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Aviation History
1953
1953 - 0069.PDF
FLIGHT, 16 January 1953 67 STOPWAYS AND CLEARWAYS I.C.A.O. Introduces a New Concept in Runway Design AS Mr. R. E. Hardingham, secretary of the Air Regis- A% tration Board, pointed out recently in his informative R.Ae.S. lecture on airworthiness, the war produced a generation of aeroplanes with higher take-off and landing speeds and, consequently, poorer prospects of safe forced- landings. In pre-war days it was possible to maintain high standards of safety without insisting that transport aircraft should conform to an elaborate set of performance require ments. It was not found necessary, for example, to enforce regulations denning the ability of an aircraft to cope with engine-failure at a critical point during take-off. By contrast, the post-war years have seen many thousands of expert man-hours devoted to this single problem. Take-off per formance is normally the most important factor influencing the amount of payload which a given aircraft can lift from a given aerodrome. In turn, the usefulness of an airport (provided, of course, that it qualifies as a traffic-centre) is largely expressed in the amount of take-off distance offered to airline operators. This all-important relationship between aircraft and airport was the subject of fruitful study at the last session of I.C.A.O.'s Aero dromes, Air Routes and Ground Aids Division, held in Montreal from October 21st to November 20th. Delegates spent several days discussing two promising methods of enabling airports to meet the requirements of traffic with minimum penalty in time and cost—namely, the use of the "stopway" and the "clearway," extensions of the runway which should enable better use to be made of present and future airports. A few definitions are essential at this point. Take-off safety speed (Vi) is the minimum speed at which the pilot is allowed to "unstick." This speed is fixed by the manu facturer of the type of aircraft concerned. Critical speed (Vi) is the speed, fixed by the operator, at which sudden and complete loss of power from the critical engine (the engine whose failure has the most unfavourable effect on per formance and handling) is assumed to occur. If the failure occurs below Vi the pilot must abandon the take-off. If the engine fails after Vi, he continues the take-off. Critical speed is normally less than, or at the most equal to, Vi\ but it must be greater than the lowest speed at which the aircraft can be manoeuvred safely on or near the ground if the critical engine fails. What if the critical engine fails at the critical speed? The pilot would have the choice of braking until the aircraft came to a standstill (the complete operation is entitled "accelerate-stop"), or of continuing to accelerate until he reached safety speed in order to take-off and climb on reduced power. The term take-off distance is applied to the distance between the start of the take-off and the point at which the aircraft, climbing on reduced power, clears a 50ft "screen."* This and other terms are illustrated overleaf. The assumption that an engine might fail at the critical point is an essential one, but the incidence of such failures is rare. It * The height of the hypothetical screen is likely to be reduced from 50ft to 30/r. follows that a runway built to the requirements of an aircraft which demands a long take-off distance will not be fully utilized. The A.G.A. session concluded that the portion of the runway used only for the accelerate-stop operation could safely be built to lower standards of strength than the main length of paved runway; this area was dubbed the "stopway." The term "overrun" was rejected as it is already used with a different meaning in the U.SA. No paving at all is needed for the second part of the take-off distance (from "unstick" to "screen" height); it was concluded, therefore, that the existence of a "clearway" upwind of the runway and stopway would be a useful and economical extension, requiring only to be free of obstructions. The following official definitions were adopted :— Stopways are portions of the aerodrome declared available for accelerate-stop purposes in addition to the take-off surface in the direction of take-off. These portions, together with the runway, constitute the take-off area as defined in Annex 6 [an I.C.A.O. document setting out standards for the operation of scheduled international services], while the runway itself constitutes the take-off and landing surface. Clearways are portions of the aerodrome augmenting the take-off area in the direction of take-off and declared suitable to be taken into account for the purpose of determining the take-off distance available, within the terms of Annex 6. The question of dividing the take-off distance available into three suitably proportioned sections—runway, stopway and clear way—is a somewhat complex one. Since Vi is a variable, its value being decided by the operator, the proportions of the three sections are dependent upon the ratio V\\Vi (assuming that take-off distance is being related to the performance of a particular type of aircraft). A low V\ means that, if the critical engine fails, the accelerate-stop distance is reduced; the take-off distance, however, is increased, since a greater proportion of the take-off is made at reduced power. In this case the emphasis is on the clearway. Conversely, a high V\ results in a better take-off performance (since the aircraft is nearer to safety speed), and a less favourable accelerate-stop distance—requiring a longer stopway. It is, of course, possible to find a V\\Vi value giving equal accelerate-stop distance and take-off distance. This equal-distance critical speed has normally been the one assumed in tailoring the length of the conventional runway to the performance of a par ticular aircraft, since it results in the shortest possible length. Realizing, however, that local conditions, both physical and economical, will sometimes affect plans to extend a runway, A.G.A. delegates evolved a simple method of proportioning the optimum stopway and clearway distance according to performance of the aircraft involved. In particular, construction of stopways at each end of a runway may frequently be an economical first stage in its extension. To take full advantage of such an extension, an operator might find it necessary to select a higher critical speed. Further calculations showed that the limiting length of a stopway would be approximately one-fifth of the length of the conventional runway for four piston-engined aircraft. In the case of an airliner powered by four gas turbine engines, the ratio is one-sixth. New commercial aircraft produced in this country, such as B.O.A.C.'s Comet 1s, are required to meet the standards set by the latest British performance code, intro duced pending the resolution of I.C.A.O. standards for airworthi ness. Take-off performance is covered by the Air Navigation Regulations, which require that provision be made for loss of power at a critical point. This article discusses the relationship between take-off and runway length, and reviews recent I.C.A.O. progress in this direction. "Flight" photograph.
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