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Aviation History
1959
1959 - 1309.PDF
636 The fail-safe, safe-life features of the 748 are achieved with a light and simple structure AVRO 748 A New DC-3 Replacement: First Detailed Figures Released THE first official figures to be released concerning theHawker-Siddeley Group's twin-Dart feederliner confirmthat the Avro 748 is intended as a contender for the DC-3 replacement market—and that it is to be a direct competitor tothe Handley Page Dart Herald and Fokker Friendship. Data for the three types, set out in the table on the right, show how closelythe specifications of the three aircraft compare. What the table cannot show is the most obvious distinguishing feature of theAvro 748: the low wing, which feature alone may make this design particularly attractive to certain operators. Structurally, the aircraft has been kept deliberately simple andlight. Magnesium or zinc-bearing aluminium alloys are not used and structural materials are limited to L.72, L.73 and L.65. Fail-safe principles are employed throughout and the airframe should also have an "almost infinite" fatigue life; in line with the develop-ing philosophy of fail-safe, the design provides for easy examina- tion of the structure and for a slow rate of crack propagation. Useis also made of duplicate load-paths and crack-stopping structural joints. Avro claim that, because of the attention that has been paidto accessibility, no part of the structure will be hidden from inspection. Bonded joints, for example, may in certain circum-stances be difficult to inspect, so none are used and structural repairs can be carried out by conventional methods. An outline of the structure is shown in the makers' drawingabove. The wing is of two-web construction embracing integral tanks, flaps and ailerons are single units and mushroom-headedrivets are used everywhere except on the upper surface of the wing. Structural joints have generally been avoided to save weight,maintenance and fatigue troubles. _ Doors under the fuselage open for access to hydraulics and IT- Viscount components, Dart engines and big elliptical windows an proven features of Avro's design ground servicing connections (in the nosewheel bay) pressuriza-tion systems (under the forward cabin) and electrics and the electric flap motor installation (aft of the wing). Choice of Rolls-Royce Dan 514 powerplants for the Avro 748has presumably been governed by the exceptionally long overhaul life which can be guaranteed—it already stands at 2,000 hr. Vickersexperience with the Viscount 800 has also been drawn upon in installing the RDa.6; the two installations are basically similarand Avro use Vickers powerplant components. Automatic water/ methanol injection to restore take-off power (1,600 s.h.p. at sealevel, I.S.A.) at high ambient temperatures is an optional fitment. Propellers are 12ft diameter Rotol, electrically de-iced, and aRotol accessory gearbox is mounted near the top of the nacelle aft of the firewall. Reverse-pitch facility is not provided, groundfine pitch being used to reduce the landing run, but automatic pitch coarsening is available. Systems. Duplicated air supply and pressure control systemsmaintain the cabin at a differential pressure of 4.2 Ib/sq in (8,000ft cabin altitude at 20,000ft) and circulate 50 lb of freshair per minute. Cabin temperature is maintained above 20 deg C and ram-air cooling is provided for hot climates. A cold airunit can be supplied. Two engine-driven hydraulic pumps energize the forwards-retracting undercarriage, nosewheel steering and brakes. For the last-named system a pressure reserve is contained in two accumu-lators. Emergency undercarriage lowering is by hand pump or by free fall. The electrical power system is 28-volt D.C. witha secondary 115-volt, 3-phase, 400 c/s system and a separate 200-volt, 3-phase variable frequency supply for engine and pro-peller de-icing. Pneumatic de-icer boots can be fitted to the wings and tail if specified. Two 550 gal integral fuel tanks, one in each wing, supply fuelto their respective engines through two electrically-driven fuel pumps on each side, but one pump alone will meet the maximumdemand of the system and the supply can be cross-fed. Over- wing refuelling is normal but underwing refuelling can be fitted. In calculating equipped weight, allowance has been made for
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