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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.

The paper provides an approach to establish compliance with current regulatory standards applicable to lightning protection of the fuel tank structure for Non-Fault Tolerant Feature Failures (NFTFF) through a numerical probability assessment. The proposed procedure is using the criteria defined in the FAA Policy Guidance for fuel tank structural lightning protection and is aligned with the regulatory path described as petitioning for an exemption. Failure modes of structural components for which fault tolerance has been shown to be impractical need to be addressed and the overall likelihood of fuel vapour ignition due to these failure modes must be shown to be extremely improbable. In order to accomplish this, the quantitative assessment of the overall probability of fuel vapour ignition is performed, along with all relevant data to support the probabilities determined for the purpose of this analysis.

The assumption usually made in the safety analysis of most systems is that the failure of any one component is independent of the failure of any other. If this assumption is not valid due to the system design and implementation, the estimated Fault Tree Analysis (FTA) event probabilities will be more optimistic than what is found in practice. One of the most important modes of failure and one, which can severely degrade the actual safety, is a common mode failure. This type of failure involves the simultaneous outage of two or more components due to a common cause. Common Mode Analysis (CMA) provides evidence that the failures assumed to be independent are truly independent. In reality, this analysis is extremely complex due to the large number of common mode failures that may be related to the different common mode types such as design, operation, manufacturing, installation and others.