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Attitude Indicator

Introduction:

  • The attitude indicator, originally called the Artificial Horizon Indicator, is a type of instrument used to reference the aircraft's pitch and bank in reference to an artificial horizon
  • Functioning off the vacuum/gyroscopic system and operating on the principal of rigidity in space, the instrument gives immediate and direct indication of pitch and bank
Instrument Flying Handbook. Figure 3-30, The dial of this attitude indicator has reference lines to show pitch and roll
Figure 1: Instrument Flying Handbook, The dial of this attitude indicator has reference lines to show pitch and roll
  • Its operating mechanism is a small brass wheel with a vertical spin axis, spun at a high speed by either a stream of air impinging on buckets cut into its periphery, or by an electric motor
  • The gyro is mounted in a double gimbal, which allows the aircraft to pitch and roll about the gyro as it remains fixed in space
  • A horizon disk is attached to the gimbals so it remains in the same plane as the gyro, and the aircraft pitches and rolls about it
  • Miniature aircraft connected to the aircraft while the gyro (horizon) remains in place
    • "Aircraft rotates around the instrument"
  • A knob at the bottom center of the instrument case raises or lowers the aircraft to compensate for pitch trim changes as the airspeed changes
  • The width of the wings of the symbolic aircraft and the dot in the center of the wings represent a pitch change of approximately 2°
  • Some attitude indicators are caged
    • Newer instruments do not have these restrictive tumble limits; therefore, they do not have a caging mechanism
  • The top half of the instrument dial and horizon disc is blue, representing the sky; and the bottom half is brown, representing the ground
  • Displays pitch from artificial horizon line in degrees (long lines: 10°, short: 5°)
  • Displays roll from bank index in degrees (10°, 20°, 30°, 60°)
  • Important for IFR but only minimal importance in VFR flight as the outside world is your attitude indicator
  • Pendulous vanes act undesirably with a turn because G's are not felt down the axis of rotation causing precession
  • Must erect itself, on the ground it uses gravity, in the air it uses pendulous vanes
  • Designed for a 360° roll and 85° pitch

Function:

  • By referencing the miniature airplane and the artificial horizon, the pilot can tell if he is flying level or in a banked or climbing turn
  • For an AI to function properly, the gyro must remain vertically upright while the aircraft rolls and pitches around it
  • The bearings in these instruments have a minimum of friction; however, even this small amount places a restraint on the gyro producing precession and causing the gyro to tilt
  • To minimize this tilting, an erection mechanism inside the instrument case applies a force any time the gyro tilts from its vertical position
  • This force acts in such a way to return the spinning wheel to its upright position

G-1000 Attitude Indicator Display
Figure 2: G-1000 Attitude Indicator Display

Limitations/Errors:

  • Attitude indicators are generally free from most errors
  • Uses pendulous vanes to correct for banking errors
  • Can tumble if 100-110° of bank or 60-70° of pitch is exceeded
  • May be caged preventing tumbling, especially during acrobatics
  • Accelerations may cause a slight pitch up indication
  • Decelerations may cause a slight pitch down indication
  • This erection can take as long as 5 minutes, but is normally done within 2 to 3 minutes
  • Attitude indicators are free from most errors, but depending upon the speed with which the erection system functions, there may be a slight nose-up indication during a rapid acceleration and a nose-down indication during a rapid deceleration
  • There is also a possibility of a small bank angle and pitch error after a 180° turn
  • These inherent errors are small and correct themselves within a minute or so after returning to straight-and-level flight
  • The older artificial horizons were limited in the amount of pitch or roll they could tolerate, normally about 60° in pitch and 100° in roll
  • After either of these limits was exceeded, the gyro housing contacted the gimbals, applying such a precessing force that the gyro tumbled
  • Because of this limitation, these instruments had a caging mechanism that locked the gyro in its vertical position during any maneuvers that exceeded the instrument limits

Preflight Check:

  • The attitude indicator should not bank more than 5° in taxi turns
  • When an aircraft engine is first started and pneumatic or electric power is supplied to the instruments, the gyro is not erect
  • A self-erecting mechanism inside the instrument actuated by the force of gravity applies a precessing force, causing the gyro to rise to its vertical position

Inertial Reference Unit (IRU), Inertial Navigation System (INS), and Attitude Heading Reference System (AHRS)

  • IRUs are self-contained systems comprised of gyros and accelerometers that provide aircraft attitude (pitch, roll, and heading), position, and velocity information in response to signals resulting from inertial effects on system components
    • Once aligned with a known position, IRUs continuously calculate position and velocity. IRU position accuracy decays with time. This degradation is known as "drift"
  • INSs combine the components of an IRU with an internal navigation computer
    • By programming a series of waypoints, these systems will navigate along a predetermined track
  • AHRSs are electronic devices that provide attitude information to aircraft systems such as weather radar and autopilot, but do not directly compute position information
  • Aircraft equipped with slaved compass systems may be susceptible to heading errors caused by exposure to magnetic field disturbances (flux fields) found in materials that are commonly located on the surface or buried under taxiways and ramps
    • These materials generate a magnetic flux field that can be sensed by the aircraft’s compass system flux detector or "gate", which can cause the aircraft’s system to align with the material's magnetic field rather than the earth's natural magnetic field
    • The system’s erroneous heading may not self-correct
    • Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi
    • Pilots are encouraged to follow the manufacturer’s or other appropriate procedures to correct possible heading misalignment before take off is commenced

Conclusion:


References: