GENERAL DESCRIPTION

Autopilot is an automatic flight control system that keeps an aircraft in level flight or on a set course. It can be directed by the pilot, or it may be coupled to a radio navigation signal. Autopilot reduces the physical and mental demands on a pilot and increases safety. The common features available on an autopilot are altitude and heading hold.
The simplest systems use gyroscopic attitude indicators and magnetic compasses to control servos connected to the flight control system. The number and location of these servos depends on the complexity of the system. For example, a single-axis autopilot controls the aircraft about the longitudinal axis and a servo actuates the ailerons. A three-axis autopilot controls the aircraft about the longitudinal, lateral, and vertical axes. Three different servos actuate ailerons, elevator, and rudder. More advanced systems often include a vertical speed and/or indicated airspeed hold mode. Advanced autopilot systems are coupled to navigational aids through a flight director.
The autopilot system also incorporates a disconnect safety feature to disengage the system automatically or manually. These autopilots work with inertial navigation systems, global positioning systems (GPS), and flight computers to control the aircraft. In fly-by-wire systems, the autopilot is an integrated component.
Additionally, autopilots can be manually overridden. Because autopilot systems differ widely in their operation, refer to the autopilot operating instructions in the Airplane Flight Manual or the Pilot’s Operating Handbook. PURPOSE

i. Autopilot reduces the fatigue of the crew on long flights by taking over the routine flight control of the aircraft at any time. ii. A single-axis autopilot manages just one set of controls, usually the ailerons. This simple type of autopilot is known as a "wing leveler" because, by controlling roll, it keeps the aircraft wings on an even keel. A two-axis autopilot manages elevators and ailerons. Finally, a three-axis autopilot manages all three basic control systems: ailerons, elevators and rudder. iii. Basically two autopilot director system is fitted into the aircraft (A/P 1 & A/P 2). Normally A/P 1 is for standard & A/P 2 is for standby. iv. Autopilot can be engaged 5 second after take off.
BASIC PARTS AND ITS FUNCTIONS

i. Sensors or gyros * Detect changes in aircraft attitude by using gyros or other sensing device. ii. Amplifier or computer * Process the signals from sensors and send signals to the servos to correct the attitude. iii. Servos * Receive signals from computer and supply force to move the flight control surface. iv. Feedback * Send signals back to the computer to indicate the motion of the flight control surface. v. Controller * Supply manual commands to the autopilot that change the pitch attitude or command a turn.

BASIC OPERATIONS

Gyros will develop signals and processed in a computer. A signal then will be send to operate the autopilot servos. Servo output moves the appropriate flying control system PFCU to move the control surface. Major control system involves is rudder, aileron and elevator. Inputs from pilot’s control panel, navigation, etc will go to computer to be digitized and summed with other inputs. Output which is analogue signal goes to either electric servo motor or solenoid to select a hydraulic servo and move the control system into the PFCU servo valve. PFCU will move the control surface, and feedback signal will be sent to the computer. As aircraft move, gyros will pickup this movement by aerodynamic coupling and provide another negative feedback.

SENSORS/GYROS AMPLIFIER/COMPUTER SERVOS FEEDBACK

PRINCIPLES

i. Inner loop control * Consists of two negative feed back loops. * Aerodynamic coupling connected by aircraft movement sensed by rate and displacement gyros. * Gyro inputs send signals to cancel the effect of the original. * Other feedback is from servo actuator position (control surface position) to autopilot computer to negate the original servo signal input. ii. Outer loop control * Non feedback loops and made up of inputs from pilot and other systems. * These signal will sent for computation before going to the inner loop.

THREE CHANNELS AUTOPILOT

Autopilot basically summation of input by computers and output signal will be amplified and sent to servo motor. There are three channels:

i. Rudder channels.
It consists of two signals which are: a) First signal * Determines when and how much rudder will move. * Signal is obtained from gyros compass system. * No signal will be develop as long as the aircraft is in the correct heading position. * Any changes will make gyro compass to send signal to rudder. b) Second signal * Rate signal will determine the rate of aircraft turn. * The faster aircraft turn, the stronger the signal produced by rate gyro.

These two signals will combined, process and amplified. Output will be sent to rudder servo to bring the aircraft to correct position. As the rudder move, feedback signal is sent to amplifier and summed with gyro input by computer until the rudder reach the desired position. The rudder will stay at new position until there is new direction heading given by pilot. This system provides cross feed input to aileron channel to produce the correct amount of bank for any rudder deflection.

ii. Aileron channels * Receives signal from roll rate gyro, turn control and navigation computers. It also has cross feed input to rudder channel for coordinated turns. * Signal produced when aileron move and summed by computer to give the output signal to servo. * When the output signal equal to input signal, servo ceased, and aileron will stop moving. * Aileron will maintain at bank condition by summation signal. * Level flight return by signal to servo become smaller, feedback signal move aileron to trail position.

iii. Elevator channel * Work similar to aileron channel.

FLIGHT DIRECTOR/CONTROLLER

i. Functions * Use during an instrument approach. Pilot can fly more accurately on ILS approach because the computer makes rapid calculations to predict the optimum heading and attitude for the approach. * Correction for wind drift. Pilot has to follow the flight director command. * Setting proper take-off pitch attitude. ii. Operations * A system that uses some of the basic components of the autopilot. * Uses sensors and computers. It does not have servos. * Uses signals from sensors which is displayed on EADI as a command for the pilot to follow. * During flight director operation, pilot will maintain manual controls and follows the commands directed by the command bar displayed by EADI.

AUTOPILOT SENSING UNIT

i. Description
Gyros either mechanical or laser is use to sense the displacement and rate of change of the aircraft. It detects aircraft pitch, roll and yaw movement. Deviations in attitude form a selected attitude will be converted into electrical error signals and send to the autopilot computer. Modern gyros are laser beam operated in one house unit and duplicated or triplicate. Older gyros use spinning gyro rotor. There are several sensing elements which are: a. Directional gyro b. Turn and bank gyro c. Attitude gyro d. Rate gyro e. Altitude control f. Other gyros ii. Operation
Ring laser gyros: * RLG has two laser beams that travel in opposite directions around a triangular course. * Sensitive detectors measure the Doppler shift or frequency change whenever the unit is rotated. * One RLG is needed for each axis measured. * They do not precess and they eliminate the moving parts that cause a conventional gyro to gradually wear out.

SERVOS

- Installed either in front of PFCU or combined within PFCU.
- Connected in such a way that input move the whole flying control system include control column called Parallel system.
- Input only affected flight control system downstream called Series system.
- This principle applies to trim actuator and yaw dampers.

- There are two types of servos: i. Electro hydraulic autopilot servo
- Connected to output shaft through reduction gear.
- Motor start, stop, reverse in response to command signals from computer.
- Utilize electrical signals from the autopilot computers to direct hydraulic fluid under pressure to a hydraulic actuator.
- The actuator supplies mechanical force to the normal linkage of the flight control system. ii. Electric motor autopilot servo
- Signals operated cause valve to cause fluid under pressure to pass a jack to move the control surface.
- Servo maybe separate unit or combined unit with PFCU by duplicate connection to different hydraulic supply. If one fail, other can continue but with half authority.

SERVO OPERATION – SEPARATE UNIT

i. Control system connected to servo via the output crank and internal crank. ii. When autopilot disconnected, two detent pistons withdrawn by spring action from roller, move freely when the system move by pilot. iii. When autopilot connected, detent piston move to engage with the roller by hydraulic pressure supply is feed to the servo piston to move control system via the cranks. iv. Pressure supply is control by EHSV under command from autopilot computer. v. Output position LVDT (Linear Variable Differential Transducer) send positional feed back data to computer.

SERVO OPERATION – COMBINED UNIT

i. PFCU and autopilot servo combined in one unit. ii. Autopilot disengaged: * Pilot’s input goes at input link by fixed pivot cause push/pull link to move, thus selecting the main servo valve. * When pilot stop input movement, the main jack will continue to move the link and cancel the main servo selection and main jack will stop. * Follow up movement take a second to give the feedback. iii. Autopilot engaged: * Autopilot on, hydraulic pressure is feed to the autopilot control jack by autopilot servo. * This will move the link via summing link to select the main servo.

GROUND CHECKS

i. With the autopilot disengaged, manipulate the flight control to see if they function smoothly and without excessive drag. ii. Check the autopilot’s alignment to the aircraft such as cable tension, torque setting, and dimensional adjustment. iii. Before engaging the autopilot for an operational check, allow the gyros to come up to speed. After engaging the autopilot, do the following checks: a. Rotate the turn knob on the controller to the left. The rudder pedals and control column should move in the proper directions to indicate a left turn. The motion should be smooth and without excessive binding, jerking or hesitation. b. Rotate the turn knob to the right and watch the controls for proper operation and motion. c. Rotate the pitch knob up and down and watch for the correct motion of the control column aft and forward. d. If the autopilot had automatic pitch trim, check the proper motion of the trim control as the control column moves fore and aft. When the control column moves back, the system should apply nose up trim and vice versa. e. With the autopilot engaged, try to overpower it by grasping the controls and applying force. It should be possible to overpower the autopilot if it is adjusted properly. f. Check all of the controls and switches for proper actuation and correct indications. g. Checks the autopilot disconnects switches to ensure that the autopilot disconnects rapidly and positively. There may be several ways to disconnect the autopilot. h. If the aircraft has a flight director, check for proper indications by the command bars in the ADI or EADI. Check the autopilot mode indicators in the ADI or EADI if so equipped.