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Aviation History
1952
1952 - 1120.PDF
492 FLIGHT, 25 April 1952 COMET FLIGHT-PLANNING . . . sponding figure in the case of the 50-kt head-wind would have been a reduction of 22 per cent. Put another way, nearly 1 per cent of overhead fuel reserve would be lost in these particular conditions for every 3 deg C rise in temperature or for every 2 kt increase in head-wind, and in order to operate the stage length at the same take-off weight, and to arrive with the same fixed fuel reserve, one less passenger would be able to be carried for every 7 deg C rise in temperature, or for every 4 kt increase in head-wind. It is thus true that for the turbojet airliner, as for its piston-engined predecessor, wind is still the determining factor on long-haul operations. Stand-off has generally been thought to be a particularly unfortunate burden for the jet aircraft, but in point of fact practical investigation by B.O.A.C. with the Comet has shown great divergence from theoretical prediction. It has been found that the most economical condition even at the higher altitudes is for stand-off to be made with two engines throttled to the flight idling condition, whilst the remaining two turn over at relatively high r.p.m. It has also been proved that, whilst some gain is experienced by holding at altitude it is not, in fact, very considerable. If, instead of being held at a given (high) level, the aircraft is stacked and descends pro gressively to lower and lower levels, some further benefit is obtained by the reduction in fuel consumed during descent: for example, it could be shown that, with the aircraft held in a stack between 10,000ft and 4,000ft based on a five-minute landing rate from the base of the stack, the mean holding consumption for a total stacking period of 30 minutes would be equivalent to level holding at about 18,000ft and, therefore, that no real benefit would be derived from high-level holding lower than about 25,000ft. It is quite evident that the stringency of flight control procedure is to a greater or less degree dependent upon the availability of sufficient meteorological data of a reliable standard, and also on the quality of navigation. The most important pre-flight forecasting requirements are for the accurate prediction of take-off conditions, both of surface wind and temperature, and also of the en route wind velocities. Three men who perhaps more than any others have contributed basic work to the establishment of a successful economic operational technique for jet aircraft: left to right, Capt. A. M. A. Majendie, John Cunningham and Capt. E. E. Rodley. Astro-navigation will play a part in Comet operations, and here Mr. F. Smith instructs future pilots of the world's first jet airliner in the use of the periscopic sextant. Temperatures at cruising heights are of secondary importance to wind but, of course, should also be covered by the forecast, and in addition, mean wind and temperature data for the climb and descent, although not so critical to an operation, facilitate the preparation of an accurate flight plan. The most important met. service required by a jet aircraft in flight is immediate notification of any deterioration in the forecast conditions for the destination airport and relevant alternatives. In this context, it is absolutely essential that, if the requisite information is to be passed rapidly and effici ently to the aircraft, a direct communication link be main tained between the aircraft and its destination. At the present time, such a direct link does not exist but, when solutions have been found to the technical and political problems involved, it is probable that long-range voice communication will pro vide the best operational answer. En route weather conditions, other than upper winds, are in general not so important to the jet aircraft as they are to those types which operate at lower levels, but there is wide agreement that the extent of radio sonde and rawind coverage over the world's trunk air routes is hopelessly inadequate for the accurate forecasting of condi tions at heights between 30,000ft and 50,000ft. So far as navigation is concerned, no serious difficulty has been encountered by B.O.A.C. in finding acceptable solutions to the various problems. The primary essential is speed of working, for it is patent that, when the aircraft is covering about 7 n.m every minute, there is not much point in trying to fix position to the nearest mile if it takes 20 minutes, or 140 n.m., to do the necessary calculation. Rather is it better to establish position to the nearest 10 miles if this can be done in but two or three minutes. It is, of course, difficult to make an accurate visual estimation of position from great heights, and most of the B.O.A.C. Comet Fleet navigational fixing has been, and will continue to be, achieved by radio aids. The degree of M.F. coverage provided on the routes so far explored is reasonably satisfactory, but by itself provides nothing like adequate service for long-range jet aircraft opera tions. The M.F. frequency band is much too susceptible to static interference, night effect, and so on, to justify being regarded as a sufficient aid, and it should, at the very least, be supplemented by static-free homing and distance measuring equipment at all the main aerodromes. The need for distance measuring from destinations is a cardinal requirement in order to permit an efficient descent from cruising altitude to circuit level, for if this descent is misjudged, an increased fuel consumption of the order of 20 per cent by weight of capacity payload can easily occur. Over those stages of the route where radio aids are sparse, astro-navigation will be used for the Comet. Even with the best modern short-cut methods and pre-computations, how ever, astro in its present form is not an ideal aid for very fast aircraft. Future techniques may, of course improve the position. That the coming of the turbojet'airliner would cause no little disturbance in air traffic control procedures was recog nized a long while ago, and when the Comet arrived joint M.C.A.-B.O.A.C. trials were carried out. As a result of these
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