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Aviation History
1953
1953 - 0540.PDF
(Left) The captain's switch panel is for landing lights, cabin signs, intercom control, audible Mach meter, engine-starter master-switch and (below) screen wiper—off, slow, fast. Beside it is the Ultra radio station-box; the other items are the pilot's oxygen supply and intercom plug. Note the nosewheel steering control on the main control column. On the central pedestal are mounted (top, I. to r.) brake, throttle levers and flap lever, just below which is the air-brake lever. At the base of these are the autopilot controls and, below again, the undercarriage selector lever, flap indicator and trimmers and their indicators (the elevator wheels are on each side) and, below again, the four H.P. cocks and the four main fuel-cocks. (Right) The controls in the roof are mainly concerned with radio but include also landing-wheel and door indicators, controls for emergency systems, lighting rheostats and alarm bells. CONVERSION to COMETS . . . Throttles are clojed, isolation switches set to "normal" and, the ground engineer being warned, and with starter master-switch "on," and low- and high-pressure cocks "on," the start is com menced. Either No. i or No. 4 engine is started first; with the selector set for the appropriate engine, the single starter-button is depressed to initiate the 30 sec starting cycle. The button is held in by an electrical relay until the cycle is complete. If the ground power supply, normally taken from a 28 V generating truck, is not suffi cient, the button may have to be pressed until the relay is ener gized by alternator output from the starting engine. It is now standard practice, incidentally, to start with the alternator switched on j it will then provide power as soon as the engine in question reaches sufficient r.p.m. If the starting cycle is interrupted by removal of ground power or failure of the relay, the probable result is a "hot start" with excessive jet-pipe temperature (maximum is 725 deg C)—and the only safe thing to do is to put the high-pressure cock off and allow drainage before a re-start. The engine may be motored with the fuel off in order to clear the gases. When all four engines are idling at 3,000 r.p.m., the starting procedure is completed by a check of flying-control operation on the "blue" (normal) hydraulic system, and the ground electrical' apply is removed. All alternators are checked for correct operation, voltage and load-sharing. Engine instruments are checked for correct temperatures and pressures, and there is a final check that a.c. failure lights and door warning- lights are out, followed by the Zero Reader check and alignment of the C.L.2 compass. Radio equipment, including the twin auto direction-finders and the I.L.S. and marker receivers, are tested and tuned for take-off procedures. Possible settings would be No. 1 A.D.F. on Dunsfold, No. 2 on Epsom radio-range, and the I.L.S. selected to the correct stud for Runway 10-Right at London. The aircraft is ready to taxi as soon as A.T.C. have given taxi clearance; the control officers offer the maximum co-operation and normally there is very little delay. With chocks away and parking-brake off, the outboard engines are opened up to about 6,000 r.p.m.. the inners remaining at 3,000 r.p.m. to conserve fuel. After a few seconds' delay, while the necessary thrust develops, the Comet begins to move. The "outers" are then throttled to a setting—say 4,500 r.p.m.— sufficient to keep a moderately high taxi speed. Light and effec tive steering is provided by a wheel, of the self-centring type, on the captain's control column. Flying-control surfaces are locked hydraulically until the actual trim or main controls are operated from the cockpit, so there is no "snatch" in the rudders as, for instance, in the DC-3, which needs the second pilot or the auto pilot to hold the rudder pedals central during taxying in a high wind. Brakes are of the toe-operated type so well pioneered by many American aircraft during the war; the Dunlop disc-and- pressure-block system gives adequate braking under all conditions with which the writer has yet had to deal. Development is pro ceeding on the application of the Maxaret type of braking to the later, heavier marks of Comet. Should the hydraulic nosewheel steering fail, the nosewheel castors, and differential brake-action will effectively steer the air craft. The toe pedals simultaneously operate two separate brake systems, providing another safeguard. Lacking the piston air craft's airscrew-drag (and in some cases reverse-thrust), and being of such clean aerodynamic shape, the Comet shows an inclination to "keep right on to the end," so brakes are a major item, and must be 100 per cent reliable. Stand-by pressure for brakes and nose wheel steering is provided by the operation of an electrical pump, which is left on for taxying and take-off. During the roll to the take-off position the "before take-off" check is completed. Flying instruments are examined for service ability, and the "blue" pressure-failure warning-horn is switched on. Flying controls are checked finally on the "blue" system; pressures should be normal. Flaps are set to 15 deg and aileron, elevator and rudder trims at take-off setting, air-brakes "off" and pitot heaters on. Critical speed (Vi) and unstick speed (V2) are then read out for the runway in use. These speeds are obtained from the operations manual, which contains charts showing regulated take-off weight, Vi and V2 for individual runways at all airfields used by Comets for every condition of wind, temperature, pressure-altitude and runway slope. These values are finalized from the last available data transmitted on V.H.F. R/T. from the control tower. At the maximum permissible a.u.w. of 105,000 lb (47,627 kg)* the critical speed at London Airport on the E.W. runway is 95 kt I.A.S. and the unstick speed 100 kt I.A.S. Under calm conditions the 3,000 yd available allow maximum weight to be used. At many airfields on the Eastern and South African routes a runway gradient factor must also be taken into account. High ambient temperatures also penalize the jet engine and, combined with high altitude, do in fact restrict the take-off weight at such hot airfields as Entebbe (3,772 ft) and Livingstone (3,230 ft). When all instruments have been checked, the engine fuel-pump isolation solenoids are switched to "isolate" and back to "normal," their operation being checked by a rise and fall in r.p.m. Pressuriz- ation controls, fuel-tank booster-pumps, fuel feed, thermal de- icing, emergency hydraulic selector, alternator, instrument (26V) and main (115V) inverters, fuel flowmeter, fuel-contents gauges and cabin air conditioning—all these will have been set correctly by the engineer as called out from the check-list by the first officer. If possible, A.T.C. will have given clearance on to the runway while the Comet has been taxying. This avoids both the noise and the fuel-penalty of using high power to get the aircraft rolling once more. With the nosewheel central and engines throttled, the parking brake is applied. If take-off clearance has been granted, the throttles are opened slowly until the jet-pipe temperature gauges reach 500 deg C; the levers may then be moved progressively forward (recommended time from idling to full opening is 10 sec). When the four Ghosts are running at 10,200 r.p.m. the isolation switches are switched to "isolate," and the r.p.m. should rise to 10,250. The first officer and engineer carry out a swift but thorough final inspection of all engine instruments. Maximum figures *Nou> certificated at 107,000 lb (48,535 kg)—Ed.
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