The piloted aircraft provides the supreme example of the highly complex machine performing a precise function in which the control loop is still completed through a human operator. Although both physically and mentally his task has been greatly assisted by the introduction of automatic pilots, auto-stabilisers, power controls, flight directors and the like, he still possesses the ultimate advantage over the machine of being able to use judgment in the face of arbitrarily changing circumstances. Thus, until it is possible in advance to define the required tasks exactly, and to measure the subsequent performance of the system in precise and unambiguous terms, the pilot will be saved from the encroachment of automation.

The performance of the human operator is described by the aptness and speed of his reaction to the pertinent stimuli, in effect his transfer function as an integral element of the overall control loop.