CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

1. Flight instrument systems and their operating characteristics
   
   
   c. Heading indicator/ horizontal situation indicator/ remote magnetic indicator
   1) Heading indicator (HI, directional gyro, DG)
   a) HI gyroscope
   i) Double gimbal mounted
   ii) Horizontal spin axis, so senses rotation about aircraft vertical axis
   iii) Rigidity in space couses HI to maintain heading indication without the oscillation and other errors inherent in the magnetic compass
   b) Not north-seeking (except slaved gyro indicator) so must be periodically set to the heading indicated by the magnetic compass
   c) DGs usually air-driven by vacuum system; filtered air blows against buckets cut in periphery of gyro wheel
   d) Bearing friction causes gyro presession, so check and reset HI to agree with magnetic compass about every 15 minutes
   e) HI precession should be less than 3° in 15 minutes
   f) HI gyro drives a vertical compass card dial (azimuth card)
   g) Heading is indicated by nose of symbolic aircraft and lubber line on instrument glass
   h) Spring-loaded knob on front may be pushed in and turned to rotate gyro and azimuth card to reset HI, then released to disengage from gimbals
   i) Check and if necessary reset HI to agree with magnetic compass about every 15 minutes
   2) Horizontal situation indicator (HSI)
   a) HSI is a direction indicator that uses output from a flux valve to drive an azimuth card
   
   
   b) Combines magnetic compass with navigation signals and glide slope to indicate location relative to a chosen course
   c) Heading indicated by lubber line
   d) Desired course (VOR/LOC/RNAV) selected by rotating course select pointer (arrowhead) using course select knob
   e) Aircraft symbol and course deviation bar display location relative to selected course
   f) TO/FROM indicator is triangular-shaped pointer
   g) Glide slope deviation pointers below aircraft symbol indicate aircraft above glide slope
   
   
   h) If, as in some HSIs, azimuth card is not a remote indicating compass, then it must be periodically checked and reset to agree with magnetic compass
   3) Remote magnetic indicator
   a) Remotely mounted magnetic slaving transmitter
   i) Usually in wingtip to eliminate magnetic interference
   ii) Remote flux valve is the system direction-sensing device
   iii) Remote heading indicator unit torque motor is operated by amplified flux valve signal and precesses gyro to align it with signal
   iv) Connected electrically to panel-mounted pictorial navigation indicator (HSI)
   b) Panel-mounted components in a typical system
   
   i) HSI
   
   
   ii) Slaving control and compensator unit
   • Slaving meter indicates left (counterclockwise) or right compass card error
     
   • Pushbutton selects slaved or free gyro modes
     
   • In free gyro mode, adjust compass card by depressing appropriate heading drive button
   c) Radio magnetic indicator (RMI)
   
   i) Compass card driven by flux valve
   ii) One pointer driven by automatic direction finder (ADF)
   iii) Another pointer driven by very-high-frequency omnidirectional range (VOR)

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