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Aviation History
1953
1953 - 0566.PDF
56o FLIGHT, i May 1953 ASSESSMENT of OPERATING COSTS Extension of Standard S.B.A.C. Method to Meet Modern Conditions EXTRAVAGANT and conflicting claims by aircraft manufacturers for the qualities of their products have, in the past, added much uncertainty and difficulty to the task of impartially comparing different types. The Society of British Aircraft Constructors took a useful step, therefore, in producing—some three years ago—their standard method for estimating the direct operating costs of aircraft. Although not the only available method, it was at that time one of the most realistic and up to date, and did enable objective assessment of different aircraft types to be made. Being based on practical operating experience, its calculations were, of course, confined to piston-engined aircraft. Now published is an amended version* of this method, which includes revised and additional information resulting from more recent operating experience. In particular—and here lies the main importance of the new version—it makes possible the calculation of performance and costs of turbojet and turboprop aircraft, which in turn now enables legitimate comparisons between turbine- powered and piston-engined machines to be made. As before, the objects of the method are stated to be "to aid a potential operator to assess the economic suitability of an aero plane for his operations, and to provide a ready means for com paring the operating economics of competitive aircraft in a standard set of conditions." There is no doubt that, for comparative pur poses especially, the method is of undoubted value to both operators and manufacturers. Mr. R. V. Perfect, chairman of the technical panel responsible for the revised method, has stated that the new turbine figures are in fact conservative; the panel's calculations have erred on this side in order that the method should not fall into disrepute through an over-optimistic assessment of the jet's 3SU 300 250 200 30 25 20 10 // \ \ s \ s. \ \ N. ipoo 2.000 3,000 STAGE LENGTH (n.m.) 4000 Curves showing speed and pay-load comparisons between the Britannia 300 (solid line) and an American aircraft—probably the DC-7 (dotted line). Using maximum-range technique, the mean cruising speeds are about 304 kt and 267 kt respectively. economy. With further operating experience, the figures are likely to be revised—in particular, turbine overhaul costs should decrease markedly in the future. The new version of the method retains the original main sub divisions of costs, and the alterations and additions to specific items are as follows:— For turbine engines the spares holding (used in calculating the power-plant obsolescence costs) is to be taken as 75 per cent of the total power-plant cost, compared with 50 per cent for piston engines, due to the closer resemblance of turbine power-plants to the bare engine. Maintenance and overhaul costs for turbine * Standard Method for the Estimation of Direct Operating Costs of Aircraft. Issue No. 2. Available from the Technical Section of the Society of British Aircraft Constructors, Summit House, Langham Place, London, W.i. Price 5s. engines are grouped together at 25 per cent of the engine first cost: for piston engines there remain separate calculations for these two factors. Ground fuel consumption assessments for turbine aircraft are more favourable than those for piston-engined aircraft for starting and final checks, but more severe for taxying. The starting allow ance is for five minutes at ground idling r.p.m. (compared with 8.5 minutes for piston-engined aircraft), while the taxying allow ance is 2 lb (turboprop) or 4 lb (turbojet) per 1,000 lb of relevant aircraft weight (compared with 1.7 lb per 1,000 lb for piston- engined types). Oil consumption for turbine engines is negligible, 55 30 20 24 5; 2 2 16 •\A 1-2 s \"-. . 1 1 1 \ \ \ \ \ \ ' \ ^ \ \ ~~ — 1 1 1 1 / 1 / / / ' 1 / 1 t i 1 I / — / / / t 1 / IpOO 2POO 3POO STACE LENGTH (am) 4.000 A comparison of costs per ton-mile (top) and per passenger-mile (bottom) for the two aircraft. The economy of the turboprop engines, especially between stage lengths of 1,000 and 3,000 miles, is evident. The S.B.A.C. method, for the first time, enables such comparisons to be made. while the assumed cost of oil for piston-engined aircraft is 4 per cent of the fuel cost. These factors, together with basic aircraft and engine data, are fed into the standard equations and the unit direct costs obtained. An interesting change from the earlier calculations is that the assumed aircraft insurance rate (for both piston-engined and turbine-powered types) has come down from 6 to 5 per cent, following a similar change in actual premiums charged due to an improvement in the record of aircraft accidents. In addition, figures for freight insurance premiums have now been incor porated. Increases in fuel, oil and landing costs have been reflected in the S.B.A.C. figures, and have resulted in generally higher flying costs. There have been similar increases in the maintenance and overhaul charges. The Method Applied.—Shown in the graphs on this page are illustrative examples of the performance of two projected airlines, calculated on the basis of the revised S.B.A.C. method. The two aircraft are the Bristol Britannia Mk 300 (assumed to carry 104 seats), and an American aircraft which, from an analysis of the performance, appears almost certainly to be the 76-seat Douglas DC-7. In terms of speed, payload and specific cost, the turboprop machine is seen to be the more efficient. The assumptions are for the maximum-range cruise procedure; calculations based on the maximum cruising-speed procedure show almost identically shaped curves, the main differences being the reduced range, and the obviously increased speed, which have comparable effects on both aircraft.
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