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Aviation History
1979
1979 - 0012.PDF
10 FLIGHT International, 6 January 1979 Mini-computers mind the tanks M ICROPROCESSORS have presented many new opportunities to avionics designers, especially those working on systems which require a large amount of repetitious comput ing. Smiths Industries has produced fuel-management systems for over 80 aircraft types and believes that micro processors will continue to increase the efficiency of these units. Fuel bills now account for about 15- 20 per cent of a civil airliner's total operating cost—compared with only about five per cent fifteen years ago— and operators are looking hard for systems which will cut their consump tion. In the years before the energy crisis a fuel-gauging accuracy of 2-5 per cent was tolerable, but now the airlines are looking for one per cent accuracy. Over the last four years Smiths has evolved and flight-tested a micro processor-based digital fuel-manage ment system. Among the many re quirements written into the specifica tion was a demand for self-monitoring plus accuracy, reliability, versatility and maintainability improvements on existing systems. Research has pro duced a fuel-management system which can be used in both military and civil aircraft and which can be adapted to different tank configura tions and instrument arrangements. The Smiths Industries system has been flight-tested in an RAE Sea Vixen specially modified for evaluation of fuel systems. Apart from units designed for small aircraft with only one fuel tank, con tents-indicating systems usually have to show the amount of fuel in several tank groups, each made up of several irregularly shaped containers. The new digital-based systems must there fore be adaptable to different aircraft types. They have to compensate for all aircraft attitudes and for wing deflection so that the indicators show the true quantity at all times. An additional complication is the variation in fuel density with tempera ture. This is why Smiths adopted "dynamic characterising" for its digital system. Use of this technique means that contents readings accurately reflect any changes in the characteristics of the fuel. Compared with an analogue system, many fewer tank probes are needed, resulting in a significant improvement in relia bility and cost of ownership. In conventional fuel-gauging systems each capacitive probe has to be individually calibrated to match its location and to account for variations in tank manfacturing tolerances. In a digital system calibration is per formed by the microprocessor soft ware. The tank probes are excited at high frequencies to protect them against water and other contaminants. The use of DC electricity means that screened leads are unnecessary, mak ing the digital systems lighter than a conventional AC arrangement. As well as the "level" probe, each tank also has a temperature compen sator and a density reference unit. Signals from the tank probes and from aircraft attitude sensors are used to obtain tank mass. To ensure the reliability required for inter national certification, these sources are cross-coupled by a high-integrity data-link. Each processor unit consists of a microprocessor with volatile and non volatile memories, and an input-out put interface. Data relating to each tank group, including sensor locations and a program for fuel mass and temperature calculations, are stored in the memories. The two-way cross link between the channels enables each to check the other's input data, recent computation history and results. Input and output data are sampled and abnormal values re jected, thereby smoothing out indi cator fluctuations. A digital fuel- management system can have ana logue or alphanumeric data presenta tion, accepting incandescent pin- lights, fibre-optics, light-emitting diodes, and liquid-crystal or gas- plasma displays. Built-in test lamps will illuminate if there is a system fault or indicate that the read-out is not within the nominal accuracy. With other types of solid- state displays the nature and location of faults can be determined by using selector buttons to interrogate the system. A digital system can not only locate its own faults but can also eliminate their effects. Monitoring and fault survival are handled by the software, so no additional hardware is needed. System integrity can be maintained by isolat ing defective elements and switching others in to take over. Single failures do not have to be notified immediately, as performance is not affected and defect rectification can be completed when convenient. The microprocessors are checked by a monitoring program which puts both independent processors through a test routine and compares their re sponses with predetermined answers. If one processor channel is lost, an automatic switch-over connects the in puts from all the tank probes to the surviving channel, which then takes on the entire fuel-gauging task. Left This Smiths Industries digital fuel-contents indicator has no moving parts. The electronic pointer has a "persistent" tail to improve readability, and a numeric quantity display Below Capacitive tank probes for new digital fuel- management systems are linear and simpler than previous types • %AKK • -:. . . :'.•••..• •.••••
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