The essence of preventive maintenance (PM) is to achieve the highest possible level of facility availability at the lowest cost. Reliability-centered maintenance (RCM) can be used to develop effective PM programs. Availability is the instantaneous probability that a system or component will be available to perform its intended mission or function when called upon to do so at any point in time. RCM is a logical, structured framework for determining the optimum mix of applicable and effective maintenance activities needed to sustain the operational reliability of systems and equipment while ensuring their safe and economical operation and support.