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Aviation History
1975
1975 - 0076.PDF
40 FLIGHT International, 9 January I97S harder to defend politically than the Australian revival of an old US con cept. Secondly, because the time- reference principle is much more widely understood than Doppler, and therefore easier to defend technically. One of the ironies of the present situation is that the unique proper ties of the commutated array and variants such as the beam-port antenna and modal array, which com prise the Doppler ground equipment, are only just beginning to be appreciated. Recent analysis of the principles of angle measurement THE DIGITAL computer in the inertial navigation system of the Boeing 747 was the first of its kind introduced into service by BOAC and the airline looked on it with some apprehension. Reliability in the early days was in deed poor but intensive development has improved it greatly, according to Mr D. A. Meredith of British Airways, Overseas Division. He was speaking at the joint Royal Aeronautical Society/ Institute of Electrical Engineers sym posium on digital avionics held in December (see Flight for December 26, 1974, page 920). In the first part of his lecture Mr Meredith discussed the installation of the in-flight entertainment and public address systems in British Airways 747s (which use digital techniques) and then described the introduction of and experience with the Delco Carousel IV inertial navigation system (INS) in the aircraft. There were several reasons for the apprehensive introduction, including memory corruption due to electro magnetic interference, program errors, crew errors during alignment, and problems with accuracy and reliability. One by one these worries have been removed and the INS is now fully accepted. In the early days, INS reliability was very poor, mean time between failure (MTBF) being below 300hr, and this led British Airways to install both Loran and a sextant in the first few aircraft. By the time the aircraft were actually in passenger service, however, reliability was already improving so that these back-ups were never used and were subsequently removed. It was unreliability in the inertial navi gation unit (INU) rather than in asso ciated equipment that caused this low MTBF, so rigorous efforts were made to categorise and analyse all failures. With three INS in each 747, operating hours have rapidly accumulated and are now well in excess of 6 million. One significant maintenance action carried out by British Airways, con tinued Mr Meredith, was the cleaning and lubrication of the edges of INU cards to reduce signal losses. Action such as this, coupled with improved manufacturing techniques, more strin gent pass-off testing and the failure reveals that it is in fact a quite different process from the classical Doppler effect and should offer par ticular advantages in respect of the problem of signal reflections from adjacent ground-based objects and overflying aircraft. When the FAA voted in favour of scanning beam (by nine votes to six) it would appear that the "users" voted for Doppler while the "R&D" mem bers of the committee voted for scanning beam. This surprising division probably arose because the Doppler system has a simpler aerial reporting noted above have helped to improve the MTBF from 300 to 1,300 flight hours in five years. This improved reliability now enables computers to carry out tasks which would have been too vital for entirely automatic systems a few years ago. The equipment cost approxi mately £200,000 per aircraft-system with maintenance costs of £1-85 per flying hour. This can be more than balanced, however, by the cost of a human navigator and the fuel saved through the use of INS. In his concluding remarks Mr Meredith said that after its success with INS the digital computer is con stantly finding new applications, par ticularly in area navigation (see Flight for December 26, 1973). Future developments should lead to a reduc tion in the complex interfaces re quired by analogue/digital and synchro/digital converters as sensors become predominantly digital. As air craft speeds and fuel costs both increase, so accurate navigation, optimum fuel management and flight planning techniques are required to minimise costs; the digital computer can provide assistance. The airlines, said Mr Meredith, now have confi dence in the airborne digital computer. The symposium was concluded by Mr R. P. G. Collinson of Marconi- Elliott Avionic Systems with a paper which looked at discernible trends as pointers to the future, and at lessons drawn from experience. Speaking first on sensors, Mr Collin son emphasised the point made by other speakers that developments are leading away from analogue sensors with complex analogue/digital inter faces towards digital sensors with their reduced costs and increased reliability. Complex digital processors, now being developed, could be linked to an inertial platform with skewed axes to give better reliability and more economical redundancy. Reception of ground-based aids can be coupled to accelerometer and gyro data to give more precise navigation information corrected for the errors inherent in a single-type system. It is possible, con tinued the speaker, that pure inertial systems may be replaced by strap- down laser gyros and accelerometers array and follows the philosophy of putting complex equipment into the aircraft. This makes the Doppler system more suitable for installation in less developed areas. Although the Doppler system is not technically appreciated, it appears to have reached the same stage of development in five years as scanning beam has reached in ten years. It seems reasonable to assume that given further development, the Doppler system will be significantly the better. It remains for Icao to make a final decision this summer. combined with Omega reception. The solid-state laser gyro offers benefits both in cost and performance; a sys tem cost of half that of a present INS seems feasible. Turning to data transmission, Mr Collinson discussed the potential weight saving from replacing cabling, looms and wiring with glass-fibre optics. This gives several major advan tages: immunity from electromagnetic interference, electrical isolation, high data transmission rate and, eventu ally, lower cost. It is estimated, said the lecturer, that glass-fibre "wiring" would become cheaper than copper before 1980, and the position will im prove as copper becomes more scarce and glass-fibre manufacturing tech nology improves. Integrated cockpit displays are being developed as the whole concept of crew/systems interface is re examined, continued Mr Collinson. Present displays are often liable to misinterpretation due to poor presen tation and low pilot confidence in contemporary technology. The require ment is to present the pilot with only the data he needs and to ensure that there is a high level of confidence that the displayed data is correct. Future civil aircraft are likely to have up to seven head-down displays with the facility to switch data from one panel to another if a failure occurs. Military head-up displays (Huds) are now commonplace and weapon-aiming accuracy has been revolutionised by this type of presentation. Head-up, low-light television or infra-red pic tures are currently available and head- down digitally controlled presentations are now being extended to attitude directors and engine management systems. Colour cathode ray tube (CRT) dis plays may not become part of an advanced flight deck, because a low production rate will probably keep prices high. It is hard to see CRTs being displaced by liquid crystal arrays or light-emitting diodes because of resolution problems, according to Mr Collinson. These solid-state devices may well find less demanding appli cations, however, bringing rapid developments in this specialised tech nology. S.R.B. Digital avionics—experience and prediction
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