Redundancy is a common approach to improve the reliability and availability of a system. Adding redundancy increases the cost and complexity of a system design and with the high reliability of modern electrical and mechanical components, many applications do not need redundancy in order to be successful. However, if the cost of failure is high enough, redundancy may be an attractive option.
In many safety-critical systems, such as fly-by-wire and hydraulic systems in aircraft, some parts of the control system may be triplicated, which is formally termed triple modular redundancy (TMR). An error in one component may then be out-voted by the other two. In a triply redundant system, the system has three sub components, all three of which must fail before the system fails. Since each one rarely fails, and the sub components are expected to fail independently, the probability of all three failing is calculated to be extremely small; often outweighed by other risk factors, e.g., human error. Redundancy may also be known by the terms "majority voting systems" or "voting logic".