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Aviation History
1964
1964 - 0716.PDF
RJGHT International, 19 March 1964 413 Testing the BAC One-Eleven By JAMES HAY STEVENS, AFRA.S IN recent years one has heard more and more about the com-prehensive testing which precedes the introduction of newaeroplanes into service. The BAC One-Eleven is no exception, and the writer recently had the privilege of learning about the pre- cautions being taken to ensure not only that "everything works," but that fatigue lives will be established in advance and that as many as possible of the teething troubles will be located and eliminated on the ground. The following notes give an idea of the thought and care behind the One-Eleven test programme. The British Aircraft Corporation has designed and produced more airliners, by type and in quantity, than has any other company outside the United States. Its experience in this field is equal to that of any civil aircraft manufacturer in the world today. This is no publicity puff, but simply a statement of fact: the series of Vickers airliners started in 1945 with the hastily made Vikings (still in profitable service today), followed by the Viscount (only European airliner ever to reach 400 sales), the Vanguard and the VC10, in an unbroken sequence, plus the Bristol 170 and the Britannia. All this back- ground of experience is designed into the One-Eleven and applied to the tests of systems and structure to establish their overall performance and reliability. There are two basic targets: a minimum service (i.e., total) life of 40,000 flights and a minimum overhaul life of 3,000 operating hours. Note that our old familiar friend "flying hours" has at last gone by the board. Since it is the flight cycle (take-off and landing thumps, temperature-change thermal stresses, cabin inflat- tion/deflation etc) which builds up fatigue life, this is measured in "flights," which for this shorthaul jetliner are assessed at 45min each. Thus we are back at 30,000hr as the minimum for a crack-free structure, with the actual tests continuing to 100,000 cycles (75,000hr) so that the 2.5 fatigue factor can be satisfied. There are two test airframes; one for static and structural-redundancy proving, the other for establishing the fatigue life. The static airframe (although complete including control surfaces) is divided into two sections (Fig 1) for convenience and to prevent different tests interfering with each other. There were five test stages: (1) proof pressure tests on front and rear cabin sections; (2) 200 per cent cabin pressure test, giving approximately 85 per cent of the structural ultimate stresses; (3) loading to 85 per cent ultimate for all major design cases; (4) redundancy (fail-safe) tests; (5) development tests as a result of experience. Ultimate-stress tests come at the end of the programme as a check on calculations, because they would otherwise interrupt the programme, and one failure can actually mask other weaknesses. The two pressure tests were needed for initial flight clearance and were done before the wings were attached to the integral centre-section of the test fuselage. There are seven design cases in the third stage. At the fourth stage critical parts of the structure will be cut or weakened to check that the designed alternative load paths worked—an example is the cutting through of one windscreen frame member followed by full pressurization. The airframe fatigue programme will take until the early part of 1967 to complete the 100,000 cycles. The most potentially catas- trophic component subject to fatigue stresses is the pressure fuselage, and so the first phase of this programme is to test the fuselage, with the fin (Fig 2) and centre section attached, to 30,000 flight cycles. In Fig 3 the set-up of the fuselage for fatigue testing is shown, together with the type of loading pattern. In April this year the wings are due to be attached and these, too, will then be subjected to the full cycle of flight and gust loads (Fig 4). The fatigue pro- gramme will be completed by artificially induced cracks, the propagation rate—another fail-safe feature—of which can be checked. In addition to the main airframe programmes there is also an extensive series of detail-part tests. Most of these were carried out before the main airframe tests. Examples of this work were: wear tests on the flap track and roller assembly; fatigue test on the lower wing skin round the fuel pump cut-out; static strength test of the undercarriage mounting brackets; fatigue tests on the flap track attachment bracket, the engine mounting beam and attachment joint, the fuel system suction valve, tailplane link bearing, etc; strength test on the leading-edge attachment bolts; ultimate Fig la This drawing, in conjunction with Fig Ib on page 414, shows how the One-Eleven static test airframe has been divided into two convenient sections and how the load cases are applied
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