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Aviation History
1960
1960 - 2238.PDF
576 FLIGHT, 7 October 1960 AVIATION ELECTRONICS ... selecter has four output devices mounted on a common shaft, butelectrically isolated, so that each autopilot and comparison channel receives its individual signal. Individual cables, sockets and plugsare also provided to isolate the outputs. The rotary mode-selecter switch has a setting for "manual"control with which it is possible to control the aircraft by means of the pitch and turn wheels. The aircraft will initially hold thecompass heading and attitude at the moment of engagement. The pitch control is disengaged except when the selecter is in "rpanual"and its synchros remain neutral until clutched in. The turn knob demands a given bank angle and its operation in a mode otherthan "manual" will cause the rotary selecter to move back auto- matically into the "manual" setting, by releasing electricallyoperated detents. A second mode is the "preset heading" one in which the aircraft will turn onto and follow the heading selectedon the course deviation indicator of the flight system. The mode-selecter may be set for following VOR or ILSlocaliaer in the normal way and a "glide-slope auto" setting will cause the aircraft to join the glide-slope beam automatically. The"glide-slope manual" setting may be used instead to initiate the entry at a moment chosen by the pilot. The "auto" selectioninitiates the push-over on to the glide-slope only when a given point before the beam has been reached at which rapid capturecan be achieved without overshoot. When most of these operating modes are selected, one of threeswitches may be set to bring in manometric locks to hold the height, i.a.s. or Mach number applying at the moment of engage-ment. Electric interlocks make these three settings mutually exclusive, so that only one may be used at a time. In addition,there is a datum adjustment knob by which the locked values may be finely adjusted without disengaging the locks. Height lockdatum may be altered by + 300ft and i.a.s. or Mach number by ± 10 per cent. The control-knob is turned through spring-loadedsteps against spring tension which slowly positions a damped output shaft so that sudden manoeuvres are not demanded. A Doppler switch may be selected to bring in Doppler navi-gator signals as an additional monitor on heading control. Selec- tion of Doppler automatically cancels any other mode selectionand smoothing similar to that of the VOR coupling mode is applied, Doppler across-track error signals being used instead ofVOR. The last position on the rotary selecter is "autoflare," whicharms the flare-out system. Prior to the actual manoeuvre, both autopilots must be engaged, and azimuth control will be dis-engaged before the flare begins. The pilot's procedures in arm- ing for an automatic flare-out allow an automatic changeover tobe made at will at any time during the approach. Immediately forward of the main control panel is the automaticthrottle panel on which servo control for each engine can be selected by a switch and either of the two independent systemschosen. The i.a.s. datum to which the throttle control is linked is selected on a veeder counter which is matched with the indica-tion of the panel instruments. Only a limited range of i.a.s. values covering approach and landing speeds may be demanded. Thethrottle servo is located beneath the flight engineer's desk and contains two independent motors and gear trains powerful enoughto drive all the throttle levers, control cables and irreversible links. Four individual electro-magnetic toothed clutches link the gearsto the control runs; and further slip-clutches allow for jammed linkages or for over-riding by the pilot if small individualadjustments have to be made. There is a dual automatic elevator trimming system in whichindependent electronic units—one in each longitudinal amplifier and computer—sense out-of-trim from independent pick-offs on the dual feel unit. The trim amplifiers operate directly into thedual hydraulic motor which positions the tailplane. Only one system can operate at a time and each has overall monitoringwhich interlocks with the associated autopilot engage circuits. An incorrect automatic trim correction can be uniquely detectedin an aircraft with an adjustable tailplane by a logical comparison with elevator movements. The interlock with autopilot engage-ment is necessary not only to ensure that the systems operate together for performance reasons, but also to ensure that, in theevent of an autopilot failure in cruising flight, automatic dis- engagement leaves the aircraft in trim. If the selected automatictrim channel is not serviceable, therefore, the associated autopilot cannot be engaged. As far as the air data and gyro references are concerned, thetwo autopilots may be separately considered, each having its own main and comparison unit of each type. The main verticalgyro of each autopilot is the Bendix unit employed in the PB-20 and has pitch and roll synchro outputs to main autopilot com-puter, comparison computer, flight system instruments and to the error monitor in which its attitude signals are compared withthose of the comparison vertical gyro. This is a stripped- down version of an instrument gyro of which more than 100,000have been produced. Its reliability is proven and it is both different from and simpler than the main gyro. Cost, weight andperformance all benefit from this factor. Its mechanical indicators have been replaced by synchro pick-offs, but mechanical erectionis provided. Each autopilot has a three-axis rate unit incorporating threerate gyros. Full monitoring is not provided for these units because it is considered that they cannot be responsible for any dangerousdemand signals, although a hard-over gimbal fault would, in fact, be detected by the comparison computer. Each autopilot containsa single Bendix Polar Path compass system integrated with the director instruments. The outputs of the two compasses areelectro-mechanically compared with each other, but independent monitors for each are not considered necessary for automaticflare-out. The layout of the air data sensing system for one of the auto-pilot and comparison channels is shown in the diagram opposite. Static and dynamic pressure (S and P-S) are sensed bycapsules frorri which position follow-up servos drive two output shafts representing log S and log P-S values. These are quitesimply mechanically subtracted to provide the third required value, log P-S/S. According to the manometric lock selected,one or other of these three shaft outputs is connected by electro- magnetic clutch to output gearing and the other to the com-parison computer. Individual synchros in the datum adjust selec- ter on the pilot's control panel are used to alter the datum ofthese synchros, as required; and the diagram shows how the electrical signals are isolated from each other, the only connec-tion being mechanical. Separate cables and connectors are used. While the main outputs from the air data and datum adjust-ment are duplicated for overall monitoring purposes, the outputs of the capsule servos themselves are compared through synchropick-offs with identical outputs in the comparison senser derived from separate capsules and pitot head. Agreement between thetwo is arranged to produce a constant electrical signal so that no signal or a wrong signal activates the disconnect circuitry. Theautopilot cannot initially be engaged unless the signal is correct. The operation of the clutches is also separately monitored. Air data is also required to adjust autopilot gains when flyingon manometric lock settings and during coupled descent along the glide-slope. Pick-offs for these are all located upstream of theelectro-magnetic clutches in the senser. Each output is dupli- cated by two synchros electrically isolated but mounted on acommon shaft, the outputs going in each case to main and com- S/iown here with top covers removed to expose the plug-in modules are, left to right, the longitudinal and lateral amplifiers and computers and the comparison monitor com- puter for both of them. These serve one of the two autopilots. The main units are in 1 ATR cases
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