Fault tolerance, the ability of a system to survive in the presence of faults, is a major research area in avionics. With the increasing use of active control technology (ACT) in military aircraft to allow reductions in wing and tail area giving greater manoeuvrability it is now the case that many aircraft are unflyable without automatic stabilisation functions. If a controller fails even for a very short time this can result in the loss of an aircraft and/or pilot. The consequences in loss of life if a civilian aircraft loses a primary flight control function are even more disastrous and this is reflected by the even higher reliability levels demanded.