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Aviation History
1960
1960 - 0298.PDF
298 FLIGHT, 4 March I960 SYSTEM SURVEY That FAA Report on Decca ON pages 65 and 84 of Flight for January 15 reference wasmade to an inexplicable press release by America'sFederal Aviation Agency concerning a report on evalua- tion of Decca Navigator—a report which, at that time, had yetto be issued. The statement remarked that FAA had tested Decca and found it sadly wanting as an airways aid. The actual report, now issued, has two sections, one being themain body of the paper entitled Final Report on the Helicopter Operations Programme, issued by the FAA Bureau of Researchand Development; the other is Supplement No 1, entitled Operational Test of Mk 10 Decca System in C-131 Fixed-WingAirplane. That part of the report which refers to the helicopter trialswas compiled under contract by the Airborne Instruments Laboratory and consists of about 40 pages of discussion and over200 pages of facts and figures on Decca performance. It seems to be a genuine effort to make an unbiased assessment of thesystem and, though its conclusions are "not altogether the same as those reached by other people, it seems reasonable andhonest. As a helicopter aid Decca is, of course, acknowledged and there is virtually no alternative at present available. New York Airways have already begun a second programme,employing helicopter airline pilots from all over the US, to investigate Decca as a permanent IFR aid. This in itself seemsto show that the results of the first trials cannot have been so bad. A further factor is that NYA have specified dual Mk 8 Deccareceivers for their fleet of Vertol 107s. Each installation will include two receivers in JATR cases and two Flight Logs to atotal weight of about 1201b—equivalent to one passenger and therefore hardly likely to be devoted to useless equipment. What has caused intense reaction is the supplement, whichpurports to summarize the reactions of a group of pilots to airways flying with Decca in a Convair C-131. Information wasgathered by making each pilot fill in a questionnaire after each flight, answering questions examples of which are listed at right.In fact seven pilots were involved, each flying for some 50hr on four carefully planned routes. Each is claimed to havereceived 12 or more hours of ground and flight training in Decca operation and in each case the Flight Log chart was retained afterthe flight. The routes were carefully planned and flown before- hand, but more than half of them were in areas where reliablecoverage could not be expected from the New York chain. Photographs show that the installation was a good one—thoughnot duplicated as VOR normally is—with the Flight Log mounted over the centre of the coaming and the three Decometersset in a sloping roof panel ahead of the co-pilot with each dial tilted to lie normal to the line of vision. Control panels weremounted in the roof immediately aft of the dials. Unexpected Results Many of the Flight Log charts show that track-keeping wasin most cases extremely good, but often the nine-mile width of the airway covered less than Jin on the chart, making trackkeeping unnecessarily difficult. Track records reproduced in the report are not effectively commented upon. The accuracy ofroute flying was certainly much better than the remainder of the report leads the reader to believe. Decca engineers flew with theFAA crews on many flights and gained the impression that pilot reaction was favourable. Decca were not shown the report before-hand. In fact, the press release was the first firm intimation of the FAA's poor opinion of the aid. Decca point out that such aprocedure is almost unprecedented. The timing of the report is worth noting. The preliminarypress statement was issued (not "leaked" as is often the case) on January 5 and appeared in the daily Press on January 6. The reportitself was dated January 15 and was actually issued on about February 6. Such timing would be curious in normal circum-stances, but in view of the ICAO background it seems more than fortuitous. The Decca chain used was that set up some time ago specificallyfor helicopter trials by New York Airways. It has relatively short base-lines between the master and three slaves and radiates at apower about one-fifth of that of the standard European Decca chains, despite a higher local noise level. Decca freely admit thatthe range coverage is not sufficient to allow accurate instrument approaches to Boston and Washington airfields—and this thereport has complained of. Some use was also made of the Quebec chain in Canada. Total flying time involved in the trials wasabout 360hr. The questionnaire filled in by pilots after each flight consistedof a series of questions to each of which "yes," "no" or "no com- ment" answers were required, the final results in the report beinggiven in the form of percentages—taken, remarkably enough, to tenths of one per cent. With seven pilots and 360hr this seemsexcessive. Moreover, the proportion of "no comments" seems to average about 20 per cent and sometimes is as high as 80 per centNo explanation is given of what the percentages might specifically indicate. The report is in fact a subjective one and extreme care in suchcases must be taken to avoid phrasing questions in such a way that the answers might be biased. No such precautions appear to havebeen taken. The questions, as will be seen, are sometimes vague, sometimes almost misleading, so that many of the "no comments''may have come from pilots who did not grasp the significance of the questions. Curious Statistics - Here are some sample questions and answers from the C-131 test questionnaires, giving the percentages of answers: — Yes No No commentWere there any discrepancies between Decometers and Flight Log after referencing? 23.9 64.0 12.1Were there any unaccountable excursions of the Flight Log?(a) During flight? 52.4 35.3 12.3 - • (b) On ground? _ 10.0 46.5 43.5 !Did the zone identification work properly? 47.5 36.9 15.6 Did the Flight Log require resetting?Present difficulties? When? (a) Entering a Decca area? ... 28.9 21.2 49.9(b) In fringe areas? 38.8 30.8 30.4 (c) In changing charts? 47.4 28.7 -23.9(d) Changing chains? 18.2 31.4 5Q.4 Were you able to use Decca in anapproach? (a) To what field minimum—200ft? 16.3 — 83.7 ' (b) —500ft? 12.4 1.7 85.9 (c) —1,000ft? 1.9 15.8 82.3 (d) —above? 3.8 8.9 87.3Were you able to stay within the omni-airway structure (9 n.m.)? ... 22.5 61.6 15.9At any time were you unable to hold in the prescribed holding patterns? ... 17.5 41.8 40.7Were you able to use Decca without resource to another navigational aid? 24.5 49.4 26.1Did you find the initial setting-up of the equipment to be difficult? 55.5 21.1 23.4At any time did you believe that the Decca equipment was operating improperly? 39.7 34.2 26.1 Apart from the difficulty of deciding quite what the percentagesrepresent, a number of these questions could better be answered by reference to the Decca operating instructions. In cases wheresome discrepancy arises there is no indication of how large it was and particularly of whether it materially affected performance.The words "difficult," "believe" and "improperly" are vague. A strange anomaly is the fact that 16.3 per cent of pilots were ableto make a simulated instrument approach down to 200ft, but only 1.9 per cent were able to do so down to 1,000ft. Who or whatwere the over 80 per cent "no comments" in each case? Flight Log resetting is known to be necessary in cases (a), (c) and (d) inthe fourth question. In few cases is the point of the question made unequivocally. Readers of both Flight and the report maydraw their own conclusions. Further detailed comment seems pointless. It seems that the FAA is not sure that two-thirds of the 21ICAO-Council nations will in fact support Vortac when they consider it in the near future. Perhaps this uncertainty is the mostsignificant aspect of the whole affair. If the council does not ratify the proposal in favour of Vortac, then the whole problem mustbe considered again. It can only be hoped that some independent body will then be given the task of making a thorough side-by-side investigation of the two systems in order to produce some answer which everyone may respect. It is certain that untilsome independent body can make a proper assessment no_ valid reports will be available. Once the immediate and unavoidablehubbub has died down, everyone would do best to foreet mis manoeuvre and hope for much better things to come. Flight forone is heartily sick of having to report this long-drawn-out and unseemly discord. C. M. L-
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