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Aviation History
1963
1963 - 0482.PDF
MINIMA AND M AN GEUVR ABILITY—2 I WOULD not have the arrogance to write about jet minima if I could find anyone else who has written knowledgeably on the performance aspect of the subject. But the fact is that today's operations are so complex that very few pilots, operations managers or designers can get together all the relevant material; and fewer still are in the position to write about it. My own picture may be a bit of a lash-up, but I have made a good few cross-checks and trust that from the result we shall get somewhere near to the kernel of the problem. As I remarked in the previous instalment [page 329. March 7], the main objection to the demonstration of landing or overshooting from 100ft (as required by the FAA for qualifying for jet transport operations down to 200ft ceiling and half-a-mile runway visual range) seems to be the risk of anunpremeditated touchdown either before or on the runway; pilots do not object to making the ap proach even if conditions of wind, ILS beam or airport lighting are something less than optimum, provided they are sure that in the final stages they have a sufficient margin for manoeuvre to jog into the beam or to execute an overshoot. I believe, therefore, that the cardinal problem is one of aircraft performance. Now the designer will say that, if the final approach speed and sink rate are correct, unpremeditated touchdowns should not occur. He has undoubtedly done his slide-rule stuff and I would not dis agree that with a recommended threshold margin of 30 per cent of the true stall there would normally be enough airspeed cushion to permit a safe pull-up (for the overshoot case) or flare (for the landing demonstration). But where I believe there is room for debate—the grey area mentioned in the earlier article—is in the accuracy with which the datum for the 30 per cent margin for man oeuvre has been measured. In the matter of measurement of the margin there are several minor components, such as inaccurate knowledge of the weight of the aircraft and the variation of performance between individual aircraft of the same type; but these will normally be small and I do not propose to say more about them than that together they will often account for about a 3kt variation. The chief factor is the stall itself, and any variation of it as imposed by the round-out or pull- up manoeuvre. I will here concentrate on the pure stall: if this has been correctly measured, the g-loading effect for flare or turn is a simple piece of mathematics, accurate if the datum is accurate. The question of the stall is historically interesting. About three years ago, when a senior representative of the FAA flight-test department was talking to a group of pilots, he did not know that there was any difference between the true or lg stall and the "mini- NEW WAY ON May 26 British United Airways together with British Railways and French Railways are to begin a rail-air-rail service between London and Paris. Known as the Silver Arrow, the service will take only 5hr from Victoria Station in London to Gare du Nord in Paris for a minimum return fare of £10 9s. Silver Arrow passengers will check in at BUA's Victoria Station terminal and travel by train to London (Gatwick) Airport. From there they will fly by Viscount to Le Touquet and on by special train to Paris. Both airports have their own railway stations as an integral part of the airport facilities. Initially the Silver Arrow will be operated twice-daily in each direction with departures in the morning and late afternoon. The FLIGHT International, 4 April i%] mum speed encountered in the stall manoeuvre" (but without regard to loss of height!), the latter being the basis of US jet certificat on. This was not altogether surprising, for at that time—and notably at the 1959 meeting of the ICAO Airworthiness Committee- the FAA was strenuously opposing any change towards an "honest stall." (I hope to deal more fully, in my next article, with the differences between the two concepts.) However, a year or so's experience with the jets in day-to-day operation saw a fundamental change in the US philosophy at FAA level and, at least in some places, at designer level—so much so that, at the 1962 meeting of the Committee, one could hear the FAA advocating the lg stall and basing their definition of it on a paper written by Boeing. In fact I believe that Boeing, having in the early days of jet transport design started off by taking maximum advantage of the anomaly,' have had a distinct change of heart; and now, in the case of the 727, they are putting all they know into a usable margin for manoeuvre which must, of-course, involve a lg stall. Thus D. A. Anderton, writing on the 727 in Aviation Week (December 12, 1962) says sig nificantly: "The only way to approach this problem was to design a wing with high wing loadings for cruise in order to minimize the drag, and with high maximum lift coefficients in order to minimize the stall and approach speeds. Boeing has not released actual maximum lift coefficients of this wing, but the company has said that it shows more than 40per cent improvement over the 720 and 707 wings." But, laudable though all this is, and successful though I expect it will be in achieving a 200-J jet operation, I suspect that the rest of the US industry is still clinging to the path of the unrighteous. I draw this conclusion from the fact that on May 4, 1962, the FAA circulated a "Notice of Conference" in which an open discussion on the stall was to have been one of the principal topics: shortly afterwards the Agency publicly endorsed the lg stall as defined by the 5th Session of the ICAO Airworthiness Committee; however, carrying the recommendation back to its own industry, it appears to have run into trouble and so, by July 5, we see issued a "Withdrawal of Notice of Conference" and, a little later, a withdrawal of the proposed new performance regulation SR422C which incorporated, among other things, the lg stall. The difference between the two stalls we will discuss more fully next time, but it is useful here to mention the order of values we are dealing with: as far as I have been able to ascertain, they range between 2kt and 9kt—though I have heard of one case much higher and I gather that the trend towards greater differences will be more pronounced as we approach the delta wing. The point is that, whatever the value of the difference, it represents a loss to the pilot's margin for manoeuvre: needing at 100ft say 30 solid knots' margin for manoeuvre (and with no slipstream to fall back on), one cannot have half-a-dozen of those knots imaginary and not notice it. By all accounts the pilots are noticing the losses in this area—not necessarily all the time, but when the odd gust, steeper ILS glide-path (with sharper round-out) or other variable also stacks up against them. It may take many landings for an adverse combination to show up, and for a variety of reasons it may never show up at all in a certification type of exercise; but operating rules need to allow for adverse circumstances and one might therefore reasonably expect a study of these to have been made and published. I am not proposing in this present series to suggest a complete set of rules, but I hope that in the next instalment I can at least assist in making sure that the datum is correct. *See ICAO Paper AIR C-WP86, July 16, 1959. TO PARIS average city centre to centre time of 5hr compares with about 2hr 45min by Air France and BEA from the West London air terminal via London Heathrow to Gare des Invalides, and 7hr 20niin by rail and sea via Dover - Calais. The maximum Silver Ano* return fare of £11 18s applies between any Friday and Sunday and compares with the normal airline return fare of between £15 1* and £17 4s depending on the season, and £9 16s for the second-class return rail fare. For some time Skyways have operated a coach-air-coach ser i& taking approximately five hours between London and Paris- they use Avro 748s between Lympne and Beauvais—for a return fare of £11.
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