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Aviation History
1956
1956 - 0621.PDF
FLIGHT, 18 May 1956 621 PILOT AND FLIGHT LOG Decca in Use: A Viscount Captains Lecture IT may be recalled that the Institute of Navigation recently held asymposium under the general title "Developments of the Decca Naviga- tor System." Three papers were given, two of which we summarizedin "Flight" of April 6. It is now possible to print—with the lecturer's five illustrations, and only slightly abbreviated—the third paper. Thiswas a talk, rather than a formal lecture, by Capt. E. R. Wright, a senior pilot of B.E.A.'s Viscount FUgnt. THE operation of modern, high-speed, high-altitude aircraftcalls for efficient and standardized crew procedures, and thisapplies also to the modern navigational aids installed in these aircraft. To get the best use from the Decca Navigator requires a mixture of technical knowledge of the equipment, plus efficient operating procedure well seasoned with common sense. The application of this system on a Viscount service out of London Airport can be shown with the aid of five illustrations of Decca charts. Important points while on the ground are, first, to allow adequate time for warming-up the equipment before carry- ing out a Decometer "reference"; secondly, while taxying out, to make a quick check of the "notching," because of possible cutting of supply when changing from ground source to aircraft source; and, finally, to make last-minute adjustments to the pen when lined up on the runway ready for take-off. Fig. 1, showing the London Control Zone chart, is a section from a roll I used on a recent service. Take-off was from runway 28R on an I.F.R. day and the outward-bound clearance was Chertsey 2,000ft, Dunsfold 3,000ft, with a radar climb en route to Seaford. At the time when the pilots are ful'y occupied with after-take-off drills, R/T., etc., and maximum concentration on instrument flying is necessary, a quick glance at the Flight Log is all that is necessary to check that the aircraft is on track, thus avoiding the additional work-load of receiving and interpreting visual or aural signals. At this stage, in the event of radar failure, an accurate position of the aircraft can be passed to Air Traffic Control at any time it is requested, and "flying well right of the centre line," and subsequent resumption of track, is easily effected. We come now to the stage where we are comfortably established on the climb, autopilot engaged, approaching Seaford and the first chart-change. Here again, standardized procedure makes for effi- Fig. 2. Fig. 1. cient operation and eliminates possibility of error. By timing the last ten miles to the changeover point and resetting the stop-watch as the pen is lifted, the latter can be replaced five, ten or fifteen miles along the track, according to the time elapsed, and the Decometers checked and final adjustments made to the pen. In practice, and after experience with the equipment, this is not strictly necessary; but it is necessary during the training period, to avoid errors. Turning now to Fig. 2, a reasonably accurate revision of the E.T.A. for the next reporting point (in this case Dieppe) can be made by carrying out a 20-mile ground-speed check within 6 to 10 min from Seaford ("reasonably accurate"—for, remember, we are still climbing). This can be quite important with over-flying traffic on the same route whose altitude one wants to climb through. By this method, the E.T.A. at Dieppe would in practice rarely be more than a minute out. Fig. 3 shows that at Dieppe we have a change to the Northern French Chain (Chain 8), again using the procedure of timing the last ten miles and setting down the pen along the track according to the time interval elapsed while completing the change, This is usually accomplished comfortably within two to three minutes. Before making final pen adjustments, the procedure is to check the Decometers through three successive lane-identification cycles. By this time we are level at our cruising altitude, which in this case was, I believe, 23,500ft, in order to take advantage of increasing north-westerly tail winds in the higher altitudes. An immediate check on the wind velocity can be carried out with a minimum of work by flying the track on the Flight Log and checking the ground speed over 20 or 30 miles—I prefer 30, as being a little more accurate. No reference to a plotting chart is necessary; the track direction and distance to run to the next point at the end of the ground-speed check are read direct from the Flight Log chart. Thus, the amount of time in /olved in calculating an accurate wind velocity, and revising the E.T.A. if necessary, is approximately two to three minutes in every 20. The mere fact of flying the track on the log ensures that any interruption of the signal or fault on the equipment is immediately noticed and the necessary adjustments made. Also, any sudden change in wind velocity, such as is encountered in jet streams, i> immediately obvious; if it is adverse the success of the steps taken to avoid the stream can quickly be checked. This can be quite important on the longer northbound sectors. After Dijon, we are running close to the edge of the normal cover on the 8 FL chart, and we begin to get an erratic trace, due to the decreasing accuracy of the system. Even so, as Fig. 3 shows, the track can be fairly closely followed to Geneva, and the ground speed checks are reasonably accurate. From Fig. 4 it will be seen that the homeward-bound trace
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