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Aviation History
1964
1964 - 1401.PDF
776 FLIGHT International, 7 May S PORT JVNP BUSINESS Gliders and Collision Risks BY CAPTAIN H. C. N. GOODHART, RN* EVER since World War 2 there has been a tendency throughoutthe world to apply ever more stringent air traffic control.This process has been carried out in the name of air safety, but unfortunately no one ever seems to have taken time out to •establish any logical basis for control. There has simply been a feeling that risk of collision is a bad thing and therefore every reasonable step should be taken to eliminate the risk. As far as it goes, this elementary feeling is right, but the two follow-up questions which must be answered are:— (1) How bad is the collision risk? (Here we want a statistical figure, not an emotional outburst), and (2) How much control is reasonable? Unless these questions can be answered at least in some degree, it is not possible even to start on a logical control system; and if •one succeeds in getting this far and inventing a particular system, the next question that has to be answered is: how effective is it in reducing collisions? Only then can a rational opinion be formed as to whether it is reasonable or not, for it must always be borne in mind that control will restrict traffic, hence an unnecessary and •unreasonable degree of control will restrict the use of the air unnecessarily and unreasonably. Logical processes such as this have hardly been used at all in introducing current systems of air traffic control. It could be that some of the present ad hoc air traffic regulations even contribute to •collisions rather than to their elimination. The number of collisions which have occurred in controlled airspace between aircraft under •control is a noteworthy percentage of all collisions. For the non-commercial air-user such as the glider or light aircraft pilot, the outlook appears black. The octopus of controlled airspace spreads its unreasoning tentacles far and wide over the land and the •space left for uncontrolled traffic shrinks alarmingly. For the glider pilot, who must go where the weather sends him rather than along man-made channels, the outlook is even worse, as the octopus stands ready to squirt the dread ink of "permanent IFR" into his path. Clearly, therefore, if gliding is to have a viable future, it is essential that logic be injected into official thinking and that at least some effort is made to ensure that the controls which are applied are necessary in order to achieve an adequate standard of safety without a disproportionate loss of freedom of all air-users. One of the purposes of this article is to explain the logical method which has been used so that other countries can, if they wish, try a similar approach. That other countries should do so is of concern to us, since our own Ministry of Aviation cannot but be somewhat biased by the opinions expressed at ICAO meetings. The method we have used is based on the random nature of cross- country glider flying. The gliders can only go where the weather permits them to go; they are not constrained to fixed heights or fixed routes. Aircraft, on the other hand, do generally fly at fixed heights and use fixed navigational facilities. There is therefore no •correlation between the movements of gliders and commercial aircraft. This being so, it is reasonable to say that the movements of one are random with respect to the other. This immediately opens up a relatively simple statistical approach to the problem of determining collision risk. The simple mathe- matics used have been set out in a series of papers, which have been made the basis of the general case presented to the Ministry of Aviation. Having determined the risk of collision between gliders and commercial aircraft, the problem then is to relate this result to specific proposals for control. The first question that has to be * Chairman of the British Gliding Association's Airways Committee.Article reproduced by permission of the Editor of "Sailplane and Gliding." answered is—what is a reasonable risk of collision between a commercial aircraft and a glider? Before answering this question, it is well to examine the present position with regard to accident rates of commercial aircraft. The current fatal accident rate for commercial aircraft is 140,000hr per accident (7 x 10"6 accident per hour of airliner flight). This level of safety is the result of attempting to hold each major cause of accident, e.g., such things as main spar failure due to fatigue, chance of failure of more than one engine on full-load take-off or more than two engines in flight, down to a figure of 1 in 10,000,OOOhr of airliner flight (1 x 10"7 accidents per hour of flight). On this basis it seems not unreasonable to accept a risk of collision with a glider at 1 x 10~8 accidents per hour of airliner flight. In considering this figure it should be borne in mind that, due to the low density of gliders, it is unlikely that a collision with a glider would in fact be fatal to the commercial aircraft. Consider- ation is being given to this point, but as yet no figure can be given for the probable proportion of collisions which would be fatal, because no collisions between glider and commercial aircraft have yet occurred. Present calculations indicate that if, in the UK, all controlled airspace other than the south-east England area was eliminated (leaving only the normal aerodrome protection "cheeses"), com- mercial air traffic would be subject to a risk of collision with gliders of 8.5 x 10-9 per hour of airliner flight, i.e., below the 1 x 10"' figure. If, on the other hand, we allow for the present distribution of controlled airspace, the risk decreases to 1.5 x 10-9. This means that a sevenfold increase in glider traffic or commercial traffic would be needed before the risk rate came up to the 1 x 10~8 figure. In the light of this it is clear that, provided logic prevails, the outlook is far from black. Gliding can and should be allowed sufficient freedom to enable it to maintain the enormous attraction it currently has, and make its valuable contribution to our total air effort. There is, furthermore, another and even brighter side to the coin. The major contribution to collision risk is provided by low-level commercial aircraft. With the advent of more and more jet aircraft, the amount of low level commercial flying is rapidly decreasing. If this tendency outweighs any increase in quantity of traffic, either commercial or glider, then the trend should, as far as gliders are concerned, be towards reduction of controlled airspace rather than increase. To put the above risk rates into perspective, it should be noted that 1.5 x 10~9 is one accident per 70,000 years of continuous airliner flight, and 8.5 x 10"9 is one accident per 13,400 years of continous airliner flight. Summaries of three relevant papers which have been submitted to the Ministry of Aviation follow. 1. A Statistical Analysis of the Heights Used by Gliders in the UK This paper starts by describing the forms of lift used in cross- countries and deciding that "the vast majority of cross-country gliding is carried out using thermal lift, and it is only necessary to consider the characteristics of this type for the purposes of this analysis." Thermal flights cannot exceed the height of the convective layer, which is generally between 3,000ft and 6,000ft in the UK but can go to 30,O0O-4O,0O0ft as shown by cumulo-nimbus clouds. "Glider cross-country flying is therefore strictly confined by height on any particular day and heights used by different glider pj-ots will all be similar, since they will all try to keep in the top ha,> o\ the effective layer." The barograph records of 54 "randomly selected" cross-country flights representing a total of 186hr—23 flights by Peter Scott, four by Capt Goodhart and 27 by members of Cambridge Univer- sity Gliding Club—are analysed, and the hours spent in each
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