Autopilot

• Small aircraft generally use position based autopilot
• Large aircraft generally use rate based autopilot
• Four basic components of autopilot are

  1. Sensing
  2. Computing
  3. Output
  4. Commands + Feedback
• Sensing Elements:

  ·         Attitude / directional gyros, turn coordinator, altitude control

  ·         Gyros may be accompanied by solid state accelerometers & magnetometers

  ·         No gyros in a rate system

  ·         Includes NAV information
• Computer & Amplifier:

  ·         Analog or digital, usually shock mounted

  ·         Interprets the sensing information, forms movement commands, and sends signals to the output elements

  ·         Amplifier is used if necessary to strengthen signal to outputs.

  ·         Amplifier is the “computer” of analog systems

  ·         Digital systems use solid state micro processing

  ·         Can contain modules to dictate aircraft type and FD type
• Output Elements:

  ·         The servos responsible for flight control movement

  ·         Cable actuated systems use electric or electro-pneumatic servos

  ·         Hydraulic actuated systems use electro-hydraulic servos

  ·         Digital fly-by-wire systems use same actuators for autopilot as manual control
• Output Elements:

  ·         Control cable actuated systems use 2 types of motor servos

  o   Motor connected to output shaft – can stop, start, reverse, at command of computer

  o   Constant running motor – geared to output through 2 magnetic clutches. Energizing each clutch separately will cause movement in the respective direction
• Output Elements:

  ·         Pneumatic servos are controlled by amplified signals from autopilot computer and actuated with air pressure

  ·         Electro-hydraulic servos use control valves for direct fluid pressure to move control surfaces

  o   They are powered by Autopilot computer

  o   When Autopilot not engaged, fluid flows freely

  o   Can incorporate feedback transducers
• Command Elements:

  ·         Flight controller, human interface controls Autopilot

  ·         Includes selecting radio NAV aids

  ·         Usually include a disconnect and CWS switch
• Feedback or Follow up Elements:

  ·         Avoids continuous over-correction

  ·         Rate system receives error signals from rate gyro, causing control surface movement. As the surface moves, follow-up signals of opposite polarity are sent back in increasing magnitude until the error is corrected
• Rudder Channel:

  • Receives 2 signals which are sent to the rudder channel amplifier, combined and strengthened, then sent to rudder servo
  • There is a follow up signal during movement
  • Signal 1: course deviation derived by compass system
  • Signal 2: Rate signal for movement about the vertical axis provided by the Turn & Bank gyro
• Aileron Channel:

  • Receives signal for the longitudinal axis from Horizon Indicator gyro
  • Signal is amplified, phase detected, and sent to aileron servo
  • As control surface moves, the follow-up signal inputs until ailerons return to streamline and level flight is achieved
• Elevator channel:

  • Receives signal from a remote altitude sensing unit
  • Controls pitch movement along the lateral axis
• Yaw Damping Channel:

  • Near continuous input is required to prevent oscillation about the vertical axis while flying at a fixed heading
  • Signal provided by the turn coordinator rate gyro
• Elevator Trim Channel:

  • Used to alleviate long term load on the elevator and pitch servos
• Flight Controller Functions:

  YAW – Provides auto control of rudder during ASI changes

  HDG & NAV – Follow a preselected heading using VOR radial or a selected RNAV (area nav) course

  APPR – Commands autopilot to capture LOC beam on approach to the airport

  REV – Use if approaching from opposite normal course signal, provides back course information

  GS – Capture and Tracking of Glideslope signal. Will display ARM once GS beam has been captured for ~20 seconds. When aircraft reaches beam center, annun displays ON, then ALT, and ARM turns off
• Flight Controller Functions:

  ALT – Maintains set barometric altitude

  NAV – Will show RNAV or ON for VOR

  Turn Knob – initiate a 12o or 24o banked turn. Disengages other lateral controls. Must be centered when AP engages

  Pitch Wheel – Spring loaded in center position, can be moved up or down to command pitch up to 20o. Disengages all other longitudinal modes, GS, ALT, GA
• Flight Controller Functions:

  GA – Commands go around action for a missed approach

  ELEV indicator – shows if elevator is neutral. Before AP engage, its shows AP signal relative to pitch axis. It should be near center sue to automatic pitch synchronization. After AP engage, meter shows force required by primary servo to hold aircraft in desired pitch altitude.

  Roll Trim – trims ailerons for aircraft level altitude. NOT to be used with other lateral modes, or with Turn control off center
• AFCS and Automatic Landing Systems:

  APFDS – Autopilot Flight Director System

  SAS – Stability Augmentation System

  SCS – Speed Control System

  PFCES – Primary Flight Control Electronics System
• The Autopilot Flight Director System provides automatic pitch and roll control, and can be visually monitored or followed by the pilot
• APFDS Modes:

  Altitude Select and Hold – Maintains altitude

  Vertical Speed Select and Hold – Controls rate of climb and descent

  Heading Select and Hold – Maintains selected headings

  Control Wheel Steering – Temporarily disengages autopilot for manual control

  AIS Select and Hold – AP and speed control system maintain airspeed

  Mach Hold – Like AIS hold, but using Mach number
• APFDS Modes:

  Localizer – Approach mode, aircraft tracks localizer beam

  VOR and RNAV – System navigates automatically using VOR or commands from RNAV computer

  Approach / Land -

  • Speed is reduced to achieve rate of descent
  • NAVsystem locks onto ILS beam
  • Rad Alt provides alt signal for landing, flare, touchdown
  • Power reduces to idle and aircraft tracks localizer
• SAS Stability Augmentation System:

  ·         Provides Yaw Damping

  ·         2 dual channel computers provide Dutch Roll damping and turn coordination. Both are engaged during runway alignment and rollout

  ·         Satisfies the “fail-operational” for Autoland, Category III fully automated blind landing

  ·         All channels receive yaw rate signal from 1/3 yaw gyros
• SAS Stability Augmentation System:

  ·         in basic SAS, turn coordination is achieved using aileron transducer signals. Signals are switched out for LOC or Autoland

  ·         Damping provided by mixing heading and heading-rate signals with yaw-rate signals

  ·         Alignment scheme removes up to 8o­­ of initial crab angle

  ·         Rollout mode is initialized at landing, uses LOC beam to stay on runway until aircraft speed slows
• Dutch Roll – Combination of slipping in the yaw and roll axis, “tail wagging”
• Crab Angle – Angle between direction aircraft it pointing and the direction it is tracking on the ground
• SCS Speed Control System:

  • Airspeed Auto Throttle Mode, used in all flight conditions through initial approach
  • Angle of Attack Auto Throttle Mode, used in final approach and landing
  • Go Around Command, Manual and Auto
  • Take Off Command, Manual take off guidance
• Speed Control System consists of:

  • 1 computer with 2 computation channels
  • Single auto throttle servo
  • Dual sensor inputs
• Speed Control System Modes of Operation:

  • Airspeed Select: Normal mode of operation in cruise configuration
  • Stall-Margin: Approach and landing configuration
  • Flare: Engaged at flare initiation altitude, provides auto closed-loop throttle retard for touchdown
  • Touchdown: Engaged at touchdown, throttles driven to idle, auto throttle disengages