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Aviation History
1960
1960 - 0240.PDF
240 FLIGHT, 19 February 1961- SYSTEM SURVEY. and can, by failing, put the whole chain out of action at a criticalmoment. At an early stage, de Havilland decided to apply fully poweredflight control without manual reyersion and also to use a single- piece horizontal tail. The latter, in fact, carries a geared elevator-like surface, but this is not separately controlled and moves in direct relationship with the tailplane. Because of these decisionssome form of multiplexed powered control system has been required and resulted in a triplex layout in which each enginedrives a pump serving a separate hydraulic system, two of which feed the utilities as well as two control channels. Each controlsurface is therefore to be operated by three actuators (made by Fairey) each supplied from a different hydraulic system. In caseof pump or engine failure, any one of the three systems may be restored to full pressure by an electrically-driven pump and, asa second stand-by, a ram-air turbine can be extended to perform the same task in the remote case of all three engines having failed. The electrical system is similarly based on three a.c. alternators,each driven by one of the engines, and each supplying one flight system. The whole may be operated either as a triplexed or asa triplicated system. Emergency d.c. supply and an inverter are included. There is therefore full provision for triplex controls, electricalsystem, flight-instruments and radio in the basic design. The triplex autopilot, which can be provided for the automatic landingstage, will naturally fit in with this arrangement. .It has been stated that the initial stage of auto-flare would probably be accom-plished with a duplex autopilot, but full provision is made for subsequent up-grading of equipment for the more advancedoperation. Auto-flare should enter service in the D.H.121 during 1964 anda period of crew familiarization and fault-clearing will follow. After that, weather minima might perhaps be reduced to some-thing below the present level of 200ft and 0.5 mile. Subsequently, the equipment will be upgraded for automatic landing and afurther period of familiarization would be required. The target for the establishment of automatic landing as a routine manoeuvrein service is stated by de Havilland to be 1970. It is remarkable that this is some 11 years after the RAF adopted Autoland. Human reaction to automation of the landing in low visibilityis a major factor in the introduction of the equipment, because each pilot must be persuaded that he is justified in committinghimself and his passengers to automatic operation which he can do very little to correct if something goes wrong in the criticalfinal stage. He must be convinced that the equipment is in fact safe and also that he can effectively monitor its operation downto that last critical stage. Provision of suitable instrumentation is one part of this task, but neither Smiths nor de Havilland haverevealed very much about their plans in this direction. But it has been strongly emphasized that monitoring must be possible andthat the whole operation must be a "simple and unfrightening" process for any pilot. From information released about the auto-flare stage it can bededuced that a normal coupled ILS approach will be flown by the autopilot down to a height of about 150ft, at which most Tripiexed hydraulic system for the D.H.121 with three separate systems for each control surface. Two systems will also operate the utilities. Fairey make the actuators present glide-paths become inaccurate or too sensitive. Th,autopilot would then provide open-loop guidance in pitch, mair- taining an attitude which has been averaged during the precedinapproach phase. At about 65ft, signals from duplicated radii: altimeters would trigger the auto-flare stage and provide information to a computer to control the flare-out on to the runway Smiths have shown that throttle setting can significantly affect theflare-out control of the autopilot and that automatic control woulc be necessary. Such a device already has been produced for BLElAutoland and would be applied to the D.H.121 to maintaii airspeed within ±2kt of a preset value during the approachDuring flare-out it would reduce power to flight idling at a pre determined rate. The pilot would control ailerons and rudder toline the aircraft up visually with the runway centre-line and to kick off drift. Airco have stressed that for both economic and technical reason;no ground equipment other than ILS should be required. Thi; almost certainly means that for automatic landing one of the new,more accurate ILS systems would be needed. Although virtually nothing has been said about the instrumenta-tion of the D.H.121, an inspection of the mock-up cockpit for BEA (see page 239), reveals quite a new approach to cockpit lay-out. Basically, the flight deck is laid out for three-crew operation, with the provision that all three men should be seated lookingforward during the first and last ten minutes of every flight. While the third man normally supervises the systems panel behind theco-pilot in cruising flight, he moves his seat up to the end of the central console during landing and take-off. Any systemfailure will be indicated to the crew in the forward field of vision, and particularly by a centralized warning panel containingilluminated notices. Red and orange attention-getting lights are located directly in front of each pilot. Failures requiring urgentaction are denoted by the red lights and those less urgent by the other. Trim, throttle, radio, and utilities controls are conven-tionally placed on the central console with further controls, including the selectors for the three hydraulic systems, in the roof. It may be deduced that several air data computers and gyroreferences are provided to supply the two flight-instrument groups on the panel and the autopilot system. Needle-and-counteraltimeters are servo-operated and the single a.s.i. pointer revolves over a logarithmic scale through two complete turns. ILS infor-mation is presented on each flight system and there is a third, miniature cross-pointer instrument beside the captain's panel.In the same way, there is a radio-altimeter dial over each pilot's panel and a third such instrument near the captain. Of the threeneedles on these dials, one appears to be an indicator and the others are yellow markers for significant heights. The heightscale reads from 0 to 700ft. The VSIs, incidentally, are of a type not seen before, reading logarithmically up to 7,000ft/min up ordown. Perhaps most significant are the central flight system instru-ments themselves. They are evidently based on the roller-blind, remote-operated principle which has been adopted by the RAF.The attitude presentation includes a director symbol and several prominent warning flags. Beneath this dial is the compass displaywith conventional compass graduations round the rim and a steering director or course-selector mark. The centre of theinstrument is taken up by a further roller-blind display on which blue and yellow areas obviously represent the ILS blue-yellowsectors and provide localizer guidance. The RAF roller-blind compass display can be shifted to display ILS or Tacan navigationinformation and it seems certain that the instrument seen in the D.H.121 mock-up provides similar facilities. Because all these instruments depend on remote sources ofcomputed information, a conventional directly-operated a.s.i., electric horizon and three-pointer altimeter are provided as stand-bys at the top of the central panel. A miniature turn-and-slip instrument is nearby. The nine basic engine instruments aregrouped within a white outline to the right of the central panel. Another significant feature of this new cockpit is the fact thatthe central panel space which traditionally has been occupied by engine instruments is taken up by a pictorial map display. Unlikethe Decca Flight Log, this map is square and can therefore display a greater area ahead of the aircraft— a useful asset for high-speedflying. DME and ADF instruments are located immediately to the left of the map, together with Doppler navigator indicators.It seems possible that it is intended to drive this map mainly from the Doppler output, using periodic fixes from the other aid^to correct any errors which might build up. Weather radar indicator and controls are mounted together above and to the leftof the map. It is remarkable that, despite triplication of so much of thebasic equipment and the addition of some entirely new elements, the cockpit should still appear neat and uncluttered. For the firsttime a new aircraft more complicated than its predecessors ha; been given a main instrument panel which appears significantlyless complicated than earlier examples. Great credit must go to BEA, de Havilland and Smiths for achieving this result.
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