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Aviation History
1964
1964 - 0604.PDF
CHT International, 5 March 1964 353 V, V2 AND ALL THAT Part 3* of CCJ's "V" Classifications 4. Decision Speed (\\) The concept that an aeroplane should have a denned minimum level of performance safety owes more to Vx speed than to anything else. Vx was introduced into early DC-3 operations and by the time the DC-4 came up for certification the criterion was sufficiently well established as defining the level of take-off safety for it to be written into the general regulatory pro- visions then being formulated in the US for the new Transport Category (Cat-T) aircraft. Moreover, since the take-off was, from the point of view of performance, usually the most critical stage, compliance with the V! take-off requirement tended to ensure that minimum standards were met (i.e., enough "climbability" margin was available) in other stages where loss of an engine could be critical, e.g., climb, and maintenance of safe en route altitude. It has suffered a certain amount of change over the years, but not substantially so; it still remains a most important criterion in every- day operations and is expected to do so at least as far as the initial SST era. It does therefore deserve a couple of articles to itself— and this seems the minimum to provide anything like a full explan- ation. In its earliest concept the criterion was rather like Topsy and was rarely referred to by name, being content to be known simply as V1# This and the fact that Vx was really a distance dissimulating as a speed gave it a certain aura of mystery which, indeed, it still to some extent holds today. However, the careful researcher could always have traced a name in the baptismal register (then CARs), where it was referred to as "critical speed." It was quite a good name, since it implied a crisis in the take-off—incidentally thereby adding an emotional content to that of mystery and ambiguity and thus giving the term all the attributes of a Delphic utterance. The more recent development of the theory of variable V1 and the use of "clearways" and "stopways" seem likely to ensure that the mystery and emotion will remain for quite a time yet, but first of all let us concentrate on the early concept, namely a fixed Vx and all distances measured over the concrete. The picture is then easy to grasp. It is simply that at some distance from the take-off threshold of the runway the aircraft can lose an engine and, at that point (called the critical point), the pilot can, with equal safety, elect either: (a) to continue the take-off, in which case the aircraft should pass over the end of the runway at 50<35)ftt; or (b), to stop, in which case the aircraft should be capable of coming to rest at the end of the runway. By selecting Vj to satisfy this "equal safety" concept, the distances from the take-off threshold to the 5O(35)ft height point or to the end of the concrete are equal whether course (a) or (b) is pursued; hence the expression "balanced field length." As for practical purposes it was inconvenient to work in terms of a distance, which would vary from one take-off to the next, the concept of "critical point" very early gave way to that of "critical speed"—which is the speed assumed to be attained at the critical point. However, in making this transformation from the Vx point to the Vt speed, some accuracy is lost in that a very slow airspeed acceleration in the early part of the run (due, for example, to poor engine response, binding brakes, puddles on the runway, tail wind) will mean that V! speed is attained late and that the true critical (Vi) point (from which it should, with equal safety, be possible to abort or to fly off) has been left behind well before Vx airspeed is called. Alternatively, an abnormally high initial acceleration will yield a premature Vx point; however, this acts on the safe side for either an abort or a continued climb-out and merely amounts to an academic loss of accuracy and payload ("academic" since an aircraft obviously cannot be overloaded on the assumption that it may turn out to have an abnormally high take-off acceleration), hcidentally, most take-off monitors aim at restoring the concept that the critical point is a distance along the runway and not as Peed\ however, no take-off monitor is currently in day-to-day commercial use and so we have to stick to the rather inaccurate Vi speed for our criterion. * Parts 1 and 2 appeared in "Flight International" of January 16 and 30 respectively. T SOft was the screen height without temperature accountability; with the certification of the jets, temperature accountability was introduced ana ">e height then made 35ft. With the introduction of the jet, take-off requirements became more demanding in terms of concrete and, as the amount of concrete was in effect usually a derivative of the critical point, the speed Vx assumed a greater and greater importance both to the manufacturer advertising his aircraft as suitable for a particular route and to the aerodrome authorities who had to fit it in. In these circumstances there was a tendency to make V! as late as possible by extending the use of such gadgets as anti-skid braking, automatic failure warning devices and automatic feathering, both the latter taking their signal from change of torque or thrust, which was instantaneous, rather than from the ASI, which introduced a lag. More recently the FAA has suggested a concession in field length for the fitting of approved arrester gear devices, which would tend to put Vx up still further. This successive pushing up of the V1 point, plus the fact that the jets operated much more frequently from limiting runway lengths, had the effect of giving the pilot the impression that he is unlikely to be able to stop from \Y For the most part he still has this impression and I would say that, in a great many cases, he is right; however, this is not the place to argue the matter and, at least for definition purposes, I propose to use the somewhat optimistic concepts applied in certification. These are simply that, to achieve the minimum runway requirement for a given set of take-off conditions (i.e., altitude, weight, temperature, wind component and runway slope), there can be only one speed for Vj. If that speed is exceeded and an attempt then made to stop, it will be found that the runway remaining is inadequate; if on the other hand a speed less than V\ is selected, the aircraft will be able to stop easily on losing an engine but will have had such a small proportion of its ground-run under the power of all engines that the distance to take-off Safety Speed would be excessive and the required height at the end of the runway would not be met. On the above basis it will be seen that any departure from V! is adverse if the aeroplane is being operated in conditions where the effective runway length is a minimum (i.e., the "balanced field length" condition). However, frequently the "effective" runway length will be greatly influenced not only by the factors of altitude, weight, temperature, wind component and slope, but will also vary with the length of "stopway" (declared as able to bear the weight of the aeroplane) or "clearway" (declared free from upstanding objects) which can be added to the end of the concrete. In these conditions quite a new set of considerations enter the picture and we find ourselves talking about "variable Vjs" and "unbalanced field lengths." But of these and the "CCJ definition" in the next number. Sufficient unto the day is the (fixed) eV^ thereof. Mr Peter Linch is to be presented with the recently established Lord Kildare Trophy for the best commercial student at British Executive Air Services's flying school at Kidlington near Oxford. Mr Linch, who is 21, will shortly join Autair International Airways at Luton as a second officer
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