Since the first decade of this century aircraft have been equipped with instrumentation to show attitude and direction and to assist navigation and to monitor the performance of the prime mover, if any. Their sophistication has grown with the aircraft but while aerodynamic shapes have apparently simplified over the years, instrument configuration has tended towards complex arrays of electro-mechanical analogues.

Fundamentally, instruments are means of data communication between the systems of the aircraft and the human crew, the latter functioning either as part of a closed loop or, alternatively as the monitor of an automatic control. When functioning as a monitor it must always be considered that the human operator may have to revert with minimum delay and disturbance to the first role, namely that of an active part of the control system. In extreme cases of multiplex automatic systems, such as those intended for Auto-land, there may be a case for denying revertion on the grounds that channel redundancy will reduce the failure rate to an acceptable value, added to the fact that the pilot might in fact be incapable of improving the situation after a certain point in time and space had been reached.