B2-13c: Autoflight PDF

Student Resource Subject B2‐13c: Autoflight  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold, or otherwise disposed of, without the written permission of Aviation Australia...

Student Resource Subject B2‐13c: Autoflight  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold, or otherwise disposed of, without the written permission of Aviation Australia CONTROLLED DOCUMENT 2020‐04‐14 Training Material Only Page 2 of 6  CONTENTS Contents 3 Study Resources 4 Definitions 5 Introduction 6 2020‐04‐14 Training Material Only Page 3 of 6  STUDY RESOURCES E. H. J. Pallett, & Shawn Coyle (1993). Automatic Flight Control, (fourth edition): Blackwell Science, London. Jeppesen Sanderson, (1974). Avionics Fundamentals, United Airlines. 2020‐04‐14 Training Material Only Page 4 of 6  DEFINITIONS Define  To describe the nature or basic qualities of.  To state the precise meaning of (a word or sense of a word). State  Specify in words or writing.  To set forth in words; declare. Identify  To establish the identity of. List  Itemise. Describe  Represent in words enabling hearer or reader to form an idea of an object or process.  To tell the facts, details, or particulars of something verbally or in writing. Explain  Make known in detail.  Offer reason for cause and effect. 2020‐04‐14 Training Material Only Page 5 of 6  INTRODUCTION The purpose of this subject is to familiarise you with general engineering drawings, parts and tooling typically used in aircraft maintenance engineering.. On completion of the following topics you will be able to: Topic 13.3.1 Autoflight ‐Fundamentals Explain the fundamentals of automatic flight control including working principle and define current terminology. Topic 13.3.2 Autoflight ‐ Command Signal Processing Explain command signal processing. Topic 13.3.3 Autoflight ‐ Modes of Operation Explain the modes of operation in the following control channels:  Roll  Pitch  Yaw Topic 13.3.4 Autoflight Systems ‐ Yaw Damping State the purpose of Yaw Dampers and explain their operation. Topic 13.3.5 Helicopter Autoflight Systems State the purpose of Stability Augmentation System (SAS) in helicopters and explain its operation. Topic 13.3.6 Autoflight Systems ‐ Trim Control State the function of Automatic trim control and explain its operation. Topic 13.3.7 Autopilot Navigation Aids Interface Explain the operation of Autopilot Navigation Aids interface. Topic 13.3.8 Flight Management System Interface Explain the operation of Flight Management Systems (FMS) including the navigation database. Topic 13.3.9 Autoflight Systems‐ Autothrottle State the purpose of Autothrottle systems and explain their operation. Topic 13.3.10 Autoflight Systems‐ Autoland Explain the principles and categories of Automatic Landing Systems:  Modes of operation;  Approach,  Glideslope,  Land and  Go‐Around.  System Monitors and Failure Conditions and  Downgrade and Upgrade Procedures. 2020‐04‐14 Training Material Only Page 6 of 6 TOPIC 13.3.1: FUNDAMENTALS Table of Contents List of Figures................................................................................................................................ 3 TOPIC 13.3.1: FUNDAMENTALS.................................................................................................... 5 Auto Pilot Purpose......................................................................................................................... 5 Inner Loop (Roll)........................................................................................................................ 7 Outer Loop (Altitude or Heading).............................................................................................. 8 Modes of Operation................................................................................................................... 9 Single Axis Autopilot................................................................................................................ 10 Two Axis Autopilot................................................................................................................... 10 Three axis Autopilot................................................................................................................. 10 Multi axis Autopilot.................................................................................................................. 10 Authority.................................................................................................................................. 10 Capture.................................................................................................................................... 11 Autopilot Principle of Operation.............................................................................................. 11 Selecting a New Heading......................................................................................................... 12 Engagement................................................................................................................................ 15 Autopilot Engagement............................................................................................................. 15 Synchronisation....................................................................................................................... 15 Amplification............................................................................................................................ 16 Turbulence............................................................................................................................... 16 Adaptive Gain Control.............................................................................................................. 16 Semi‐adaptive Gain Control..................................................................................................... 17 Self‐adaptive Gain Control....................................................................................................... 17 Wash Out................................................................................................................................. 17 Artificial Feel............................................................................................................................ 17 Control Surface Actuation........................................................................................................ 18 Controls................................................................................................................................... 18 Servomotors and Servo Actuators........................................................................................... 18 Actuators and Servos............................................................................................................... 19 Electro‐Pneumatic Servo.......................................................................................................... 22 Electro‐Mechanical Servo........................................................................................................ 23 Electrohydraulic Servo‐Actuators Power Assisted....................................................................... 24 Powered Flight Controls........................................................................................................... 24 2020‐04‐14 B2‐13.3.1 Fundamentals Page 1 of 44 Training Material Only  Power Assisted Control............................................................................................................ 24 Power Operated Control.......................................................................................................... 25 Electro‐Hydraulic Actuators..................................................................................................... 26 Manual Operation.................................................................................................................... 27 A Summary of Actuator Operation to this Point...................................................................... 28 ON/OFF Solenoid..................................................................................................................... 30 ON/OFF Solenoid Operation.................................................................................................... 31 Transfer Valve.......................................................................................................................... 32 Transfer Valve Operation......................................................................................................... 33 Autopilot Actuator Operation.................................................................................................. 34 Artificial Feel............................................................................................................................ 35 Electrohydraulic Servomotors.................................................................................................. 36 Duplex Servomotors................................................................................................................ 37 Quadruple Redundant Duplex Hydraulic Actuator................................................................... 38 Quadruple Redundant Duplex Hydraulic Servo Actuator Schematic........................................ 40 Interlocks................................................................................................................................. 43 2020‐04‐14 B2‐13.3.1 Fundamentals Page 2 of 44 Training Material Only List of Figures Figure 1: Autopilot basic principles................................................................................................... 5 Figure 2: Basic autopilot (yaw control).............................................................................................. 6 Figure 3: Basic autopilot (pitch mode).............................................................................................. 6 Figure 4: Inner loop roll control........................................................................................................ 7 Figure 5: Outer loop.......................................................................................................................... 8 Figure 6: Outer loop control.............................................................................................................. 8 Figure 7: Basic 3‐axis autopilot.......................................................................................................... 9 Figure 8: Autopilot capture............................................................................................................. 11 Figure 9: Selecting a new heading................................................................................................... 12 Figure 10: Autopilot coupling.......................................................................................................... 13 Figure 11: Couple related to mode of operation............................................................................. 13 Figure 12: Auto land runway........................................................................................................... 14 Figure 13: Autopilot gain................................................................................................................. 16 Figure 14: Self‐adaptive gain control............................................................................................... 17 Figure 15: Electronic flight control system...................................................................................... 19 Figure 16: Actuators and servos...................................................................................................... 20 Figure 17: Pneumatic and electric servos........................................................................................ 21 Figure 18: Electro‐pneumatic servo................................................................................................ 22 Figure 19: Electro‐mechanical servo............................................................................................... 23 Figure 20: Electrohydraulic servo‐actuators ‐ power assisted......................................................... 24 Figure 21: Electrohydraulic servo‐actuators ‐ power operated....................................................... 25 Figure 22: Electrohydraulic actuator No. 1...................................................................................... 26 Figure 23: Electrohydraulic actuator No. 2...................................................................................... 27 Figure 24: Electrohydraulic actuator No. 3...................................................................................... 28 Figure 25: Electrohydraulic actuator No. 4...................................................................................... 29 Figure 26: Electrohydraulic actuator No. 5...................................................................................... 29 Figure 27: ON/OFF solenoid............................................................................................................ 30 Figure 28: On / off solenoid operation No. 1................................................................................... 31 Figure 29: On / off solenoid operation No. 2................................................................................... 31 Figure 30: Transfer valve................................................................................................................. 32 Figure 31: Transfer valve operation No. 1....................................................................................... 33 Figure 32: Transfer valve operation No. 2....................................................................................... 33 Figure 33: Autopilot actuator operation No. 1................................................................................ 34 2020‐04‐14 B2‐13.3.1 Fundamentals Page 3 of 44 Training Material Only Figure 34: Autopilot actuator operation No. 2................................................................................ 35 Figure 35: Artificial feel unit............................................................................................................ 35 Figure 36: Electrohydraulic servomotor.......................................................................................... 36 Figure 37: Duplex servomotor......................................................................................................... 37 Figure 38: Quadruple redundant duplex hydraulic actuator........................................................... 38 Figure 39: Servomotors duplex with no command input................................................................ 39 Figure 40: Command input and transfer valves respond correctly................................................. 39 Figure 41: Duplex actuator remaining non‐failed half operates normally....................................... 40 Figure 42: Quadruple redundant duplex hydraulic servo actuator................................................. 41 Figure 43: Hydraulic duplex servo actuator with control and monitor circuitry.............................. 42 Figure 44: Hydraulic duplex servo actuator with analogue interface.............................................. 42 Figure 45: Interlocks........................................................................................................................ 43 Figure 46: Troubleshooting schematic No. 1................................................................................... 44 Figure 47: Troubleshooting schematic No. 2................................................................................... 44 2020‐04‐14 B2‐13.3.1 Fundamentals Page 4 of 44 Training Material Only TOPIC 13.3.1: FUNDAMENTALS Auto Pilot Purpose The aircraft autopilot controls the aircraft in vertical speed, attitude and heading to reduce workload and fatigue on the flight crew and to provide improved flight comfort and stability. The basic principles of an autopilot are:  Error sensing  Correction  Follow‐up  Command Figure 1: Autopilot basic principles 2020‐04‐14 B2‐13.3.1 Fundamentals Page 5 of 44 Training Material Only  Figure 2: Basic autopilot (yaw control) Figure 2 shows an aircraft which is on‐course and encounters a crosswind. The gyro senses the heading change and the amplifier provides a signal to the servo to move the control surface. A position sensor on the control surface provides a signal back to the amplifier to either null the system or to give negative feedback to dampen the system. Figure 3: Basic autopilot (pitch mode) 2020‐04‐14 B2‐13.3.1 Fundamentals Page 6 of 44 Training Material Only All autopilots follow the basic principles of error sensing, correction, follow‐up and command, although different types of sensors and servos are used. The auto‐flight system, as with any control system, has an “inner loop” and an “outer loop” The inner loop typically handles internal conditions such as:  Attitude sensing  Attitude changes in terms of error signals  Processing of error signals  Conversion of error signals into movement of flight control surfaces Inner Loop (Roll) Figure 4: Inner loop roll control 2020‐04‐14 B2‐13.3.1 Fundamentals Page 7 of 44 Training Material Only Outer Loop (Altitude or Heading) Raw data inputs such as air speed, altitude, magnetic heading and interception of ground‐based radio aids for instance, can also constitute “outer loop” control. Figure 5: Outer loop Figure 6: Outer loop control 2020‐04‐14 B2‐13.3.1 Fundamentals Page 8 of 44 Training Material Only Modes of Operation Aircraft operating in automatic flight mode are capable of maintaining set operating parameters depending on the stage of the flight. These can include:  attitude hold  heading hold  turbulence  vertical speed hold  airspeed hold  altitude hold  control wheel steering  navigation: o VOR o ILS o Aux or several other modes as selected by the pilots to suit the stage of the flight. A flight director system provides cues for the pilot to navigate and fly the aircraft, but a flight director cannot control the aircraft. Only the pilot or the autopilot system can fly the aircraft. Figure 7: Basic 3‐axis autopilot 2020‐04‐14 B2‐13.3.1 Fundamentals Page 9 of 44 Training Material Only Single Axis Autopilot A Single axis Autopilot is:  Normally only roll axis and is the most basic concept  Is used in small light aircraft for heading hold and basic radio coupling  Originally known as a wing levelling device Two Axis Autopilot A Two axis Autopilot is:  Typically roll and pitch  Allows heading hold and radio coupling  Has altitude and attitude hold capabilities. Three axis Autopilot A Three axis Autopilot provides attitude control about all three axes and is typically a fully Automatic Flight Control (AFCS) System. Multi axis Autopilot Multi axis Autopilot is a system which controls an aircraft about the roll and pitch axis (two axis) or roll, pitch and yaw axis (three axis). Sometimes the rudder has a yaw damper incorporated but this is not considered a three axis system. Authority Limits may be placed on control signals that are demanded to prevent excessive attitude changes or harsh manoeuvring. It is necessary to monitor what is known as the authority of the AFCS. This means that limits must be placed on any control signals that are demanded to prevent excessive changes to the attitude of the aircraft, or to cause any harsh manoeuvring. For example, a typical roll channel incorporates a bank angle limiter and a roll rate limiter. The bank angle limiter limits any input signal to a value which is required by a particular mode of operation. The maximum bank angle limit is 25 degrees to 30 degrees but can be changed automatically depending which mode is selected. For example, 20 degrees for VOR on course mode. The roll rate limiter limits the rate at which the aircraft changes its bank angle by limiting the rate at which the servo motor turns. Typical roll rate limit outputs are 1 1/2 degrees per second to 7 degrees per second. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 10 of 44 Training Material Only Capture Radio deviation signals are sensed by the AFCS but are not used until capture occurs. The term capture means the point at which the radio deviation is used by the AFCS as a reference to fly the aircraft. Figure 8: Autopilot capture In the case of the ILS the capture point is determined by the vertical beam sensor in the pitch channel and the lateral beam sensor in the roll channel. The beam sensors are voltage level sensing circuits that satisfy certain switching functions and apply radio deviation to the signal chain. Autopilot Principle of Operation To control autopilot steering the flight control computer can utilise many references. Heading, attitude and instrument landing system commands. The basis of operation to maintain a selected reference is typically conducted by selecting a reference and then having the flight control system generate corrective attitude changes whenever a deviation from the selected parameter is selected. For example, the pilot flies onto a heading and engages heading hold. The actual heading and desired heading signals are compared in an operational amplifier and any variation from the desired heading will produce a differential between the two signals (phase or amplitude), which will be applied to a servo amplifier to correct for the deviation. Similarly, a parameter can be selected on an autopilot control box, e.g. rate of climb. When auto pilot is selected the difference between the actual rate of climb and the selected rate of climb will produce an error signal (from an Op Amp), which will only be nulled when the aircraft is climbing at the same rate as selected. This principle is the basis of all automated flight management. Aircraft actual parameters are applied to Op Amps (or something similar) and are compared with desired parameters whenever automatic pilot is engaged. Whenever a differential between selected parameter and actual parameter is detected the aircraft attitude will be corrected to re‐align. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 11 of 44 Training Material Only In a fully computerised system heading changes can be programmed in advance. Assume an aircraft is programmed to fly from Dubai to Sydney (Australia). The parameters are typed into a Flight Management Computer (FMC). The aircraft takes off and heads for Singapore. When the Inertial Reference System (IRS) determines the aircraft is over Singapore a signal will trigger the change of heading required and the aircraft flight control system will respond and turn onto the new selected heading for Sydney. The maximum rate of turn permissible is typically programmed into the flight control computer so as not to throw the aircraft into violent manoeuvres. Selecting a New Heading With heading hold engaged, when the pilot decides to select a new heading, he selects a new heading by turning the heading marker on the HSI dial. The aircraft is in position “A” flying on the original heading. The pilot selects a new heading by turning the heading marker on the HSI. The new heading is compared to the aircraft’s actual heading. The AFCS will immediately notice the difference and send an error signal through to the aileron servo actuator to deflect the ailerons and the aircraft rolls and turns onto the new heading. Once the aircraft has reached the new heading, the error signal is nulled, and the aircraft returns to its straight and level attitude. Figure 9: Selecting a new heading 2020‐04‐14 B2‐13.3.1 Fundamentals Page 12 of 44 Training Material Only  Figure 10: Autopilot coupling Couple – Related to the mode of operation – it is the provision of raw data input to the AFCS relevant to a particular flight path. Figure 11: Couple related to mode of operation 2020‐04‐14 B2‐13.3.1 Fundamentals Page 13 of 44 Training Material Only Runway Visual Range (RVR). The range over which the pilot of an aircraft on the line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line. Decision Height (DH). A specified altitude or height in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. Categories of precision approach and landing operations: Category I DH 200 feet and RVR 550 metres Category II DH 100 feet and RVR 300 metres Category IIIa No DH or DH below 100 feet and RVR not less than 175 metres Category IIIb No DH or DH below 50 feet and RVR less than 75 metres Category IIIc No DH and no RVR limitation. NOTE: Special authorization and equipment required for Categories II and III. Figure 12: Auto land runway 2020‐04‐14 B2‐13.3.1 Fundamentals Page 14 of 44 Training Material Only Engagement Autopilot Engagement The basic principle of autopilots is to hold the aircraft in basic heading, pitch and roll channel attitude at the time of engagement. An autopilot system is designed so there will be a gradual transition when it is engaged, therefore if heading hold is engaged when aircraft is 90° from selected heading, the aircraft will not immediately throw itself into a violent bank to capture the commanded heading. The aircraft will be limited in its rate of heading change to perhaps 3° per second, thereby taking 30 seconds or more to align to the commanded heading. Typically, the rate of change of heading can also be selected by the pilot. The same gradual engagement is replicated for any autopilot function. The Autopilot Control Panel provides for engagement for the range of autopilot options, e.g. Heading hold, Roll stabilisation and Vertical speed hold all engaged simultaneously to control a climb to assigned altitude. Often autopilot cannot be engaged until pre‐set conditions are met, e.g. roll stabilisation cannot be engaged until bank angle less than 10°, Autoland can only be engaged if Radar Altimeter system functioning, Radar altitude hold, and barometric altitude hold cannot be engaged simultaneously. The Autopilot is engaged by selecting the appropriate switches and buttons to select the autopilot functions desired. Autopilot will not engage if:  Interlock circuit not complete  Circuit breakers not engaged  Switches in incorrect positions Synchronisation The Synchronisation of signals is normally performed as a pre‐engage function. This is to remove any standing signals that may cause snatching of the controls when initially engaged and allows the autopilot to take control in a smooth manner. The attitude sensing elements, such as the vertical gyro are continuously monitoring the aircraft’s attitude and supplying signals to the servo motors. If the aircraft has been placed, for example, in a nose down attitude prior to engagement, on engaging the autopilot would immediately receive an error signal from the vertical gyro. The computer would then signal the servo motors to move the control surfaces to provide a nose up attitude. It is therefore necessary to oppose the vertical gyro error signal and reduce it to zero before engaging the autopilot. This ensures that the system is synchronised with the attitude of the aircraft. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 15 of 44 Training Material Only Amplification Any error signal derived from a detection device or command signal must be amplified, as they do not have the magnitude to operate a servo actuator. The type of aircraft and its handling characteristics will have a great bearing on the way it responds to control surface movement. For example, an empty aircraft will respond quite differently from the same one that has a full load of fuel and passengers on board. It is therefore necessary for the AFCS to have a system which will adjust the ratio of the output signal to the input signal to achieve the desired rate of response. The response of an aircraft to a command or detection signal will therefore be determined by the gain of the amplifier circuits in the flight control computer. The gain or transfer ratio of an amplifier is the ratio of the output to the input. To put it another way, the gain is equal to the transfer ratio, which is equal to output divided by the input. Figure 13: Autopilot gain Turbulence An aircraft experiencing turbulent air conditions in flight will have varying degrees of load applied to its structure. In these circumstances, the pilot will adjust speed, power and use of controls to suit the prevailing conditions. If the aircraft encounters turbulent conditions while the AFCS is engaged, the AFCS will correct for any movement that takes place. However, this correction, in these conditions, could possibly lead to additional loads being applied to the structure due to the rate at which the AFCS responds. This can cause the AFCS response rate to get out of phase with the disturbance rate. The pilot would normally disengage the AFCS in these conditions, but in some AFCS there is a mode selection which reduces the gain of the pitch and roll channels thereby softening the response of the AFCS to the disturbance (Turbulence Penetration). Adaptive Gain Control This term refers to the way in which the sensing of the handling characteristics in flight is carried out by a system which is classified as either:  Semi‐adaptive gain control  Self‐adaptive gain control 2020‐04‐14 B2‐13.3.1 Fundamentals Page 16 of 44 Training Material Only Semi‐adaptive Gain Control This is a system of adjusting the gain of the amplifier using external signals. It is called gain programming or gain scheduling. As an example, a radio deviation signal could be used to control the gain within the computer. Self‐adaptive Gain Control This is a system whereby it monitors its own operation and automatically adjusts its own gain for the best aircraft performance. The system is controlled by a model reference which will adjust the system to give the best response regardless of the flight conditions. Command signals are fed through the model to the system, and as the gain is increased, the error signal is amplified and fed to the control servo. The overall system is known as high loop gain because maximum gain is needed to get the best response. Figure 14: Self‐adaptive gain control Wash Out Wash out is the automatic sensing and opposition of any standing signals existing from attitude signal transmitters. This enables engagement of the Autopilot system without any sudden control surface position change or ‘snatching’ of control surfaces. Artificial Feel Artificial feel is used to provide an artificial mechanical feedback between the control surfaces and the pilot. This feel is provided by a variety of methods depending on aircraft complexity and sophistication. The feedback is provided so the pilot can determine what aerodynamic loads are on the control surfaces. This is necessary in some situations, as without this feedback, the pilot could demand a control surface movement that exceeds the allowable aerodynamic loading. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 17 of 44 Training Material Only Control Surface Actuation On small light aircraft the power to move the control surfaces is provided by the pilot. The control column is physically connected to the control surfaces by cables and the pilot moves the control surfaces by repositioning the control column or rudder pedals. As the control column or rudder pedals are displaced, the movement is mechanically transferred to the control surface, the aircraft attitude changes and the pilot re‐centres the control column or rudder pedals when the desired attitude or heading is achieved. Any autopilot function in this simple system is nothing more than the use of trim tabs to trim the aircraft to eliminate excessive drift and to relieve the pilot of the necessity to continually maintain a force on the control column or rudder pedals in order to maintain straight and level flight. Controls When the control column is moved backwards, the elevators are raised thereby decreasing the lift of the horizontal stab so that the aircraft is displaced by a pitching moment about the lateral axis into a nose up or climbing attitude. Forward movement of the control column lowers the elevators to increase the lift of the horizontal stabiliser and so the pitching moment causes the aircraft to assume a nose‐down or descending attitude. Pitch displacements are opposed by aerodynamic damping in pitch and by the longitudinal stability and as the response to elevator deflections is a steady change of attitude. Elevators are essentially displacement control devices. Control column and control wheel movements are independent of each other so that lateral and longitudinal displacements can be obtained separately or in combination. This concept of having separate control movements for each axis goes right through to the most complex automatic flight control systems with multi axis control. Servomotors and Servo Actuators On larger aircraft it is physically impossible to move the control surfaces by muscles alone. All aircraft, with the exception of light aircraft or very old aircraft, will incorporate some form of power assistance (like power steering in a car) to move control surfaces. The power assistance is provided by actuators or servos and these devices can operate from either mechanical input (like your cars power steering) or electrical input. Once a flight control system is capable of repositioning control surfaces by use of electrical signals, these signals can be provided by a number of sensors to control the aircraft’s flight path. Instead of the pilot detecting an uncommanded roll or heading change and moving the stick or pedals to counteract it, gyros, accelerometers and other sensor equipment can detect the uncommanded attitude changes far more accurately and then provide an electrical output to an actuator or servo. This is the basis of a fly‐by‐wire or automatic flight control system. The sensors detect uncommanded attitude changes and counter them. When the pilot moves the control column or rudder pedals (a commanded attitude change) an electrical signal from the stick or pedal transducers is transmitted to the electrically operated actuator and the control surface is deflected by pilot input to achieve an attitude change. In this electrically operated system, it is electrical signals not mechanical inputs which control the actuator or servo operation. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 18 of 44 Training Material Only Going one step further, the electrically operated flight control system can be programmed to fly a specific route, at a specific altitude and then the pilot is simply along for the ride. The avionic systems of the aircraft provides the flight control computer with inputs of heading, altitude, waypoints, etc. and the flight control computer repositions the actuators with electrical signals to maintain the aircraft on the programmed flight path. This attribute in an automatic flight control system is called an autopilot. Figure 15: Electronic flight control system Actuators and Servos The components used in an automatic flight control system (AFCS) to move the aircraft’s control surfaces are called servomotors, servo actuators, or by the name of the control surface or channel that it controls, for example rudder servo or pitch actuator. The signals received from the AFCS computer are electrical. Therefore, the control of the actuators is electrical. The servo actuators convert these electrical signals into control surface movement by converting the electrical signal into mechanical motion which is usually done by torque motors or solenoid controlled valves (electro‐hydraulic valves). The three main types of servomotors are:  electromechanical  electro‐pneumatic  electro‐hydraulic Electromechanical and electro‐pneumatic actuators or servos are more suited to smaller aircraft, and the typical installation in a modern commercial aircraft is an electro‐hydraulic system. Even though some of the surface actuators may be electro‐hydraulic, it is not uncommon for others to be electromechanical or electro‐pneumatic, e.g. flap motors may be electrically driven and throttle boost actuators may be pneumatically driven when the remaining actuators may all be hydraulically driven (aileron, rudder, elevators). 2020‐04‐14 B2‐13.3.1 Fundamentals Page 19 of 44 Training Material Only Servomotors may be connected in series or parallel with the AFCS. A series servomotor is one that moves the control surfaces without moving the pilot’s controls, whilst a parallel servomotor moves the control surfaces and the pilot’s controls. The most common actuator used on commercial aircraft flight control systems is the electro‐ hydraulic actuator. Figure 16: Actuators and servos Pneumatic and Electric Servos1 Simple servos found on light aircraft use vacuum sources like those which operate the gyroscopic instruments. Pneumatic pressure is obtained from either an engine driven pump or from a tapping at one of the engine compressor stages. The vacuum is directed to pneumatic servos that are mechanically connected to the normal mechanical flight control linkages. The pneumatic servo is an airtight housing which contains a moveable diaphragm. When vacuum is applied to the servo the diaphragm is displaced pulling on the cable to reposition the flight control surface. Two of these servos would be needed for each control surface, a “pull” and a “push” actuator. Another method of driving the control surface of light aircraft is by use of an electric motor. These servomotors may be powered by either AC or DC depending on the type of automatic flight control system used. An electric motor servo can use a reversible DC motor and reduction gearing to supply the force to move the flight control surface in both directions. Alternatively, a constant direction motor can be used with magnetically switched clutches to engage a mechanism to apply force to a control cable. The servomotor consists of an electromagnetic clutch, gearbox and drive mechanism. It may also include an amplifier to amplify the command signal and a feedback system such as a potentiometer or tachogenerator. The constant drive type has the advantage that the inertia forces in starting and stopping the motor are eliminated so it can be engaged and disengaged more rapidly and precisely. When the AFCS computer sends a signal to the motor of the electric servomotor, it will drive the gear train and subsequently the control surface in the desired direction. At the same time, it drives a tacho generator to provide feedback to the computer for speed limiting and smoothing. A follow‐up synchro is also driven by the motor which will send a signal back to the computer indicating the actual position of the control surface. The synchro signal is of a phase opposite, but in proportion to the control surface displacement and will null the output signal of the computer when both signals are equal. Thus, control surface movement will cease. 1 (Pallett Automatic Flight Control pg. 200 and 201) 2020‐04‐14 B2‐13.3.1 Fundamentals Page 20 of 44 Training Material Only Both the pneumatic and electric servos are only power assisting servos, with the flight control system still fundamentally powered by the pilot’s muscles. The pneumatic and electric flight control servos are limited to use in only light and simple aircraft. Neither of these examples are a fly‐by‐wire system. The actuators used in larger and more modern aircraft are typically electro‐hydraulic. Figure 17: Pneumatic and electric servos Pneumatic Servo – when vacuum is applied the diaphragm pulls the cable repositioning the flight control surface. Electric Motor Servo – can use a reversible DC motor or a constant direction motor with magnetically switched clutches. Electric Motor and Pneumatic (vacuum) powered servos are normally only incorporated in light aircraft. Neither of these are fly‐by‐wire systems – large modern aircraft typically use electro‐hydraulic actuators which are entirely electrically controlled 2020‐04‐14 B2‐13.3.1 Fundamentals Page 21 of 44 Training Material Only Electro‐Pneumatic Servo An electro‐pneumatic servo consists of an electro‐magnetic valve with dual poppet ports connected via pressure ports and orifices to two cylinders containing pistons sealed against pressure loss. The valves are controlled by electrical command signals from the auto pilot and with no signal present both valves are open. Figure 18: Electro‐pneumatic servo 2020‐04‐14 B2‐13.3.1 Fundamentals Page 22 of 44 Training Material Only Electro‐Mechanical Servo This mechanism consists of an AC or DC motor and gear train, coupled to the flight control system via an electro‐magnetic clutch, gear train and a sprocket and chain drive. Feedback is provided by a potentiometer in a DC motor, and a CX synchro and a tachogenerator to provide position and rate feedback signals for AC motors. Figure 19: Electro‐mechanical servo 2020‐04‐14 B2‐13.3.1 Fundamentals Page 23 of 44 Training Material Only Electrohydraulic Servo‐Actuators Power Assisted Powered Flight Controls These are used in high performance aircraft and consist of two main types:  Power assisted  Power operated The main difference in the two systems is the way in which the actuators are connected to the control surfaces. Power Assisted Control In the power assisted system, the pilot’s control stick is connected to the control surface via a control lever. When the pilot pulls back on the stick to begin a climb, the control lever pivots about point X and commences moving the control surface up. At the same time, the control valve pistons are displaced allowing hydraulic fluid to flow to the left hand side of the actuating jack, which is secured to the structure of the aircraft. The pressure exerted on the piston causes the whole servo unit and control lever to move to the left and, because of the greater control effort produced, the pilot is assisted in moving the control surface further. Figure 20: Electrohydraulic servo‐actuators ‐ power assisted 2020‐04‐14 B2‐13.3.1 Fundamentals Page 24 of 44 Training Material Only Power Operated Control In this system, the control column is connected to the control lever only, whilst the servo unit is directly connected to the control surface. The effort required by the pilot to move the control column is that needed to move the control lever and control valve piston. The power required to move the control surface is supplied solely by the servo unit’s hydraulic power. As there are no forces transmitted back to the control column, the pilot has no feel of the loads acting on the control surfaces and a means of artificial feel must be introduced at a point between the control column and the connection to the servo unit control lever. Figure 21: Electrohydraulic servo‐actuators ‐ power operated 2020‐04‐14 B2‐13.3.1 Fundamentals Page 25 of 44 Training Material Only Electro‐Hydraulic Actuators The transfer valve is an electrically controlled hydraulic valve which operates a piston assembly called the autopilot actuator, which in turn operates the main control valve for the actuating cylinder. The movement of the actuator is monitored by the output of a Linear Variable Differential Transducer (LVDT). This will provide the follow up signal back to the computer. Direct operation of the hydraulic power unit has two main advantages; one is the very low computer power output required and the other is that it is more sensitive and accurate, due to the absence of cable slack, stretch and drag. Figure 22: Electrohydraulic actuator No. 1 2020‐04‐14 B2‐13.3.1 Fundamentals Page 26 of 44 Training Material Only Manual Operation In manual operation, the control column moves the control quadrant and we will assume this relates to a back stick input to move the elevators. The pilot pulls the stick backwards to start a climb (moves the cockpit control to the left on the slide), the control cables will turn the control input quadrant which will move the upper end of the control valve actuator (long green arm) to the left. At this point before the surface begins to move the control surface actuator is hydraulically locked in position (shown still centred on the diagram, because this is before it begins to move) so the control valve actuator (long green arm) is anchored at the bottom. The end result of the top of the arm moving left is that the control valve will be displaced left. When the control valve moves left hydraulic supply pressure is ported to the left hand side of the control surface actuator, which will force the piston to the right. Figure 23: Electrohydraulic actuator No. 2 The pressure applied to the left of the control surface actuator will force the piston to the right moving the control surface. This will move the bottom anchor point of the control valve actuator (long green arm) to the right and this time the top of the arm is held stationary (pilot still has control column pulled back). The control valve spool will be moved to the right, thus synchronising again and causing a hydraulic lock on either side of the control surface actuator piston, locking the control surface in the commanded position (whilst ever the pilot maintains the back stick input). 2020‐04‐14 B2‐13.3.1 Fundamentals Page 27 of 44 Training Material Only A Summary of Actuator Operation to this Point When the pilot applies back stick, the control valve ported hydraulic pressure through to the control surface actuator, thus hydraulically driving the control surface to the commanded position. The pilot input is only moving the control valve; it is the hydraulic pressure which drives the control surface actuator. While the pilot retains the stick input the control surface will remain in the position it has been driven to (just like a pilot holding back stick on a cable system – the control surface will remain in the back stick position until the stick is re‐centred). If no hydraulic pressure were applied, you can move the stick all you want, but the only effect it has is to move the control valve. The control surface will not move without hydraulic pressure applied. Figure 24: Electrohydraulic actuator No. 3 The control surface will remain in the back stick position until the stick is re‐centred. When the back stick is released, the same process as previously described occurs again. The bottom of the control valve actuator (long green arm) is locked in place because the control surface is initially still hydraulically locked in the extended position. With the bottom of the control valve actuator (long green arm) locked, the control valve will be displaced to the right, porting hydraulic pressure to the right side of the control surface actuator. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 28 of 44 Training Material Only  Figure 25: Electrohydraulic actuator No. 4 With the bottom of the control valve actuator (long green arm) locked, the control valve will be displaced to the right, porting hydraulic pressure to the right side of the control surface spool piston. The hydraulic pressure applied to the control surface spool piston will force the piston to the left and retract the control surface. This will also reposition the control valve to the left re‐ centring it and again hydraulically locking the control surface in the central position until the control column is again displaced. This description of the flight control system operation still refers to a manually operated system. No electrical inputs have been described yet. As you can see, actuator operation is dependent upon control valve position. If we can electrically drive the control valve, we can control the actuator with electrical signals alone. Figure 26: Electrohydraulic actuator No. 5 2020‐04‐14 B2‐13.3.1 Fundamentals Page 29 of 44 Training Material Only ON/OFF Solenoid An ON/OFF solenoid is simply a hydraulic relay. With no electrical power applied, hydraulic pressure is shut‐off because the solenoid spring holds the seat against a seal, preventing pressure from being felt downstream. When power is applied, the solenoid coil magnetises and unseats the valve (overpowers spring pressure) and permits hydraulic oil to flow. In the hydraulic actuator, the ON/OFF solenoid provides pressure to the transfer valve when autopilot is activated. Power to the ON/OFF solenoid is typically controlled through a series of monitors which detect any failures in the autopilot system. In the event that an autopilot failure is detected, the ON/OFF solenoid is de‐energised, isolating autopilot inputs from the actuator. Figure 27: ON/OFF solenoid 2020‐04‐14 B2‐13.3.1 Fundamentals Page 30 of 44 Training Material Only ON/OFF Solenoid Operation When the ON/OFF solenoid is energised, the transfer valve is provided with hydraulic pressure. Typically, the pressure provided to the transfer valve nozzle will also be provided through the ON/OFF solenoid, but that is not shown here to reduce the complexity of the diagram. When the transfer valve is provided with hydraulic pressure, it is primed to convert the electrical inputs into hydraulic outputs which will drive the autopilot actuator, controlling the control valve and control surface actuator electrically instead of mechanically as previously described. Figure 28: On / off solenoid operation No. 1 Figure 29: On / off solenoid operation No. 2 2020‐04‐14 B2‐13.3.1 Fundamentals Page 31 of 44 Training Material Only Transfer Valve Before looking at the operation of the actuator, we must understand the workings of the transfer valve, which transforms electrical signals from the computer into hydraulic pressure. A transfer valve is also often called an Electro‐Hydraulic Valve (EHV), or a hydraulic servo which can be driven by torque motors directly connected to the Autopilot Actuator spool. As the torque motor style is a high current application though, so the transfer valve and EHV style of electrical interface to the hydraulic actuator are more common in modern flight control installations. In Figure 28, on the right‐hand side, there is a coil of windings around a C shaped core. If a signal is presented to this coil, it will move the permanent magnet armature up or down about its pivot. The computer outputs a DC signal and the polarity of the signal determines the direction of movement. Hydraulic fluid is fed into the unit through the feed pipe, passing through a flexible tube which then divides across the pointed divider, just under the flexible tube. The feed pipe provides full hydraulic system pressure to the transfer valve nozzle, but it is supplied through only very narrow gauge plumbing because the work it has to perform to manipulate the spool valve is minimal, so a high rate of flow of pressurised hydraulic fluid is unnecessary. If there is no electrical signal to the coil, the flexible tube remains in the neutral position, due to spring loading (represented by the two black lines connecting the nozzle point and the spool valve piston assembly). In this position, the spool valve and feedback springs sense equal hydraulic pressure at both ends and take up the neutral position, closing off both control ports. Figure 30: Transfer valve 2020‐04‐14 B2‐13.3.1 Fundamentals Page 32 of 44 Training Material Only Transfer Valve Operation When the flight control computer sends a signal to the coil windings (either drawing the permanent magnet attached to the nozzle up or down) the permanent magnet will rotate, moving the nozzle in a direction dependent upon polarity of the input signal from the computer. In the first illustration, this will cause a greater pressure to be directed to the top of the spool valve than at the bottom. This moves the spool valve down. The spool valve will continue to move down until the force of the feedback springs is sufficient to bring the flexible tube back almost to the neutral position. With the spool valve moved down, hydraulic supply pressure is ported out through the upper control port. This pressure is used to control the autopilot actuator spool valve, which will be explained next slide. With the spool valve down, hydraulic pressure is ported out the top control port and the bottom control port is opened to the hydraulic return line. If the electrical signal is of reversed polarity, the spool valve will move up instead of down, porting hydraulic pressure through the bottom control port and opening the top control port to the hydraulic return line. Figure 31: Transfer valve operation No. 1 Figure 32: Transfer valve operation No. 2 2020‐04‐14 B2‐13.3.1 Fundamentals Page 33 of 44 Training Material Only Autopilot Actuator Operation On engaging the Automatic Flight Control System (AFCS), the ON/OFF solenoid opens and supplies hydraulic pressure to the transfer valve. When an AFCS command signal is supplied to the coil windings in the transfer valve, the spool valve nozzle is displaced. The hydraulic pressure is applied to the right hand side of the autopilot actuator, causing it to move to the left. The control valve actuator (long green arm) pivots on the control surface actuator and moves the control valve to the left. This is the same movement as shown in the manual operation, except that the input to the control valve actuator is provided by the autopilot actuator, not the cockpit control. When the command signal is of the opposite polarity, the transfer valve nozzle moves down forcing the spool valve up, which ports hydraulic pressure to the left side of the autopilot actuator. When the autopilot actuator moves right, the control valve is forced to the right providing pressure to the control surface actuator to drive the control surface. On the actuator illustrated below, the electrical inputs driving the transfer valve and the autopilot actuator reposition the control surface and also reposition the control column in the cockpit. Any corrections made by the flight control computer will be felt at the control column. Other actuators are designed so that electrical inputs only move the control surface and have no effect on the cockpit controls. Damper signals are typical of this method of operation. When aircraft oscillations are detected by a gyro, it outputs a signal through the flight control computer to the actuator to counter the oscillations, but the rudder pedals or control column will not be moved. This design of the actuator is not that different from the type described here, but the differences will not be covered in this lesson. As the autopilot actuator moves to the left, the autopilot LVDT produces an electrical output which is sent back to the computer to null the command signal input. Figure 33: Autopilot actuator operation No. 1 2020‐04‐14 B2‐13.3.1 Fundamentals Page 34 of 44 Training Material Only  Figure 34: Autopilot actuator operation No. 2 Artificial Feel Because the pilot has no feel of the aerodynamic loads acting on the control surfaces, it is necessary to incorporate an “artificial feel” device at a point between the pilot’s controls and their connection to the servo‐unit control lever. Figure 35: Artificial feel unit 2020‐04‐14 B2‐13.3.1 Fundamentals Page 35 of 44 Training Material Only This is commonly known as a “q” feel unit in which the feel force varies with the dynamic pressure of the air sensed by the Pitot‐static system. Q=1/2ΡV2 A “q” feel unit monitors hydraulic pressure and produces control forces dependent on the amount of control movement and forward speed of the aircraft. An artificial feel unit is connected at a point between the pilot’s controls and their connection to the servo‐unit control lever – it restricts movement of the control column. Electrohydraulic Servomotors Figure 36: Electrohydraulic servomotor  When the autopilot is engaged the A/P SOLENOID ACTUATOR energises and allows fluid through to the A/P ENGAGE MECHANISM and up to the A/P SELECT VALVE.  The SELECT VALVE compresses under fluid pressure and allows fluid to flow to the SPOOL and TRANSFER VALVES  The PCU receives signals from the A/P COMPUTER moving the TRANSFER VALVE thus sending fluid to the MAIN ACTUATOR to move the AILERONS. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 36 of 44 Training Material Only Duplex Servomotors In some aircraft the possibility of a hard over or runaway condition resulting from automatic flight control malfunctions is prevented by utilising two independent control systems which displace control surfaces via duplex servomotors and differential gearing. The pitch and roll servo motors are of equal authority and torque and their outputs are summed by their respective differential gearing. The yaw servomotor is of a single type with torque limiting. When the commands from AFCS computer 1 and 2 to a servomotor are identical (normal operation), the motions of both motors within the servo are identical, so providing doubled authority to operate the appropriate control surface. If, however, a malfunction in one system occurs such that a hard over roll is commanded by that system, then it will turn the differential gear in the direction commanded. The other system however, will, at the outset, detect the undesired attitude change and will command its motor to rotate the differential in the opposite direction with the net result that the deflection is prevented. Each motor is coupled electrically to a sensor known as a speed monitor, which in turn is connected to braking units. The purpose of the monitor is to identify a runaway motor, which it does within about 2 milliseconds, and then to apply a signal to the respective brake thus locking out half of the differential gear and enabling the good motor to drive the control surface through its half of the gearing. Since the servo power is halved, then any hard over risk in the remaining control system is reduced. Additional redundancy typically incorporated is for 3 or 4 computer channels. Figure 37: Duplex servomotor 2020‐04‐14 B2‐13.3.1 Fundamentals Page 37 of 44 Training Material Only Quadruple Redundant Duplex Hydraulic Actuator Figure 38: Quadruple redundant duplex hydraulic actuator Common command is applied to all EHVs from all 4 channels and all signals are identical and complimentary. The DP (Differential Pressure) Sensors monitor the EHV outputs and when all pressures are balanced (normal operation), no failure signal is generated. If any detected computer fails (in a single channel), it turns off the command signal and the remaining signals still command the actuator with no loss of efficiency: e.g. LVDT fail, cross channel mismatch, wiring short or open circuit. If a DP sensor detects EHV failure, the SOV controlling the EHV is turned off removing hydraulic pressure from EHV. Pressure is still provided to the MCV and the main ram actuator continues to function on the remaining good EHV with no efficiency loss. The bypass damper ports return pressure to MCV to prevent hydraulic lock with the EHV de‐energised. If the second EHV fails, the second SOV turns off and the actuator reverts to mechanical operation for input directly to MCV. 2020‐04‐14 B2‐13.3.1 Fundamentals Page 38 of 44 Training Material Only If there is no command input, the transfer valves are in the neutral position and the pressures/flow rates P1 + P4 = P2 + P3. The Failure Sensor is centred and LVDT O/P (output) is normal Figure 39: Servomotors duplex with no command input With the command inputs equal and the transfer valves response equal, P1 + P4 still equals P2 + P3, the Failure Sensor is centred and the LVDT O/P is normal. Figure 40: Command input and transfer valves respond correctly 2020‐04‐14 B2‐13.3.1 Fundamentals Page 39 of 44 Training Material Only If a transfer valve is blocked or misaligned, P1 + P4 > P2 + P3 for example, the Failure Sensor piston is driven from neutral – LVDT O/P to FCC indicates failure and the SOV is de‐energised. The primary function of the failure sensor is to detect faults and disengage the EHV before any uncommanded flight control input is made. In a duplex actuator, the remaining non‐failed half of actuator continues to function normally driving the control surface. Figure 41: Duplex actuator remaining non‐failed half operates normally Quadruple Redundant Duplex Hydraulic Servo Actuator Schematic The Schematic diagram Figure 42 is a hydraulic duplex servo‐actuator. Its components are:  SOVs  EHVs (Transfer Valves)  DPs (Failure Sensors)  System Selector valve  Mechanical input – mode selector valve  MCV  Bypass Damper  CAS LVDT – Rate feedback  RAM LVDT – position feedback 2020‐04‐14 B2‐13.3.1 Fundamentals Page 40 of 44 Training Material Only  Figure 42: Quadruple redundant duplex hydraulic servo actuator 2020‐04‐14 B2‐13.3.1 Fundamentals Page 41 of 44 Training Material Only  Figure 43: Hydraulic duplex servo actuator with control and monitor circuitry Figure 44: Hydraulic duplex servo actuator with analogue interface Gain scheduling, interface with sensors and input devices is performed by the FCC processor. The circuitry illustrated represents analogue interface with the actuator (just 1 channel is shown). 2020‐04‐14 B2‐13.3.1 Fundamentals Page 42 of 44 Training Material Only Interlocks Before an automatic control system can be engaged with an aircraft’s flight controls, certain preliminary operating requirements must be fulfilled to ensure that the system is in a condition whereby it may safely take control of the aircraft. The principal requirements are that the connections between system power supplies, the elements comprising the system, and the appropriate signal and engage circuits are electrically complete. It is the practice, therefore, to incorporate within any automatic control system a series of switches and/or relays, known as interlocks, which operate in a specific sequence to ensure satisfactory engagement and the coupling of input signals from outer loop control elements. Figure 45: Interlocks 2020‐04‐14 B2‐13.3.1 Fundamentals Page 43 of 44 Training Material Only  Figure 46: Troubleshooting schematic No. 1 Figure 47: Troubleshooting schematic No. 2 2020‐04‐14 B2‐13.3.1 Fundamentals Page 44 of 44 Training Material Only TOPIC 13.3.2: COMMAND SIGNAL PROCESSING Table of Contents List of Figures................................................................................................................................ 4 TOPIC 13.3.2: COMMAND SIGNAL PROCESSING........................................................................... 7 Introduction.................................................................................................................................. 7 Autopilot Control Panels............................................................................................................... 7 Command Signal Processing.......................................................................................................... 8 Autopilot Sensors.......................................................................................................................... 9 E and I Bar Sensors..................................................................................................................... 9 Moving Vane Sensors............................................................................................................... 10 Dynamic Vertical Sensor.......................................................................................................... 11 Displacement Gyroscopes........................................................................................................ 12 Rate Gyro................................................................................................................................. 12 Accelerometers........................................................................................................................ 14 Torque Balanced Accelerometer.............................................................................................. 15 Modern Accelerometers.......................................................................................................... 16 Air Data Sensor........................................................................................................................ 17 Force Transducers....................................................................................................................... 18 Control Wheel Steering............................................................................................................ 18 CWS Transducer....................................................................................................................... 19 Linear Variable Differential Transformer (LVDT)......................................................................... 23 LVDT Operation........................................................................................................................ 23 Trim Tabs..................................................................................................................................... 25 Operation of Trim Tabs............................................................................................................ 25 Fixed Trim Tabs........................................................................................................................ 25 Controllable Trim Tabs............................................................................................................. 26 Trim Indicators............................................................................................................................ 27 Automatic Trim Control Devices.................................................................................................. 28 Cable Tension Trim Sensor....................................................................................................... 28 Servomotor Sensor.................................................................................................................. 29 Electro‐Hydraulic Servo Actuator................................................................................................ 30 Transfer Valve.......................................................................................................................... 30 AFCS Principles............................................................................................................................ 32 Fly–By–Wire Control System....................................................................................................... 33 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 1 of 68 Training Material Only  Fly–By–Wire Benefits............................................................................................................... 33 Sidesticks................................................................................................................................. 33 Servomotors and Servo Actuators........................................................................................... 33 Closed Loop Feedback............................................................................................................. 35 Position Feedback.................................................................................................................... 36 Rate Feedback......................................................................................................................... 36 Feedback and Damping............................................................................................................ 37 Automatic Flight Control System (AFCS) Elements.................................................................. 38 Inner Loop................................................................................................................................ 38 Synchronising........................................................................................................................... 39 Gain and Adaptive Control....................................................................................................... 41 Limiting.................................................................................................................................... 44 Damping Systems..................................................................................................................... 45 Turbulence Penetration............................................................................................................... 46 Flight Control Surfaces............................................................................................................. 46 AFCS Operation........................................................................................................................... 48 Autopilot Principle of Operation.............................................................................................. 48 Autopilot Engagement............................................................................................................. 48 Autopilot Override................................................................................................................... 49 AFCS Failure Monitoring.......................................................................................................... 50 AFCS Failure Monitoring Circuits and Servo Amplifier............................................................. 51 Single Axis Auto Pilot............................................................................................................... 52 Single Axis Autopilot Components........................................................................................... 52 Three Axis Autopilot................................................................................................................ 54 Basic Autopilot System................................................................................................................ 56 Typical Autoflight Components................................................................................................ 56 Roll Channel............................................................................................................................. 58 Pitch Channel........................................................................................................................... 59 Turn Coordination....................................................................................................................... 60 Versine..................................................................................................................................... 61 Pitch Computer Versine Processing............................................................................................. 62 Roll Computer in Attitude Hold Mode..................................................................................... 63 Selecting a New Heading......................................................................................................... 63 Control Wheel Steering (CWS) Mode....................................................................................... 64 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 2 of 68 Training Material Only  Autopilot Coupling....................................................................................................................... 65 ILS Localiser Element............................................................................................................... 65 Glide Path Element.................................................................................................................. 66 Autoland.................................................................................................................................. 67 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 3 of 68 Training Material Only List of Figures Figure 1: Autopilot controller – Beech King Air................................................................................. 7 Figure 2: Autopilot control panel – Boeing 777................................................................................. 7 Figure 3: Autopilot control panel – Airbus A330............................................................................... 8 Figure 4: E and I bar sensor operation.............................................................................................. 9 Figure 5: Moving vane transducer with two square wave inputs.................................................... 10 Figure 6: Moving vane transducer with offset vane........................................................................ 10 Figure 7: Dynamic vertical sensor................................................................................................... 11 Figure 8: A laser ring gyro schematic diagram................................................................................. 12 Figure 9: Rate gyro.......................................................................................................................... 13 Figure 10: Rate gyro response......................................................................................................... 13 Figure 11: Different types of accelerometers.................................................................................. 14 Figure 12: Accelerometer operation............................................................................................... 15 Figure 13: Torque balanced accelerometer.................................................................................... 16 Figure 14: Modern day accelerometers.......................................................................................... 16 Figure 15: Air Data Computer (ADC)............................................................................................... 17 Figure 16: Practical use of a force transducer................................................................................. 18 Figure 17: CWS selector switches.................................................................................................... 18 Figure 18: Mode select panel with CWS option.............................................................................. 19 Figure 19: Roll CWS location........................................................................................................... 19 Figure 20: CWS installation............................................................................................................. 20 Figure 21: CWS transducer.............................................................................................................. 20 Figure 22: Pitch CWS operation...................................................................................................... 21 Figure 23: Pitch CWS circuit operation............................................................................................ 22 Figure 24: LVDT principles of operation.......................................................................................... 23 Figure 25: LVDT signal generation................................................................................................... 24 Figure 26: Fixed trim tab................................................................................................................. 25 Figure 27: Controllable trim tabs.................................................................................................... 26 Figure 28: Trim indicator................................................................................................................. 27 Figure 29: ECAM trim indicator....................................................................................................... 27 Figure 30: Pitch trim control schematic.......................................................................................... 28 Figure 31: Electrically sensed pitch trim control circuit................................................................... 29 Figure 32: Electro‐hydraulic servo actuator.................................................................................... 30 Figure 33: Transfer valve................................................................................................................. 31 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 4 of 68 Training Material Only Figure 34: Simplified AFCS principle of operation........................................................................... 32 Figure 35: Powered flight control.................................................................................................... 33 Figure 36: Using electrical and hydro‐mechanical power for flight controls................................... 34 Figure 37: Input applied to closed loop system............................................................................... 35 Figure 38: Response to closed loop input....................................................................................... 35 Figure 39: Simplified position feedback schematic......................................................................... 36 Figure 40: Simplified rate feedback schematic................................................................................ 37 Figure 41: Position and rate feedback comparison......................................................................... 37 Figure 42: AFCS elements................................................................................................................ 38 Figure 43: AFCS inner loop.............................................................................................................. 39 Figure 44: AFCS synchronising prior to engagement....................................................................... 39 Figure 45: Gain control.................................................................................................................... 41 Figure 46: Model referencing gain control...................................................................................... 42 Figure 47: Radio altitude control of GS and LOC signals.................................................................. 43 Figure 48: Gain programming with invalid radio altimeter signal................................................... 43 Figure 49: Bank angle and bank rate limiting.................................................................................. 44 Figure 50: Damping systems........................................................................................................... 45 Figure 51: Primary control surface operation................................................................................. 47 Figure 52: Autopilot mode control panel........................................................................................ 49 Figure 53: Shut Off Valve (SOV) interlock circuit............................................................................. 50 Figure 54: Servo amplifier failure monitoring circuits..................................................................... 51 Figure 55: Single axis autopilot....................................................................................................... 52 Figure 56: Autopilot components.................................................................................................... 52 Figure 57: Basic single axis autopilot schematic.............................................................................. 53 Figure 58: ’Mechanical Mike’ (the evolution of the modern airplane autopilot)............................ 54 Figure 59: Sperry automatic pilot diagram...................................................................................... 54 Figure 60: 3‐axis autopilot components.......................................................................................... 57 Figure 61: Roll channel circuit......................................................................................................... 58 Figure 62: Pitch channel circuit....................................................................................................... 59 Figure 63: Yaw channel sensor inputs and processing required for turn coordination................... 60 Figure 64: Rudder displacement command processing................................................................... 60 Figure 65: Versine command signal processing – level flight.......................................................... 61 Figure 66: Versine command signal processing – bank angle of 30 degrees................................... 61 Figure 67: Pitch computer versine processing................................................................................. 62 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 5 of 68 Training Material Only Figure 68: Roll computer attitude hold mode................................................................................. 62 Figure 69: Heading hold function.................................................................................................... 63 Figure 70: LOC signal schematic...................................................................................................... 65 Figure 71: G/S signal schematic...................................................................................................... 66 Figure 72: Autoland configuration.................................................................................................. 67 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 6 of 68 Training Material Only TOPIC 13.3.2: COMMAND SIGNAL PROCESSING Introduction Autopilot systems have various modes of operation that can be selected depending on the function (e.g. maintain altitude, follow ILS glideslope, climb, descend etc.) that the pilot requires the system to perform. Each different function may require input signals from different sensors. The selected mode also determines how these outputs will be processed to produce the command signals that will perform the selected autopilot function. Autopilot Control Panels Most autopilots have a mode control panel where the pilots can select the various functions available to operate the aircraft. Terminology varies as does the number of functions,both of which are dependent on the type of autopilot fitted and the complexity and requirements of the aircaft in which it is installed. Figure 1: Autopilot controller – Beech King Air Figure 2: Autopilot control panel – Boeing 777 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 7 of 68 Training Material Only  Figure 3: Autopilot control panel – Airbus A330 Command Signal Processing The signals produced by error signal sensors, in whatever form the sensors may take, cannot be applied directly to their associated servo‐actuators for 2 principal reasons:  Further computation of signals is necessary particularly when outer loop control is adopted, and  the power of the signals is very low. They do not have sufficient power to control the servomotors, solenoids, relays or the actuators that are designed to cope with the aerodynamic loads acting on the flight control surfaces. Therefore, in any one flight control system it is necessary to incorporate within the corresponding servo control loops a signal processing system having some, or all, of the following functions: 1. Differentiation, e.g. deriving simulated rate information from a vertical‐axis gyroscope signal sensor. 2. Demodulation, i.e. converting AC error signals into DC control signals where polarity represents phase relationship. 3. Integration, e.g. to obtain simulated attitude information or to correct any sustained attitude errors. 4. Amplification, e.g. to increase sensor signals to a level high enough to operate the servomotors. 5. Limiting, e.g. to ensure that certain parameter changes are kept within prescribed limits. 6. Gain adjustments, e.g. to allow pre‐setting and/or automatic programming to adapt system response to suit the handling qualities or flight path of an aircraft. 7. Programming, e.g. to produce a precise manoeuvre, e.g. when selecting a particular outer loop control mode. 8. Applying feedback, e.g. signal processing to ensure that corrective control is proportional to command signal inputs. 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 8 of 68 Training Material Only Autopilot Sensors E and I Bar Sensors The E and I bar sensor requires an AC signal for excitation of the primary winding. They operate using the same principles as an AC transformer. The primary differences are:  An adjustable component that can vary the output signal, and  Two ‘counter wound’ secondary windings that counteract each other in the null position. In the right image of Figure 4 the movement of the I bar relative to the E section varies the air gap resulting in the output signals illustrated. Figure 4: E and I bar sensor operation Another application of the E and I bar is illustrated in the left image of Figure 4 showing the ‘I’ bar as have a lateral motion as would be used in an accelerator and side slip sensor. When an aircraft maintains an attitude change which is cannot be sensed by the gyros, an acceleration sensor can provide an output in a direct relationship to the attitude change. An I bar, suspended between calibrated springs with the sensing axis aligned to sense acceleration in that plane. 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 9 of 68 Training Material Only Moving Vane Sensors Moving vane transducers differ from E and I bar sensors in that they use two square wave signals as excitation inputs. Two square wave A.C. signals that are 180 degrees out of phase are applied to coils 1 and 3. In the null position there is no output to the computer/amplifierError! Reference source not found.. Figure 5: Moving vane transducer with two square wave inputs If two moving vane transducers were mounted on the inner and outer gimbal rings of a vertical gyro to sense pitch and roll attitude changes, then during level flight there is no output. If there was an attitude change detected by the gyros the transducer vane would have relative movement to the coil assembly with the resultant output used as a command signal to the computer or amplifierError! Reference source not found.. 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 10 of 68 Training Material Only  Figure 6: Moving vane transducer with offset vane 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 11 of 68 Training Material Only Dynamic Vertical Sensor The dynamic vertical sensor is a pendulum actuated synchro transmitter the axis of which is aligned with the fore and aft axis of the aircraft. The unit is oil damped and in a coordinated turn the pendulum aligns with the resultant of centrifugal and gravitational forces. At the dynamic vertical there is no signal output but if the aircraft is slipping or skidding, the pendulum will be displaced from the vertical with a signal output. Figure 7: Dynamic vertical sensor 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 12 of 68 Training Material Only Displacement Gyroscopes Aircraft attitude information is provided by sensors mounted to a stabilised or referenced platform. Gyroscopes are commonly used to provide the stable reference and provide the primary element of inner loop stabilisation. The outputs of a gyroscope are used to provide attitude information to autopilot systems in a similar manner to the way gyro outputs are displayed on gyro instruments. They detect aircraft pitch, roll, and yaw movement and deviations in attitude or in the rate of change from a selected attitude are converted into electrical error signals and sent to the autopilot computer. On small aircraft, the autopilot sensors are commonly part of the attitude and heading gyro display instruments. Larger modern aircraft and autopilot systems use sensors that employ laser beams instead of a spinning gyro rotor. These laser sensors are called Ring Laser Gyros (RLGs) or Fibre Optic Gyros (FOGs). These devices are more expensive than mechanical gyros, but they do not precess, and they eliminate the moving parts that generate errors and cause a conventional gyro to gradually wear out. Figure 8: A laser ring gyro schematic diagram Rate Gyro Where a displacement gyro utilises a gyros property of rigidity in space and measure displacement around it, a rate gyro relies on a gyro being subjected to precessive forces against spring pressure to determine rate of movement. The higher the rate of movement the greater the inertial force applied to the gyro resulting in precession. The higher the rate of turn, the greater the precessive force, the greater the movement against spring pressure. 2020‐04‐14 B2‐13.3.2 Command Signal Processing Page 13 of 68 Training Material Only  Figure 9: Rate gyro These units are used to measure the aircraft’s rate of turn. If the nose of the aircraft is deflected, the gyro will sense this, and signals will be sent to the rudder to move the aircraft back to the correct heading. The rate gyro output shown below the aircraft represents the 400 Hz synchro signal developed during an