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Aviation History
1961
1961 - 0408.PDF
416 At far left is a Vanguard main undercarriage leg, near the top of which is a box containing four Honeywell micro- switches. Left, flap-pro- tection switch (visible near centre of picture). Right, a Honeywell Type HS hermetically sealed switch in diagrammatic section (see text below) FLIGHT, 30 March 1961 GAS • " CHAIV3 GLAS: POTTC[ CHAM.! The Industry MICROSWITCHES ON THE VANGUARD THE operation of a large modern airliner involves the simul-taneous control of so many actions that the flight crewcould neither think nor act quickly enough to handle all individually. Many functions are therefore left to the care of auto- matic devices, ranging from complex instruments like autopilots and navigational aids to apparently simple things like electric switches which carry out essential actions at the correct time and in the correct sequence. The description "apparently simple" is used advisedly, because aircraft requirements make the design of switches a much more difficult matter than designing an industrial control device. Air- borne switches have to be as small as possible, yet carry a heavy electrical load; they must have a very long operational life without the need for servicing; they must work in difficult and widely varying environments; and, most important of all, they must be absolutely dependable. Specially developed switches are now available to meet these requirements. As an example of the reliance which is placed in them, the Vickers-Armstrongs Vanguard uses no fewer than 70 microswitches of various types, produced by Honeywell Controls Ltd, of Greenford, Middlesex. This follows experience on a smaller scale with the Viscount, in which the dependability of the switches has been amply proved. Problems of environment have been solved by the Type HSseries of switches, in which the contacts are contained within a hermetically sealed chamber, filled with inert gas (see diagramabove). The leads enter this chamber through glass-to-metal seals, which are further protected by a second chamber in which theleads are potted in epoxy resin. Operation of the contacts by the actuating lever is through a brazed-in metal diaphragm.This type of switch is used on the Vanguard undercarriage. Four are mounted in the box at the top of each main leg, where they areoperated by the rod connected to the knuckle at the bottom and actuated by the weight of the aircraft. The chassis switches close arelay circuit which variously reduces the de-icing current to a value which will not cause overheating when the aircraft is on the ground;prevents accidental retraction of the undercarriage; ensures that engine ignition cannot be inadvertently selected "on"; and isolatesthe stall-warning device and the heated element for the ice detector. Similar switches in the three wheel-wells provide cockpit indica-tion of undercarriage up- and down-lock, and ensure that the nose landing light is switched off when the undercarriage is retracted.All the undercarriage switches have to operate in the very worst conditions. They must work without fail at very high altitudes andvery low temperatures; successive landings and take-offs may expose them, within the space of an hour or two, to a blast ofsand-laden tropical air and an icy spray of corrosive water from a salt-treated frozen runway.Although the weight of each complete switch is only 1.5oz, the d.c. rating is 24A with a purely resistive load, and 10A with aninductive load. An even smaller hermetically sealed switch, work- ing under conditions only slightly less arduous than the under-carriage switches, is illustrated above. Seven of these Type EN switches are fitted to the flap mechanism and come into operationshould part of it fail. They trigger electrical locks which prevent runaway of the flap and operate a warning lamp in the cockpit.Perhaps the most important of all the microswitches in the Vanguard are those concerned with engine-fire procedure. Fourhandles at the top of the instrument panel each carry out for the pilot the complete emergency sequence of operations for one engine.Four switches are built into each handle. When a handle is pulled, the propeller is feathered, low-pressure fuel is shut off, and thehydraulic shut-off valve is closed; an engine-bay fire-extinguisher is This switch in the engine bay, controlling the Godfrey control valve for cabin air, is designed to withstand very high temperatures Fire emergency switches at the top of the instrument panel operated when the handle is turned anti-clockwise. Should thefire not be extinguished at the first attempt, a clockwise twist of the handle brings in a second extinguisher. This simplification not onlyensures that all vital operations are carried out in the correct sequence, but allows the pilot time to concentrate on trimminghis aircraft to the new situation and, if necessary, thinking out the next move. The microswitches in this case are of the standardplastic-cased V3 type which, although mounted in a well-protected position, are satisfactory for use at temperatures up to 180°F.A high-temperature version, with an asbesto-filled phenolic case, and suitable for temperatures up to 250cF, is also available. For really high temperatures, the Type HT microswitch ismounted in a ceramic and Monel case, which allows an operating temperature range of from -50°Fto l,000°F. Fitted to the engine-mounted cabin-air blowers, devices of this type act as limit switches on the butterfly valves which are co-ordinated with the cool airsupply from the ducts. Other applications of Honeywell microswitches on the Vanguardinclude those concerned with equalization of cabin and external pressure before cabin doors can be opened; control of local lightingin the under-wing refuelling bays and other areas when the access doors are opened and closed; selection of emergency flap andundercarriage operation; and control of warning lights to indicate hydraulic and other service failures. In addition to those described above, which apply to the BEAVanguards, many more switches of the same kind have been speci- fied for the Vanguards supplied to TCA. Almost all of the normaltoggle switches are by Honeywell, as is a special array of switches mounted in the control cabin roof and operated in one movementby a rotary cam, to act as power supply master-switches.
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