moment Capability

The hydraulic actuators are used to power flight control surfaces of the aircraft and to ensure surface movement. A system of two or three actuators is usually designed depending on the surface and intuitively these actuators are considered as a redundant architecture from a reliability and functionality point of view. The proper reliability modeling of the system of actuators must consider the system's functionality and design constraints for the remaining available actuator hinge-moment in the event of a partial or total actuator failure. As a result, this will affect the reliability assessment of that design. Furthermore, this system of actuators is also designed to provide a second function involving an assurance of the surface stiffness and damping. Generally, this second function does not require necessarily the same number of available actuators in order to be fully provided. Therefore, the criteria for the reliability model might be different depending on the failure definition and severity related to the specific function and the assumed redundant architecture for one function might not be valid for the second function. This will obviously result in a significantly different level of computed reliability. Additionally, this difference in the reliability modeling might lead to some difficulties in the assessment of potential latent failures at the actuators' system level related to the various functions of the actuator. This paper provides an approach to reliability modeling and latent failures designation for the system of actuators designed to perform two or more different functions. Specific actuator failure modes and their respective combinations resulting in a reduction in hinge-moment capability are discussed in regards to the criticality of the related failure conditions associated with both functions. The resulting failures at the system of actuators' level might be latent depending on the system's configuration in a partially failed state and on the specific failure mode. However, this latency may be designed out if the actuator is accurately specified. Some factual application examples are provided in order to illustrate the discussed topics.