Aircraft manufacturers had, for some four decades, attempted to design the elusive DC-3 replacement with different powerplant types, including the piston-engined Convair 240/340/440 and Martin 2-0-2/4-0-4 series and the turboprop Vickers Viscount, Fokker F.27 Friendship, and Hawker Siddeley HS.748. The latest attempt had been made by the British aircraft industry when both de Havilland and Hawker Siddeley had conducted market research and formulated designs for a small-capacity, short-range airliner powered by pure-jet engines during 1959 and 1960.
Of the two, de Havilland, with its previous Rapide, Dove, and Heron pistonliners, had had considerable regional aircraft experience and had designed the world’s first pure-jet airliner in the form of the quad-engined DH.106 Comet. An initial study for such a DC-3 replacement, designated the DH.123, had featured a 60.6-foot overall length, an 81.3-foot wingspan, two 1,150 shp Gnone turboprops attached to a high wing, and a 22,100-pound maximum take off weight. So configured, it would have accommodated between 32 and 40 passengers, or slightly more than the DC-3’s standard 21 to 28.
De Havilland, subsequently taken over by Hawker Siddeley and redesignated the “de Havilland Division,” had forcibly discontinued design work on the DH.123 because it would have competed too closely with Hawker Siddeley’s own Rolls Royce Dart-powered Avro 748 which had seated 44. Nevertheless, existing turboprop competition, coupled with de Havilland’s belief that pure-jet technology would attract considerable passenger appeal, resulted in the mid-1960 DH.126 design proposal, which featured the later-standard configuration of most low-capacity, short-range twin-jets, Nashville limo service such as the SE.210 Caravelle, the BAC-111, and the DC-9, with a swept wing, aft-mounted engines, and a t-tail. Powered by two 3,860 thrust-pound de Havilland PS92 jet engines, it had featured a 60.3-foot length for accommodation of 30 passengers and a 62-foot wingspan.
Several iterations had introduced progressive, although moderate wingspan, thrust, and gross weight increases by 1964, but further development had been hampered by four fundamental obstacles:
1. Suitable pure-jet engine availability.
2. Discontinuation of promising engine development because of several mid-1960s British engine manufacturer mergers.
3. Higher seat-mile costs over DC-3-like sectors for which the new design had been intended.
4. The inability to exploit a pure-jet airliner’s speed over relatively short sectors.
Hawker Siddeley, believing that the turboprop engine had only been interim-step technology, had equally embarked on a pure-jet airliner design program of its own long before the de Havilland merger, although its low-wing, aft-engined, t-tailed configurations had strongly resembled its former competitor’s.
Attempting to minimize development costs by utilizing the cockpit, forward fuselage, systems, and passenger cabin of its own Avro 748, it had proposed the HS.131 in 1964, which had featured similar 62.8-foot overall lengths and 67-foot wingspans as de Havilland’s comparable DH.126, but its projected 5,000 thrust-pound Rolls Royce RB.172 engines had enabled it to offer a higher, 30,000-pound gross weight and a 32-passenger capacity.
Faced, like de Havilland, with engine unsuitability and unavailability, Hawker Siddeley devised iterations round projected powerplants. A radical configuration change, introduced by the HS.136 of 1967, for instance, had resulted in a low-wing aircraft powered by two 9,730 thrust-pound Rolls Royce Trent engines with a conventional tail accommodating 57 passengers in a five-abreast cabin and offering a 54,000-pound maximum take off weight. Although the arrangement would have eliminated the aft-mounted, t-tail’s propensity toward deep-stall and flame-out conditions, and its close ground proximity would have facilitated passenger, aircraft servicing, and maintenance access, the potential for foreign object debris (FOD) engine ingestion had yielded to the proposed HS.144 two years later, which had once again reverted to the now standard aft-engine configuration.
Progressive design evolutions and dimensional and thrust increases had intermittently resulted in an airplane whose passenger capacity had been double that of the DC-3’s, and with the Rolls Royce bankruptcy-sparked discontinuation of Trent development in 1970, the DC-3 replacement, now powerless, had become ever more elusive.
This low-capacity, short-range jetliner had, like never before, hinged upon a powerplant for its existence, and the only potential lay with a small turbofan being developed by Avco Lycoming in the US. Based upon the 7,000 thrust-pound F102 which had powered the Northrop A-9A, the engine, a derated civil derivative designated ALF-502, had been launched in 1969 for the Canadair CL-601 Challenger business jet and had first run two years later. In order to offer commercial application, it had been of modular construction.
Because the type’s 6,500 thrust-pound rating had been inadequate for the latest aircraft design, the HS.146 of 1971, and because no other suitable powerplant had been in the development stage, the ultimate DC-3 replacement had been forcibly designed round four, not two, engines and it featured neither the standard, aft engine-mounted, t-tailed nor alternative wing-mounted configuration. Instead, it would sport two high, modestly swept wings to which the four turbofans would be pylon-mounted. Accommodating 88 passengers, or three times as many as the DC-3, the airliner, with an 86.2-foot length and 84.10-foot wingspan, had a 70,000-pound gross weight and 700-nautical mile range.
Nevertheless, the HS.146 offered several advantages over the earlier, standard-arrangement de Havilland and Hawker Siddeley design studies. Short-field performance, fully the equivalent of the turboprops it had intended to replace, had been attained by its thrust-to-weight ratio and wing, which, with 78-percent coverage of its trailing edge with Fowler flaps, had obviated the need for leading edge devices, and simplification and weight reduction had been further achieved with the elimination of thrust reversers. The t-tail, remaining from the earlier designs, had been retained in order to avoid engine and wing turbulence interference.
The four engine pods, which had been interchangeable with each other, housed modular construction cores built up of the basic fan, the accessory gearbox, the gas producer/compressor, and the combustion turbine sections.
An 11.8-foot fuselage diameter had permitted an internal, six-abreast coach seating arrangement, which had been double that of the DC-3’s.
In order to cater to different route demands, Hawker Siddeley offered an initial, 88-passenger HS-146-100 and a stretched, 102-passenger HS.146-200 version, both at maximum, six-abreast densities, although capacity could be reduced with varying class, seat pitch, and abreast arrangements.
Fully intended as a pure-jet counterpart to the turboprop Viscount, HS.748, and F.27, the HS.146 had been optimized for multiple daily, high frequency, short-range sectors from short and unprepared, gravel runways, yet achieve 15-percent lower direct-operating-costs than these aircraft. Slow, controlled approach speeds, of just over 100 knots, had been attainable by its aft fuselage, petal airbrakes and 40 degrees of trailing edge flap, permitting operation from 5,000-foot runways.
Hawker Siddeley had estimated a market of 1,500 aircraft of its type by 1982.
HS.146 program launch, based upon a 40 million British pound government backing and the manufacturer’s own investment, had occurred on August 29, 1973, and the first flight, of the short-fuselage HS.146-100, had been targeted for December of that year with certification following in February of 1977, while the stretched HS.146-200, coinciding with the seventh airframe, had been targeted for certification in August of 1978. A full-scale wooden mock-up had been intermittently built at Hatfield.