Flight Control PDF

Flight Control Axes of an Aircraft An aircraft in flight is controlled within three stabilised planes. Movement within each plane is about an axis, rather than centred on an axis. All three axes pass through the Centre of Gravity (C of G). The three principal axes are: Longitudinal (roll or X axis),...

Flight Control Axes of an Aircraft An aircraft in flight is controlled within three stabilised planes. Movement within each plane is about an axis, rather than centred on an axis. All three axes pass through the Centre of Gravity (C of G). The three principal axes are: Longitudinal (roll or X axis), which runs nose to tail through the C of G. Lateral (pitch or Y axis), which runs parallel with a line from wing tip to wing tip and intersects the X axis at the C of G. Normal/vertical (yaw or Z axis), which runs perpendicular to the other two axes, intersecting them at the C of G. Roll pitch and yaw axis Roll Movement around the longitudinal axis is called rolling; its control or stability is called the lateral stability, and this is controlled by ailerons. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 197 of 356 Pitch Movement around the lateral axis is called pitching; its control or stability is called the longitudinal stability, and this is controlled by elevators. Yaw Movement around the vertical axis is called yawing; its control or stability is called the directional stability, and this is controlled by the rudder. Flight Control Surfaces In straight and level flight, a flight control surface can be considered as an extended aerofoil. When the control surface is deflected, the amount of lift produced by the extended aerofoil either increases or decreases depending on the direction of the control surface. Movement If the control surface is deflected down, the shape of the aerofoil is extended, so the amount of lift produced increases. If the surface is deflected up, the aerofoil shape is distorted, and the amount of lift is decreased. Primary control surfaces consist of the following: ailerons, elevators and rudder. Control surface deflection Ailerons/Spoilers The roll attitude of an aircraft is controlled by ailerons and is operated by sideways movement of the pilot’s control column. They are found on the trailing edge of the wing near the wing tip. Placing them near the wing tip gives greater leverage, meaning a small deflection will create a large amount of roll. They are connected in opposition to each other so that when the left aileron is raised, the right one is lowered. When the aileron is lowered lift is increased and the wing will rise. The raised aileron will assist the lowered one to roll the aircraft. Movement of the control column is instinctive. If you move the column to the left, the aircraft will bank to the left, and vice versa. Spoilers can be used as a substitute for ailerons, and they operate on the same principle as ailerons, except they only extend upwards. When the spoiler of one wing is raised, the wing drops due to the loss of lift. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 198 of 356 Elevators These control the pitch of the aircraft and are coupled together and are found on the trailing edge of the horizontal tail plane. They are operated by means of fore and aft movement of the pilot’s control column. Pulling the column back will cause the elevators to rise, which will decrease the amount of lift on the tail, which will drop, causing the nose to rise. Thus, the aircraft will climb. Pushing the column forward has the opposite effect, and the aircraft will dive. Rudder This controls the direction of the aircraft or yaw in the same way as the rudder on a boat. The rudder is hinged at the rear of the vertical fin and is operated by the pilot’s rudder pedals. Pushing the right pedal forward causes the rudder to deflect to the right, causing the aircraft to yaw to the right. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 199 of 356 Manually Operated Flight Controls On small light aircraft, the power to move the control surfaces is provided by the muscles in the pilot’s arms and legs. 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’s attitude changes and the pilot re-centres the control column or rudder pedals when the desired attitude or heading is achieved. Any automatic function in this simple system is nothing more than the use of trim tabs to trim the aircraft. Correct trim tab setting relieves the pilot of continuously maintaining a force on the control column or rudder pedals to maintain straight and level flight or a desired heading. Flight control surfaces 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 200 of 356 Powered Flight Controls Introduction to Powered Flight Controls On larger aircraft, it is physically impossible to move the control surfaces by muscles alone. These 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, and these devices operate from mechanical input. Hydraulically powered actuators 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 actuator 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. Power-assisted actuator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 201 of 356 Power-Operated Control In this system, the pilot’s stick is connected to the control lever only, whilst the actuator unit is directly connected to the control surface. The effort required by the pilot to move the pilot’s stick 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 actuator unit’s hydraulic power. As there are no forces transmitted back to the pilot’s stick, 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 pilot’s stick and the connection to the actuator unit control lever. Power-operated actuator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 202 of 356 Fly-by-Wire System Hydraulically operated actuators can be modified to operate under the control of an electrical signal. Using electrical signals to reposition control surfaces, a pilot can input a commanded attitude change by moving the control column or rudder pedals. Electrical signals from stick and pedal transducers are processed for output to the appropriate electrically operated actuator or servomotor, resulting in control surface deflection. The term fly-by-wire (FBW) implies a purely electrically signalled control system. This fundamental point of difference means that there is no mechanical link between the pilot’s control column and rudder pedals and the actuators that positions the flight control surfaces. At all stages of flight, a computer system is interposed between the pilot and the flight control surfaces, and the computer modifies the manual inputs of the pilot in accordance with pre-set control parameters to maintain the stability and structural integrity of the aircraft. To provide redundancy and maintain full capability, an aircraft might have up to four independent channels, each responding to pilot and system input and operating the control surfaces. An FBW aircraft can be lighter than a similar design with conventional controls. This is partly due to the lower overall weight of the system components and partly because the flight control surfaces can be made smaller, as the natural stability of the aircraft can be relaxed. Automatic Flight Control System Sensors like gyros and accelerometers can be added to the fly-by-wire system to detect any uncommanded attitude changes. These signals are processed to provide an electrical output to actuators or servomotors to control the aircraft’s flight path. An Automatic Flight Control System (AFCS) can overcome a stability or control deficiencies (Dutch roll), improve the handling and ride qualities or perform a manoeuvre that a pilot is unable to perform due to lack of visual cues. By integrating more horizontal and vertical navigation systems into the AFCS system, we can control the aircraft’s attitude automatically. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 203 of 356 Damping Systems Damping systems are used to automatically correct for any non-pilot induced attitude changes. For example, an elevator damping system can counter porpoising, and a rudder damping system can counter Dutch-roll or turbulence-induced motion. Swept wing airplanes tend to oscillate at a very low frequency when disturbed in the yaw axis. This movement about the yaw axis will, if uncorrected, result in a rolling action known as Dutch Roll. As the airplane yaws, the outside wing becomes less swept back in relation to the air flow, and its speed is effectively increased. Therefore, it has more lift. The forces on the opposite wing are decreased, and the airplane rolls. This yawing of the airplane is automatically controlled by use of a yaw damper control system. Dutch roll Artificial Feel For a pilot to input a command to a hydraulically operated control surface actuator, the only effort required is that which is necessary to move the control valve spool. This makes it very easy to deflect the control surface to its limit regardless of the aerodynamic loads that might exist on the control surface. This contrasts with mechanically operated flight controls where the pilot experiences increasing difficulty in exerting enough force as a function of airspeed and the amount of deflection necessary. The pilot needs to be able to feel the aerodynamic loading on the flight controls to properly and safely operate the aircraft. An artificial feel unit replicates the opposition forces that occur in a mechanically driven control surface, which does not have the assistance of a hydraulic actuator. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 204 of 356 The artificial feel system, shown below, is for providing elevator feel. It uses both spring and hydraulic pressure to generate artificial feel as a function of the position of the horizontal stabiliser and the aircraft’s airspeed. To move the column, the pilot must compress the spring and overcome the force exerted on the hydraulic piston. The spring feel unit is adequate at low speeds, and the cam against which it runs also provides centring of the control column when it is released. At high speeds, more resistance to control movement is required to prevent overstressing the airframe. Pitot pressure is delivered to one side of the airspeed diaphragm and static pressure to the other. The diaphragm deflects against spring pressure set by position of the horizontal stabiliser cam and the spring on the relief valve of the metering valve. Hydraulic pressure supplied to the metering valve is regulated by the balance of the metering valve spool spring and the tension of the relief valve spring. The regulated pressure is supplied to the hydraulic feel cylinder. As airspeed increases, the downward force on the metering valve spool increases, and the regulated pressure value is also increased. To move the control column, the pilot must force the hydraulic feel piston up into the cylinder. The hydraulic fluid is displaced through the relief valve to maintain the regulated pressure. If the horizontal stabiliser is near its outer limits of movement, the control surface position cam increases the spring pressure on the yellow valve and increases the regulated pressure to increase the force opposing the control column. Elevator artificial feel computer 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 205 of 356 Autopilot Introduction to Autopilot The original use of an autopilot was to provide pilot relief during tedious stages of flight, such as highaltitude cruising. Advanced autopilots are now incorporated as part of the Automatic Flight Control System (AFCS) and can do much more, carrying out highly precise manoeuvres, such as landing an aircraft (Autoland) or operating throttles (Autothrottle). An autopilot is classified by the number of axes about which control is implemented: Single axis, or single channel autopilot, in which attitude control is about the roll axis only. This is common on light aircraft and is known as a wings leveller. The pilot can command turns or engage a heading hold or radio course mode. Multi axis, with attitude control about the pitch and roll axis or about the pitch, roll and yaw axis. A multi-axis aircraft may incorporate automatic pitch trim, a yaw damper system and a flight director system. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 206 of 356 Basic Capability When the autopilot is engaged, it is designed to hold the aircraft in the same pitch and roll attitude that existed at the time of engagement. To clarify this statement: If the aircraft is climbing, descending or in level flight, this attitude will be maintained. If a bank angle exists at the time of engagement, the autopilot will command a gentle return to wings level. The autopilot will respond to any uncommanded changes of attitude to retain the above conditions. Typical auto flight components 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 207 of 356 Closed Loop Servo System Automatic Pilots (A/P) and Automatic Flight Control System (AFCS) are based on a closed-loop servo system. Basic AFCS Loop An autopilot channel consists of two servo loop systems: An inner loop, where the actual attitude and the desired attitude are compared, and any error sensed used to develop a command for control surface movement. This is sometimes referred to as an Attitude Loop. A yaw damper system or stability augmentation system operates on the inner loop principle. An outer loop, where sensors of external systems are used by the autopilot to command attitude changes. Most autopilots will use some form of external input, from a heading hold function to an Autoland function. Both loops rely on aerodynamic feedback for the autopilot to operate successfully. Descriptions about the operation of an autopilot channel may refer to a closed-loop system, which means the aerodynamic feedback forms an important part of the successful operation of the autopilot. For a ground engineer carrying out maintenance on an autopilot system, this lack of aerodynamic feedback (open loop) creates some unusual responses in that the control surfaces are driven to their limits or appear to drift continuously. These effects are detailed in the maintenance manual test procedures. Autopilot servo loops 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 208 of 356 When an aircraft flying with the autopilot engaged drifts from its original attitude, the following sequence of events occurs: An input signal originates from a gyro stabilisation signal (attitude gyro). The signal is amplified to provide an increase in signal strength to power the correction unit (actuator or servomotor) to move the aircraft control surface. A follow-up system (position feedback) counters the input signal which reduces the amplifier output to zero (null), and the control surface movement stops. Control surface movement causes the aircraft to respond with a change in aircraft attitude, and the original input signal is reduced back to zero. The follow-up system (position feedback) is now providing the only input to the amplifier, the output of which now drives the servomotor and the flight control surface back to its neutral position. The aircraft drift has therefore been corrected by the sensor output, and when the aircraft is returned to the selected attitude by the control surface displacement, the control surface is returned to the neutral position. Closed loop servo system 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 209 of 356 Autopilot Elements A basic autopilot consists of the following major elements: The sensing element consists of attitude gyros, rate gyros, accelerometers and pitot static systems. They detect the uncommanded movement of the aircraft about its three flight axes and the rate of that movement. The command element consists of the autopilot mode select panel, flight controller, control wheel steering and navigation systems. The computer amplifier computes, amplifies and processes the signals from the detecting and command elements and directs the output element to respond to the pilot’s and/or system’s requirements. The output element consists of the units which move the control surfaces of the aircraft in response to the computer demands. These can be pneumatic actuators, electric servomotors or electromagnetic transfer valves. Autopilot elements 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 210 of 356 Sensing Elements Depending on the autopilot type, sensors may be dedicated to the autopilot or sensors may share information with other systems. As a bare minimum an autopilot requires: Pitch and roll attitude supplied by a vertically referenced gyro. Heading information supplied by a directional gyro or compass system. Autopilot functionally is improved by adding: Pitch, roll and yaw rate sensing by using rate gyroscopes, accelerometers or through the integration of the displacement gyro’s output. Sensing vertical acceleration using an accelerometer. Sensing flight control and autopilot actuator position to improve feedback and operational stability. Using airspeed and altitude information to alter the gain programming of the pitch, roll and yaw autopilot channels. A fully operational AFCS (Automatic Flight Controls System) uses the following: Flight path and navigational data from inertial navigation systems, radio receivers, global positioning systems and flight management systems. Air data information for airspeed and Mach speed control, vertical speed control and altitude hold. An angle of attack sensor for approach path computations. Radar altimeter information for automatic landing control. Multiple autopilot engagement for approach and landing. Autothrottle operation. Autopilot operation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 211 of 356 Computer Flight Control Computer An aircraft may be fitted with one, two or three independent autopilot systems, depending on the operator’s requirements. The flight control computer of each autopilot will receive dedicated inputs and provides its own outputs. The computer units are generally rack mounted and may consist of a single unit or multiple units. Additional rack-mounted units for autopilot control logic and operational monitoring and automatic trim control may form part of the autopilot system. Autopilot flight control computers An aircraft capable of an Autoland requires a minimum of two totally independent autopilots. Two definitions that relate to multiple autopilots operating in the Autoland mode are: Fail Passive – a dual autopilot installation which provides a warning and allows the aircraft to default to a stable condition should a single input failure occur. The automatic landing cannot continue with the disengagement of one autopilot. Fail Operational – a triple autopilot installation which allows the automatic landing to continue should a single input failure occur causing one autopilot to disengage. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 212 of 356 Command Elements Autopilot Mode Select Panel The Mode Select Panel (MSP) is the primary interface between the flight crew and the automatic flight control systems. The panel contains the switches and logic circuits for on/off control and mode selection for the autopilot/flight director channels and the Autothrottle system. The MSP shown below manages the engagement of two autopilot systems, either of which can be selected to manual mode or command mode. In manual mode the pilot can change the attitude and heading of the aircraft by using a knob and wheels mounted on a flight controller. Under normal flight conditions, only one autopilot is engaged; however, during approach it is possible to select both autopilots to command mode. Lateral and vertical navigation modes, including VOR/ILS, heading hold and heading select, altitude hold, altitude select, vertical speed, indicated airspeed, Mach or vertical speed hold modes are selectable through the MSP. As the operation of the flight director reflects the operation of the autopilot, the flight director function is incorporated into the MSP, as it relies on the same computers and selections. Autothrottle operation is also controlled through the MSP. The autopilot system shown does not integrate with the Flight Management System (FMS). The autopilot relies on the pilot inputting the parameters generated by the FMS. Most modern systems do integrate auto flight with FMS. Autopilot/flight director mode select panel 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 213 of 356 Flight Controller The flight controller provides a means for the pilot to alter the aircraft’s attitude without having to disengage the autopilot. With the autopilot engaged to manual, moving the turn knob will signal the roll channel of the autopilot to command a roll manoeuvre. The knob has a wings level detent position which also incorporates an engage interlock. The autopilot will not engage unless the knob is in its detent position. The two pitch wheels are mechanically locked together and do not have a detent position. Moving the pitch wheel during autopilot engagement will change the pitch attitude of the aircraft. Autopilot flight controller Some autopilots have the Control Wheel Steering (CWS) option fitted. On aircraft fitted with CWS, there is no need for the flight controller shown above. Also, the mode-select panel autopilot engage switches have CWS in place of MAN. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 214 of 356 Control Wheel Steering The normal autopilot engaged mode, known as the command mode, is where the pilot does not touch the controls because the autopilot is flying the aircraft. The pilot can set the autopilot to control wheel steering mode, and the force created as the pilot moves the control in the normal fashion is measured and used as an input signal to the flight control computer. In effect, the human pilot is flying the aircraft, but the autopilot stays engaged and is helping to move the control surfaces. The transducer on the control wheel which converts the input force into an electrical signal is either an E-and-I-bar assembly, a LVDT or a piezoelectric-type sensing device. Control wheel steering transducer Autopilot Override Any autopilot can be overpowered at any time by the pilot. The autopilot design allows the pilot to instinctively apply a reasonable amount of pressure on the respective flight control to override the actions of the autopilot should there be a malfunction. A typical overpowering force would be 25 to 35 pounds of turning force on the control wheel or 40 to 50 pounds of force on the control column. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 215 of 356 Trim Indicator The trim indicator provides a visual display of the magnitude and polarity of the signal being applied to the flight control actuator or servomotor. Each pointer represents a primary flight control, and a pointer is deflected each time a servo amplifier command signal is applied to the actuator. When the servo commands are satisfied, the pointers return to their zero trimmed positions. If a pointer remains continuously deflected, it means the servo amplifier needs to produce a continuous correction signal to keep the attitude of the aircraft. The pilot can manually trim the aircraft until the pointer returns to the zero position. Autopilot trim indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 216 of 356 Autopilot Disengage Switches To enable the pilot to rapidly disengage the autopilot, a disengage switch is mounted on each control yoke. Pressing either switch when the autopilot is engaged will release the autopilot from operating the flight controls. Autopilot disengage switch 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 217 of 356 Flight Director Display On some aircraft, the flight director information is generated by the same computing circuits used to command the autopilot. When the flight director mode is selected on the autopilot mode-selector panel, information is displayed on attitude director indicator and can be used for monitoring autopilot activity. Attitude director indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 218 of 356 Flight Mode Annunciator The flight mode annunciator is mounted close to the attitude director indicator and gives the pilot a visual indication of the status of selected modes used for flight director, autopilot and autothrottle operation. Flight mode annunciator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 219 of 356 Output Elements Autopilot Actuators The components used to move an aircraft’s control surfaces are called actuators, servomotors, servo actuators or by the name of the control surface or channel that it controls, for example, rudder servo or pitch actuator. The servo actuators convert the electrical signal into mechanical motion by using AC- or DC-powered motors, torque motors or solenoid-controlled valves. The actuator output moves the control surface mechanically, pneumatically or hydraulically. The three main types of servomotors are: Electro-pneumatic Electro-mechanical Electro-hydraulic. Electromechanical and electro-pneumatic actuators are more suited to smaller aircraft that do not need powered controls. Modern commercial aircraft use hydraulic-powered flight controls and the actuator is most likely to be electro-hydraulic with secondary systems being electromechanical. AFCS actuators may be connected in series or parallel. A series actuator is one that moves the control surfaces without moving the pilot’s controls, whilst a parallel actuator moves the control surfaces and the pilot’s controls. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 220 of 356 Electro-Pneumatic Actuator Simple actuators 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 pneumatic actuator is an airtight housing which contains a moveable diaphragm. When vacuum is applied to the actuator by a controller, the diaphragm is displaced, pulling on a bridle cable clamped to the normal flight control cables to reposition the flight control surface. Two of these actuators would be needed for each control surface: a ‘push’ and a ‘pull’ actuator. © Jeppesen Pneumatic actuator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 221 of 356 Electro-Mechanical Electro-mechanical systems use electric servomotors powered by either DC or AC. A DC-powered servomotor can be one of two types: An intermittent operation motor supplied with a reversible DC supply. A continuously running motor supplied with a fixed DC supply. Intermittent DC Motor The motor unit includes a reduction gearbox and an electromechanical clutch. The assembly mounts onto a frame and engages with a capstan fitted with bridle cables clamped to the flight control cables. Whenever the autopilot is engaged, the clutch is energised, allowing the motor output to be transferred to the flight control cables. With the autopilot disengaged, the flight control cable is free to move in the normal manner. © Jeppesen Reversible DC motor 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 222 of 356 Continuous DC Motor The motor drives two contra-rotating magnetically operated clutch assembles. Each clutch is independently and progressively engaged by two DC signals from the flight control computer. The amount of engagement and the transfer of torque from the motor to the gearbox is determined by the strength of the DC signal. The direction of the applied torque is determined by which DC signal is used. The output from the gearbox is applied to the control surface operating mechanism. Whenever the autopilot is turned on, the motor is energised, but there is no transfer of torque to the flight controls. When the autopilot is engaged, clutch signals from the amplifier will energise the clutches, as required, to allow the transfer of motor torque to the flight control. 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. © Jeppesen Constant drive DC servomotor 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 223 of 356 AC Servomotor AC-powered autopilots with AC-powered servomotors use a reversible squirrel cage motor that drives a reduction gearbox and DC-powered clutch assembly. When the autopilot is engaged, the clutch is energised, and motor torque is transferred to the flight control cables. AC signals from the amplifier vary in phase relationship and magnitude to drive the motor. AC servomotor assembly Electro-Hydraulic Actuator An electro-hydraulic actuator is a modified power-assisted or power-operated hydraulic actuator. Electro-hydraulic actuator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 224 of 356 Electro-hydraulic actuator cross-section Aileron Autopilot Electro-hydraulic Actuator Cross-section: 1. On/off solenoid valve 2. Pressure line closing valve 3. Return line closing valve 4. Transfer valve (servo valve) 5. Autopilot actuator 6. Damping orifice 7. Check valve 8. Return relief valve 9. Fluid reserve 10. Control surface actuator LVDT 11. Autopilot actuator LVDT. To engage the servo, the on/off solenoid must be activated. This is only done after a series of interlocks and fault-detecting monitors are satisfied. When these units are satisfied, the solenoid will engage. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 225 of 356 When the autopilot is engaged, the on/off solenoid is energised, and hydraulic fluid is ported to the transfer valve. The transfer valve is an electrically positioned hydraulic valve that responds to a small DC signal of variable polarity and magnitude. The hydraulic output of the transfer valve positions a spring-restrained spool valve assembly called the autopilot actuator. The autopilot actuator ports hydraulic fluid to the control surface actuating cylinder. The two LVDTs record the positions of the autopilot actuator and the control surface actuator piston for the autopilot computer feedback circuits. When the autopilot is disengaged, the on/off solenoid prevents hydraulic fluid from reaching the transfer valve and autopilot actuator. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 226 of 356 Torque Limiting Introduction to Torque Limiting The AFCS needs to have a limiting force. Without force limiting, it would be very easy for the AFCS or the pilot to overstress the aircraft by carrying out manoeuvres outside the aircraft’s stress limits. Need for torque limiting 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 227 of 356 Methods of Limiting Limiting is provided in the pitch, roll and yaw channels. The maximum output of an autopilot servo amplifier is limited with power source limiting resistors usually placed in the output amplifier for the servomotor. In addition, the mechanical force is limited by using mechanical clutches that slip or disengage. Force limiting is commonly referred to as torque limiting. Another term used is gain scheduling. The computer servo-amp signal to the actuator is limited by airspeed, attitude (high AOA), altitude, all up weight, etc. By limiting the command signal to the actuator, it does not matter how far the pilot pulls the control column back, the aircraft control surface will only be displaced as much as is allowed by the flight control computer. This gain scheduling is not to limit the capabilities of the aircraft, but the software program loaded into the FCC limits control commands to only what is permissible to remain within the aircrafts flight envelope. That is, the system should prevent a pilot from stalling, overstressing or losing control of the aircraft by overriding their inputs to remain within the flight envelope. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 228 of 356 Autopilot Operation Autopilot Engagement Before an autopilot system can be engaged, certain preliminary operating requirements must be met. The principal requirements are the following: Valid power supplies. External and internal elements are operating and serviceable. Appropriate signal and engage circuits are electrically complete. This series of switches and/or relays which ensure systems are operational before allow ing engagement of the autopilot system are known as interlocks. When the aircraft is powered, the attitude sensing elements and signal processing elements of the autopilot are maintained in a standby condition even though the engage switch is in the off position. Autopilot interlock schematic 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 229 of 356 If the pilot wishes to engage the autopilot, the following conditions must be met: Captain and co-pilot disengage button switches must be closed. Power monitoring relays must be closed. Mach trim coupler relays, the auto-trim cut-out switch and the vertical gyro contacts must be closed, indicating serviceability of these systems. The turn control switch must be in detent, and the mode selector switch must be in MAN. This ensures that when the autopilot is engaged, it will initially maintain the attitude of the aircraft. Movement of the engage switch to autopilot energises the engage interlock relay to provide power to the three servo clutches, the engage switch hold-in coil and the engage relay. The mode selector switch and the turn control switch can now be used without causing autopilot disengagement. Any loss of power or operation of a disengage switch or loss of serviceability of Mach trim, auto trim or gyro will relax the engage interlock relay, which de-energises the engage switch hold in coil, causing the engage switch to return to off and all the servo clutches to de-energise. Note that the engage switch provides an earth return for the servo clutch and engage switch coils when the engage switch is in the off position. This is to ensure that there is no possibility of a clutch or hold-in coil receiving power from another source in the event of an electrical fault. If the pilot wishes to engage the yaw damper, a much-simplified interlock system is used. When the engage switch is selected to damper, the engage interlock relay will be energised, provided both disengage button switches are closed and the DC and AC power monitoring relays are energised. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 230 of 356 Autopilot Modes The basic capability of an autopilot can be supplemented with additional features designed to reduce the pilot’s workload. All autopilots have the provision which allows the pilot to manually command an attitude change. A flight controller panel with a turn knob and a pitch wheel allows the pilot to operate these controls any time the autopilot is engaged to MAN. The turn knob must always be returned to its detent position once the selected roll attitude is acquired. Manual autopilot command Aircraft with Control Wheel Steering (CWS) allow the pilot to input commands by applying pressure to the controls in the same manner as if he were flying the aircraft manually. The autopilot engage switch has CWS instead of MAN and is not fitted with a flight controller. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 231 of 356 Additional modes require the autopilot to be selected to “command,” and the pilot uses a flight mode annunciator panel to confirm the status of some of the modes selected, and these include: Heading Hold – this is the most common mode and often automatically operational once the wings are level after autopilot engagement. Heading Select - the autopilot responds to the angular difference between the aircraft’s heading and a heading select bug, on the Horizontal Situation Indicator (HSI), which the pilot has positioned. Altitude Hold – this will maintain the aircraft’s altitude existing at the time of selection. If the aircraft is climbing or descending, it will level off accordingly. Altitude Select – this allows the pilot to preselect a desired altitude, and the aircraft will level off when the selected altitude is reached. Vertical Speed Hold – the autopilot will maintain the vertical speed existing when this mode is selected. This mode is not compatible with altitude hold and will disengage automatically at the selected altitude if altitude select is in use. Airspeed or Mach Hold – this will maintain the aircraft’s speed as existing at the time of selection. The mode will change the aircraft’s pitch attitude to achieve this. Turbulence – the autopilot will return to MAN mode, as the pitch and roll channels are referenced to a purely stable attitude and the response of the autopilot to changes in attitude is dampened during violent turbulence. Autothrottle – this allows the pilot to select a speed which is maintained by the autothrottle adjusting the thrust levers. Radio Navigation – this is a series of modes and sub modes which allow the autopilot to guide the aircraft in response to radio navigation signals, or inertial reference signals or the flight management system for en-route navigation. Land – when selected, the landing sequence is commenced and engagement of the second autopilot is possible, and progress can be observed on the flight mode annunciator. The graphic below shows a dual-channel fail-passive autopilot and gives an indication of the selections made on the mode select panel and the presentations that will show on the flight director and flight mode annunciator. Both autopilots are engaged, and the localiser and glideslope signals are captured. The aircraft is in a 5-degree nose-down attitude and tracking both beams with no error. The flare mode is in the armed condition. The autothrottle is not being used. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 232 of 356 Autoland armed for flare 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 233 of 356 Autothrottle A fully optioned autothrottle (A/T) system is designed to operate in conjunction with the autopilot to maintain an aircraft’s speed and vertical path for all flight stages from take-off roll to flare out at landing. When the autopilot is controlling the vertical path of an aircraft, the autothrottle maintains airspeed through thrust control. Engines fitted with digital fuel control have a thrust management computer, which programs the power setting of each engine in response to cockpit controls and inputs from the flight management system, engine sensors and air data computers. Autothrottle system On aircraft using engines with mechanical fuel control, the autothrottle output drives a single servo motor to position the thrust levers collectively, with the crew adjusting each engine’s parameter individually. Provided the autopilot is engaged, the autothrottle can be selected by a switch on the autopilot mode select panel. If the autothrottle engage interlocks are valid, the switch will be magnetically held in the on position, and a green light next to it will illuminate. If the autopilot is disengaged, the autothrottle will also disengage, the switch drops and the green light extinguishes. A speed select knob and indicator permits the crew to select a desired airspeed which the aircraft will maintain during the climb, cruise and descent phases of flight. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 234 of 356 Speed select and autothrottle engage The crew can disengage the autothrottle by pressing either of the A/T disconnect switches (1) mounted on the outer sides of the thrust lever pack. Two go-around switches, used on aircraft without a thrust management computer, are mounted on the inner thrust levers to allow the crew to abort an approach and have the thrust levers advance automatically to provide the predetermined go around thrust setting. Aircraft with thrust management computers have take-off/go-around (TOGA) switches (2), which the pilot selects when the aircraft has take-off clearance. Autothrottle disconnect switches (1) and TOGA switches (2) 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 235 of 356 Flight Management System A Flight Management System (FMS) has the capability of automatically controlling the aircraft from just after take-off through to roll out after landing at the destination airport. The pilot must then take over to turn off the runway and taxi to the gate. The system may not be used to its fullest capability on any or all flights. A single FMS consists of the following: A Flight Management Computer (FMC) A Control Display Unit (CDU) A data bus. Flight management system 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 236 of 356 The FMS system is used by the flight crew to reduce the workload when planning, undertaking, monitoring and executing a flight plan. Aircraft may carry two or three fully functional and integrated systems, with each one being able to provide: Flight planning, where the entire flight can be programmed into the computer using the Control Display Unit Performance management, where the system provides optimum flight profiles, at minimum cost, for thrust settings, climb rates, cruise altitudes, descent rates and holding and approach patterns Automatic navigation guidance, where the system can calculate great circle routes to any destination, designate waypoints, avoid prohibited areas, provide alternate airports, avoid weather hazards and tune navigation receivers to the required frequencies for the route. The FMC interfaces with many aircraft systems to aid in its primary functions, and is in effect the master computer which integrates the functions of: The fuel quantity and fuel flow systems Radio navigational systems Inertial reference systems Air data systems The thrust management computer The flight control computers EICAS/ECAM and EFIS systems Air/ground data link systems The central maintenance computer The weight and balance system. The aircraft autopilot and the thrust management computer use appropriate outputs from the FMC for automatic guidance for engine power settings. With the autopilot and autothrottle systems engaged, the aircraft will follow the flight plan as entered by the crew prior to the flight commencing. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 237 of 356 Graphical representation of flight plan 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 238 of 356