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Aviation History
1964
1964 - 0947.PDF
540 FLIGHT International, 2 April 196 FLYING THE TRIDENT... weather minima than those existing today and at a consistently high safety factor. With the final goals of autoflare and autoland in view, a higher standard of integrity is demanded of the autopilot than ever before. The S.E.P.5 is controlled by an autostabilizing system consisting of duplex pitch and roll channels which drive the ailerons and pitch trim systems by means of servo motors, demand signals being fed in from the flight control system to produce the required automatic manoeuvres. When no other form of pitch guidance is selected the autopilot will maintain the attitude obtaining at the time of its engagement, and this can be varied by the pitch control. In addi- tion, height, airspeed and Mach locks are available. During the descent, 2,000ft was preselected on the height-acquire system, the speed lock selected to throttle, and 190kt dialled in. The outboard throttles then began to move of their own volition, resulting in the airspeed varying plus or minus 3kt either side of the desired figure. As 2,000ft was approached the descent lock tripped out, and the aircraft flared off to level flight, the throttles advancing to maintain the selected speed. Undercarriage was then lowered, droop and flap already having been run out, and the aircraft was turned on to an interception course for the ILS. The radio signals were switched in, the QDM and assessed drift set up and the flight controller prime switch set to GLIDE. The speed was then dialled back to 150kt and the throttles moved back obligingly to achieve the new speed. The auto-throttles are capable of controlling between 10 per cent above ground-idle to 10 per cent below maximum power. Rate of pitch as well as IAS signals are fed into the system to give it anticipatory powers. Should a power-supply failure occur, or a discrepancy arise between the air data computer comparators, then the system cuts out, and the central warning system flashes. The pilot can disengage the auto- throttle by pressing the instinctive cutout buttons on either of the outer throttles, turning the speed lock to IAS or Mach, pushing off the speed lock engage switch, or manually operating the throttles as permitted by the slipping clutches until the limit switches operate. This auto-throttle facility has now been cleared down to autoflare height. By this time the aircraft had lined itself up on the beam, which it was holding accurately. Autopilot response to demand signals in roll is limited to 3°/sec, resulting in extremely smooth operation. As the glide-path was intercepted further flap was lowered, and the nose of the aircraft lowered itself obediently to follow the slope. The almost total absence of pitching moments during flap extension is a great help during applied instrument exercises, whether the aircraft is being flown manually or automatically. The centre throttle was now set at 10,800 r.p.m. and from then on there was nothing to do but monitor the equipment. Both localizer and glide slope were held smoothly and accurately, with no tendency to hunt despite the somewhat turbulent conditions. The throttle movement is a little coarse, but airspeed bracketing was impressive. The azimuth display on the .flight compass is also of the roller- On final approach, with full fop selected, tht Trident's speed is reduced to HOkt blind type, and varies through a simple line when no radio aids are fed in, two convergent lines to indicate a VOR radial, and yellow and blue areas representing an ILS, the dividing line between the colours representing the beam itself. The autoflare facility has not yet been cleared for general use but, when under control of the radio altimeters, bank is limited to 5°. At a height of 65ft to 70ft the flare is initiated, and at 10ft bank is eradicated. Many autoflares and automatic landings have already been made with this equipment, but a further extensive programme is planned to prove beyond doubt the integrity of the system before it is cleared for normal use. At 200ft the aircraft was on the glide slope and slightly right of the centre line, this no doubt being due to an incorrect assessment of drift on my part, while the airspeed was at the required figure. During the let-down a tail-wind component of some 15kt had existed, but the equipment had shown itself to be sufficiently versatile to cope effectively with the situation. The cut-out button on the control column was then pressed, and the aircraft overshot manually for a landing in the opposite direction. Weight prior to landing was now 40,700kg, resulting in a threshold speed of 131kt, and the wind 18kt at 30° to the runway. The first circuit was flown manually, and the powerful and positive tailplane control facilitated accurate height keeping. There was a tendency to over-correct on ailerons, as these are light and powerful, while the aircraft is not over-stable laterally. On the final approach, with full flap selected, the speed was reduced to 140kt, and the impression was gained that the aircraft was very slightly unstable speed-wise, though the effects of turbu- lence made this difficult to assess with any accuracy. Certainly, the flat lift slope of the Trident wing required fairly large angle-of- attack changes in order to achieve the requisite variations in rates of descent to maintain the approach path. However, this did not prove unduly difficult, merely requiring a slight change in technique from the handling of an unswept wing. The Flare and Touchdown Flare-out was started at 100ft, speed having been reduced to the threshold value, and it was found that a considerable rotation was required to negate the rate of sink. Having flared, plenty of elevator control was still available to continue rotation and to touchdown with a minimum sink rate. The high angle of attack achieved on landing was underlined when the nose was lowered on, for it had a long way to go. There was no tendency to swing or to lift a wing as a result of the cross-wind component. On the second approach the auto-throttles were switched in while the aircraft was otherwise flown manually. It was then merely necessary to keep on the approach path, while the auto compensation device dealt with the changes in attitude, which is noticeably nose-up at these speeds. As the threshold was approached, the auto-throttles were disengaged, and reverse thrust selected on the outer engines. This operation is not hazardous, as the elevator control remains unaffected; in fact, in the event of an overshoot, it is a distinct advantage, for the outer engines are already "wound up" and, once the reversing deflectors are re- tracted, thrust is immediately available. With the use of reverse thrust at this comparatively high speed, and the effectiveness of the braking system, the landing distance was impressively short. The Trident is an intriguing aeroplane, designed for an exacting task and embracing new concepts in the field of automatics. Its high-speed handling characteristics are delightful, though like most swept-wing turbojets, it makes rather more demands at the low-speed end of the scale. It has been thoughtfully planned and skilfully engineered, so that not only is it a good proposition now, but it also has immense possibilities for development. Thirty-two 1, IE and IF Tridents are on order; negotiations are proceeding for a further 24; and options are outstanding on one more. For the record, eight have so far flown. Nos 1, 3 and 4 are being modified to final delivery standard at Bitteswell, while No 2 is being used in an autoflare programme. No 5 carried out the tropical trials and No 6 was used initially for BEA training. These two are now undergoing modifications. No 7 is fully employed in training BEA pilots and No 8 is undergoing its production testing. During this month BEA should have four machines in service, and will thus be in the fortunate position of being able to offer the newest equipment giving the highest cruising speeds and with rear-engined comfort.
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