In addition, reliability began to fall in spite of improved hardware and design technology, or at best was only maintained by the efforts of maintenance teams. At that time, the required utilisation was around 8-10 hours per day for five days per week. This was soon extended to six days per week, even then, the requirement of today, for a training utilisation of virtually 24 hours per day for seven days a week could be foreseen. It thus became obvious that the demands for increased fidelity of simulation and reliability could no longer easily be met with analogue machines even with the use of the new solid state elements which had appeared. Around this time the second generation of digital computers, started to materialise, and were able to satisfy the speed and cost requirements of flight simulation. As a consequence, there was an almost total swing to digital simulation for all but the simplest trainers.
It was realised from the earliest days of programmable electronic digital computers that a potential application would be in real-time digital simulation. The advantages of digital computers, improved flexibility, repeatability and standardisation, were approached by the U.S. Navy who initiated a research program at the University of Pennsylvania in 1950. The general purpose computers of the time could not be used directly for real-time flight simulation, due to their poor arithmetic and input-output capabilities. A special machine therefore, was designed at the University for their simulator, which was named UDOFT (Universal Digital Operational Flight Trainer). This computer was manufactured by the Sylvania Corporation and completed in 1960. The UDOFT project had demonstrated the feasibility of digital simulation and was mainly concerned with the solution of the aircraft dynamic equations. In the early 1960's Link developed a special purpose digital computer, the Link Mark I, designed for real- time simulation. This machine had three parallel processors for arithmetic, function generation, and radio station selection. In the late 1960's general purpose digital computers designed for process control applications were found to be suitable for simulation, with its large input - output requirement, and the use of special purpose machines declined. Today special purpose digital computers are only used in applications demanding very high speed processing, such as computer generated imagery.
Nearly all of the simulators produced up to the mid 1950's had no fuselage motion systems. This was justified by the statement that modern pilots did not fly "by the seat of their pants", but the fact remained that fixed-base simulators did not feel like aeroplanes to fly. It was found that a handling improvement could be made by empirical adjustment of the control loading and aircraft dynamics simulations which, in part compensated for the lack of motion. Proposals were made by the manufacturers for motion systems, but it was not until the late 1950's that the airlines decided to purchase them.
In 1958, Redifon received a contract from BOAC for the production of a pitch motion system as part of a Comet IV simulator. More complex motion systems were designed capable of producing motions in two and three degrees of freedom, and with the introduction of wide-bodied transport aircraft, such as the 747, a lateral acceleration was required which led to the introduction of four and six degrees of freedom systems. Six degree of freedom motion systems are now the most common. The perception of motion and its effect on training is one of the less understood aspects of simulation and research is still active in this area.