Comprehensive Study Notes on Flight Director Systems (FDS) and Electronic Flight Instrument Systems (EFIS)

Definition and Scope of Flight Director Systems (FDS)

  • Flight Director System (FDS) or Integrated Flight System (IFS): Defined as a system that integrates multiple signal inputs to provide a consolidated output to a display system.
  • Integral Signal Inputs: The system processes data from the following sources:
    • Attitude reference.
    • Aircraft heading.
    • VHF Omni Range systems (VOR).
    • Localizer beams.
    • Glide-slope (GS) beams.
    • Marker beacons.
    • Radio altimeters.
    • Inertial Navigation Systems (INS).
    • Doppler navigation systems.
    • Distance Measuring Equipment (DME) systems.
    • Area navigation systems (RNAV/VNAV).
    • Speed control signal information.
  • Principal Display Units: An FDS typically comprises two primary instruments:
    • Attitude Direction Indicator (ADI): Historically referred to as the flight director or approach horizon.
    • Horizontal Situation Indicator (HSI): Also known as a course deviation indicator.
  • Manufacturer Variations: Display configurations depend on the specific manufacturer's design and the extent of functional integration required.

The Attitude Director Indicator (ADI)

  • Visual Representation: Displays a view from behind the aircraft looking forward. It presents steering commands and attitude around a fixed aircraft symbol.
  • Aircraft Symbol: Positioned in the center of the display. It is adjustable via a knob located at the lower right of the instrument.
  • Attitude Sphere:
    • Physical layout: Usually colored blue/grey for sky and brown for ground, separated by a white horizon line (solid or dashed).
    • Pitch Ladder: Painted on the sphere to indicate pitch angles from 0∘0^{\circ} to 90∘90^{\circ} for both climb and dive maneuvers.
    • Range of Motion: The sphere moves 360∘360^{\circ} in roll. Depending on the instrument type, it allows for either 90∘90^{\circ} or 360∘360^{\circ} in pitch.
  • Bank Pointer: Displays the aircraft's bank angle, which is read against a scale on the instrument casing.
  • Command Bars: Consist of two bars for pitch and roll guidance. These are called command bars because the pilot is instructed to "fly" the aircraft symbol toward them. Signals are provided by the Flight Director Computer (FDC).
  • Glideslope Pointer:
    • Location: Left side of the ADI.
    • Function: Used during landing after capturing runway glideslope beams; shows vertical position relative to the beam.
    • Center line alignment: Indicates the aircraft is exactly in the center of the glideslope.
    • Operational Trigger: When the pointer reaches the dot closest to the center line, the pitch command bar appears, and the pilot must fly toward it.
  • Localizer Deviation Indicator: Features a pointer showing aircraft position relative to localizer beams. Centering the pointer indicates the aircraft is in the heart of the beam.
  • Slip and Skid Indicator: Provides visual data on the type of turn (coordinated, slip, or skid) being encountered.
  • Rate of Turn Indicator: Shows the speed of the aircraft's turn.
  • Warning and Status Flags:
    • Off Flag: Indicates loss of power or incorrect phase rotation.
    • Course Warning Flag: Appears when bank steering information is unreliable.
    • Glideslope Flag: In ILS mode, indicates ILS signals are not being received.

Operational Mechanics and Circuits of the ADI

  • Electrical Control: While appearing similar to a conventional gyro horizon, the ADI's pitch and roll elements are electrically controlled from a remote vertical gyro unit.
  • Signal Flow: Attitude changes trigger signals from pitch and roll synchros to corresponding synchros in the indicator. Error signals in the rotors are amplified and fed to servomotors.
  • Servo-Driven Mechanism: Servomotors position the pitch bar and horizon disc/sphere/cylinder and simultaneously drive synchro rotors to the "null" position.
  • Synchro/Servo Integration: The housing is full of synchro/servo devices rather than internal gyros. Command bars have dedicated servomotors to drive them to positions processed by the FDC.
  • ILS Logic:
    • If the aircraft is above the glide path, signals cause the glideslope pointer to deflect downward against the scale.
    • If the aircraft is left of the localizer/runway center-line, the localizer pointer deflects right, directing the pilot to bank right.
    • The glideslope pointer is referenced against the pitch bar; when matched at the horizontal center, the aircraft has captured the path correctly.

The Horizontal Situation Indicator (HSI)

  • Navigational Overview: Provides a plan view (view from above) of the aircraft's navigation situation, including heading on an azimuth card.
  • Compass Card and Lubber Lines:
    • The compass card rotates relative to the aircraft symbol based on heading.
    • Upper Lubber Line: Reference line for accurate heading at the nose of the symbol.
    • Lower Lubber Line: Extension of the upper line on the lower dial.
  • Deviation Section and Course Arrow:
    • The deviation section rotates with the compass card but can be adjusted via the course set knob.
    • Course Arrow: Indicates the selected course. Rotating the course set knob turns a synchro, producing an error signal that positions the CDI bar and ADI steering bars.
    • Course Arrow Tail: An extension opposite the arrow head.
  • Course Deviation Indicator (CDI):
    • Moves perpendicular to the course arrow to show deviation from the selected path.
    • Logic: If the CDI bar is left, the aircraft is to the right of the course (fly left); if right, the aircraft is to the left (fly right).
  • Course Deviation Scale: Uses four dots. The two inner dots represent 5∘5^{\circ} of deviation, while the two outer dots represent 10∘10^{\circ} of deviation in VOR mode.
  • TO/FROM Indicator:
    • White triangle appearing near the arrow head signifies "TO" (bearing pointer and arrow head within 90∘90^{\circ}).
    • White triangle near the tail signifies "FROM" (bearing pointer and arrow tail within 90∘90^{\circ}).
  • Heading Marker: A rectangular marker with a black center line positioned by the heading set knob. Rotating the knob produces a synchro error signal sent to the ADI bank steering bar.
  • Additional Indicators:
    • Bearing Pointer: An arrowhead showing the bearing to a radio navigation aid.
    • Course Selector Window: Counter display at top right mirroring the course arrow setting.
    • Range Indicator: Counter at top left showing slant range to the navigation aid in nautical miles.
  • HSI Warning Flags:
    • Off Flag: Indicates AC power failure or incorrect phase rotation.
    • Range Failure Flag: Covers range counters when signal is unavailable.
    • TO/FROM Warning Flag: Visible when the system is not locked on to the radio navigation aid.

Flight Director System (FDS) Operating Modes

  • OFF Mode: Command bars are stored out of sight. The ADI and HSI serve as basic attitude and heading references only.
  • Heading (HDG) Mode:
    • Provides lateral guidance to maintain a selected compass heading.
    • Vertical guidance is from a preselected pitch attitude.
    • Steering Logic: Positioning the heading marker (e.g., to 50∘50^{\circ}) develops an error signal in differential synchros. This is combined with bank angle to position the ADI bank steering bar.
  • NAV/LOC Mode: Lateral guidance to capture and track a VOR radial or localizer beam.
  • Altitude Hold (ALT): Provides vertical guidance to maintain desired altitude using signals from the Central Air Data Computer (CADC).
  • Approach Manual (APPR MAN): Used for Category I ILS approaches to track GS and LOC beams. Also known as Glide Slope (GS) mode.
  • Approach Auto (APPR AUTO): Provides tighter tracking for Category II ILS approaches. Also known as GS Auto.
  • Go-Around (GA): Selected after a missed approach. Commands a wings-level, pitch-up attitude. IAS and HDG modes are typically selected afterward.

Flight Director Computer (FDC) Channels

  • Functional Goal: The FDC processes all sensor inputs, mode selections, and pilot-set references to drive the command bars on the ADI.
  • Pitch Channel Inputs:
    • Vertical gyro (pitch attitude).
    • CADC (barometric height).
    • Radar altimeter (radar height).
    • ILS receiver (glideslope).
  • Roll Channel Inputs:
    • Vertical gyro (roll attitude).
    • ILS receiver (localizer).
    • DME receiver (slant range).
    • VOR receiver (navigation data).
    • Compass system (heading information).
  • Feedback Mechanism: When a pilot satisfies a command by steering the aircraft, the current attitude signals oppose and cancel the initial command signals, reaching a state of equilibrium.

Electronic Flight Instrument System (EFIS)

  • Purpose: Replaces multiple analogue instruments with electronic displays to reduce cockpit clutter and improve reliability/serviceability.
  • System Components:
    • 44 interchangeable CRT displays (Captain EADI/EHSI and First Officer EADI/EHSI).
    • 33 Symbol Generators (SGs) (Left, Right, and a spare Center unit).
    • 22 Control Panels.
    • 22 Source Selection Panels.
  • Redundancy Switching: The third (center) SG can be switched to either display set via electromechanical relays if a primary SG fails.
  • Remote Light Sensor: A photodiode that adjusts CRT brightness based on ambient flight deck light.
  • Data Communications: Systems are linked via digital data buses. On computerized aircraft, symbols are generated digitally from sensors using ARINC 429429, ARINC 629629, or MIL-STD 15531553 protocols.
  • Instrument Layout: Displays often conform to a standard "T" layout. Early models used dedicated CRTs, while modern systems allow all 66 displays to be multi-function and interchangeable.

Electronic ADI (EADI) and HSI (EHSI) Specifics

  • Electronic ADI Features: Displays pitch/roll, FD commands, ILS deviations, airspeed, ground speed, flight modes, and radio altitude.
  • EADI Scanning Methods:
    • Raster Scanning: Used for background shading (Cyan for sky, Yellow for ground).
    • Stroke (Vector) Scanning: Digital stroke pulsing technique for symbols and characters.
  • EADI Height Alerts:
    • Digital display at 25002500 and 10001000 feet.
    • Below 10001000 feet, it switches to a white circular scale with 100100-foot increments.
    • Decision Height (DH): A magenta marker on the scale. At DH plus 5050 feet, an aural warning sounds. At DH, the scale turns amber and flashes.
  • Electronic HSI (EHSI) Modes:
    • MAP Mode: Dynamic display with a moving map background integrated with the FMC flight plan. Displays weather radar, wind data, and waypoints.
    • PLAN Mode: Static map background displaying route data for verification before entry into the FMC.
    • ILS/VOR Modes: Heading orientation against an expanded or full compass scale. Weather radar is only visible in expanded scale.
  • Thermal Management: Both EADI and EHSI generate significant heat and require air circulation from an equipment cooling system.