In the earlier military aircraft designed since the last war, equipments and systems were generally isolated, each performing its own function independently of the others. Methods of checking correct behaviour of the aircraft system were generally by means of switching on and operating the equipment under reasonably known conditions—that is, for example, observing that indicators showed the correct situation. The methods of flight testing where there was no ready display indication were simply go, no-go methods which generally speaking merely indicated that power was supplied to the equipment under examination. In the Canberra for instance although fairly reasonable space was available, airborne checking in-flight was fairly minimal, most of the system tests taking place prior to flight and in some cases prior to installation of the equipment. With the advent of more miniaturised electronics it was possible to achieve more integrated systems and also system testing, although the natural trend towards smaller and faster aircraft rendered space a premium and therefore such functions more difficult. In the Lightning aircraft there appeared for the first time a fully integrated flight control system which was potentially integrated with the radar system. Figure 1 is a flow diagram which shows a considerable amount of system inter-relationship. In addition there originated a requirement for an automatic test system which was developed and gave a first experience of the use of a complex computer in the testing field. The main problems associated with its use lay in the difficult and time-taking connection to the aircraft and the fact that build up of adverse tolerances in the equipment could result in the rejection of systems which were themselves flight worthy. These factors resulted in the system being only viable at second line. A considerable number of lessons were learnt, however, which have since been applied on a smaller but equally complex scale.