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Aviation History
1964
1964 - 0473.PDF
288 FLIGHT International, 20 February (96, FLIGHT SYSTEMS SURVEY Strictly for the Record ... multiplexing is achieved by a solid-state multiplexer. The only moving parts in the system are the transducers and recording deck. Play-back and analysis of the recorded information, some companies claim, presents its own problems, not the least of which is that no yard-stick may yet exist against which to assess much of the new details which will emerge. Crash-recorder records will in any case probably be examined only after an accident, being otherwise erased after retention for the statutory after-flight period. For maintenance assessment it may be very difficult to set a level beyond which performance of a recorded system or component is unsatis- factory, and many levels become significant only in relation to several other levels. A given reading may be routine in one flight regime and critical in another. There is the frustrating possibility that the significance of all the readings in a complex system in one type of aircraft may not finally come to be understood before the aircraft is obsolete. Exactly what factors indicate impending failure of one piece of equipment, and are they the same in all the types of aircraft in which the unit is installed? So much information is going to become available, and yet so much remains to be assessed and understood. How many heavy landings, of what severity, for example, justify what main- tenance action on the undercarriage or other parts of an aircraft ? Certainly, the "quick-look" run through of a tape at an en route stop is a far from straightforward procedure. Careful analysis at home base over a period of days is more likely at first. Much more profitable as an immediate step will be "quality control of flight," the assessment of how aircraft are actually handled during normal flying. The parameters, such as heading, speed, height, radio deviation and so on, are relatively unequivocal and can yield valuable information fairly easily, as already stated by MoA. Pilot reaction to such surveillance is naturally mixed, but the records can just as easily exonerate as blame a pilot. Cockpit conversation recording, now becoming mandatory in the USA, is a more difficult prospect. Crew conversation is not necessarily, and need not always be, germain to the flight, and in emergency has often proved surprisingly unrevealing, or even misleading. Royston have considerable play-back experience and have made installations capable of running the tape at ten times recording speed, with level gates producing selected types of output, printed or plotted, and for various types of analysis. In this aspect of flight recording, actual experience is again a valuable asset. One of the major difficulties is that each operator must grasp the significance of flight recording, and then each must decide how best this new technique can be fitted into his particular operations. It is only too clear that no two are going to come to quite the same con- clusions and that none will be spared the energetic attentions of the salesmen—who differ among themselves even more than the operators. This commentary has, if nothing else, illustrated the multiplicity of policies, problems and contentions. One of the main factors encouraging the present proliferation of systems and interests is quite simply that "there's gold in them thar hills." Orders for fleet- wide equipment, both civil and military, are the prize, and no one with skill to offer and company money available will lightly opt out of the race for orders. Somewhere in the technical smoke, there is a small, bright flame of clear comprehension and sure knowledge. 1964 may finally see it emerge into the light of day. Digital Computers for Aircraft By P. A. HEARNE, DCA«, DLC, AFRAOS* A NEW type of equipment, the airborne digital computeris rapidly becoming prominent in the planning of newaircraft. To understand its appeal to the system designer one should start by considering the well worn definition that "analogue computers measure while digital computers count." The world in which we live is basically an analogue one and digital events or happenings occur relatively infrequently. Measure- ment of quantities by analogous means has consequently been the easiest way of obtaining an accurate knowledge of their value. It was natural that, when aircraft started to fly, the measurement of their behaviour in this new world of flight followed the same pattern. Airspeed and altitude, for example, were measured by an analogue of pressure, which was subjected to a simple form of mechanical analogue computation in the airspeed indicator and altimeter to derive the speed and height equivalents of the pressures. On the other hand, where two or three measurements of different quantities have to be combined to obtain results, man has not always constructed a complicated measuring instrument, but has instead taken the numerical values from analogue measurements and used them in numerical calculations to obtain the desired result. A close parallel is found in modern aircraft. Where the infor- mation required for display or transmission to automatic controls must increasingly be derived from a combination of outputs from several different sensors, such as the inertial platform, forward- looking radar and air data system. Analogue computers have been built to carry out the difficult task of integrating many different sensors into a co-operative system, but this often involves straining analogue techniques to their limit to achieve acceptable accuracies, reliabilities and weights. What then are the characteristics of the digital computer, and what does it offer in this difficult airborne systems area? Digital Computers A digital computer is basically an electronic machine which manipulates electrical pulses under the control of a programme prepared and inserted in the computer's store. In crudest terms, the manipulation or computing is carried out by controlling through the programme the operation of a series of transistor switches, known as gates or flip-flops, which open and shut. The store containing numerical values as well as the instruc- tions is similarly simple in concept, though difficult in realization. The medium is usually magnetic storage and each element has only two states, magnetized and demagnetized. Thus the digital com- puter is not dependent on the problems that beset analogue com- puters, such as linearities, scale factors and component tolerances. Its elements are essentially two-state devices, ON and OFF, and this gives some important advantages, the first of which is accuracy. Airborne analogue computers, for the reasons mentioned above, can consistently achieve at best 0.1 per cent accuracy but, because the accuracy of the digital machine is mainly dependent on the number of pulses and bits allocated to the number and on the organization of the programme, much higher computing accuracies can be achieved simply by proper organization of machine and programme. Moreover, the two-state characteristic eliminates the difficult calibration or drift problems, which often arise in high- accuracy analogue systems. Second advantage is the reprogramming capability. A powerful digital computer is capable of carrying out any type of calculation, the actual calculations and values used being inserted respectively by a paper or magnetic tape programme and, in on-line systems, by the signals from any sensors to which the computer is connected. Thus the entire function of the computer may be altered by inserting a different programme tape, without any of the hardware and circuit modifications which would be required in an analogue computer. .As discussed later on, this is a particularly important advantage for military aircraft. Programming capability is illus- trated by the fact that the Autonetics Verdan computer has been used, without any changes, for navigation and control systems in Polaris submarines, the aircraft carrier USS Enterprise, A-5 Vigilante attack aircraft and GAM-77 Hounddog missile, and was intended for Dynasoar. The third advantage is reliability. A digital computer mainly contains a large number of repetitive circuits of a few different types; and the small number of different types of components allows the component and equipment manufacturer to use extensive and • Asst General Manager Elliott Flight Automation Ltd, member of the Elliott Automation Group.
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