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 extraordinarily 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". Forms of redundancy  There are four
major forms of redundancy, these are: - Hardware redundancy,
such as DMR and TMR - Information redundancy, such as error
detection and correction methods - Time redundancy, including
transient fault detection methods such as Alternate Logic -
Software redundancy such as N-version program