TM 1-1510-225-10 3C-23 shows  aircraft  position  and  heading  with  respect  to  a radio course and rotating heading dial. (13)   Aft Lubber Line  –  Fixed  heading  mark  at the aft position. (14)   Reciprocal  Course  Pointer   –  Positioned on the heading dial in a direction 180 from the course pointer. (15)   Heading  Knob   –   Positions  the  heading bug   on   the   rotating   heading   dial   and   displays   pre- selected compass heading. (16)   Vertical  Warning  Flag    –  When  in  view, indicates the vertical information is unreliable. (17)   Vertical    Deviation    Pointer    –     Displays glideslope   deviation.      Pointer   is   in   view   only   when tuned to a localizer frequency.  Aircraft is below glide path   if   pointer   is   displaced   upward   and   each   dot represents approximately 0.4 displacement. (18)   To-From Pointer  –  Two flags, 180 apart.   One always points in the direction of the station along the selected VOR radial. (19)   Course Pointer  –  Like  the  heading  bug, the course pointer also rotates with the heading dial to provide  a  continuous  readout  of  course  error  to  the computer.    When  one  of  the  radio  modes  is  selected, the  vertical  command  bar  in  the  ADI  will  display  bank commands to intercept and maintain the selected radio course. (20)   Heading Bug And Heading Knobs  –  The notched  orange  heading  bug  rotates  with  the  heading dial  so  the  difference  between  the  bug  and  the  fore lubber   line   index   is   the   amount   of   heading   error applied  to the flight director computer.  In the heading mode, the ADI will display the proper bank commands to turn to and maintain this selected heading. c.     HSI   Failure   Warning   System.      The   HSI failure  warning  system  monitors  and  compares  lateral steering signals for VOR, ILS, localizer, TACAN.  The system  also  monitors  and  compares  vertical  steering signals for ILS glideslope.  Lateral steering failures are displayed  by  the  HSI  NAV  flag,  and  vertical  steering failures  are  displayed  by  the  HSI  VERT  (glideslope) flag. The system provides independent failure warning  capabilities  and  failure  warning  test  switches for  both  pilot  and  copilot.    Failure  warning  priority  is provided  for  failures  within  the  active  NAV  receiver.   The    pilot    and    copilot    test    switches    are    on    the autopilot/flight  director  transfer  panel  located  on  the extended pedestal. The  circuit  receives  28  Vdc  power  from  the  #1 and #2 avionics buses routed through each respective navigation  receiver,  i.e.,    NAV   1   /   NAV   2,   TACAN.    This   design   provides   a   built-in   power    redundancy since  only  one  navigation  receiver  is  required  to  be operating at any given time to provide sufficient power for  effective  failure  warning  to  exist.    Once  active,  the failure warning system automatically monitors steering signals   for   the   navigation   system   selected   by   the operator and displayed on the HSI. The EFWS monitors the failure prone circuits of the Course Deviation Indicator (CDI) system.  The CDI system   receives   input   from   the   VOR/LOC,   TACAN, and   GLS   (Glideslope   systems)   through   a   common printed circuit (PC) board.  The EFWS compares these input   signals   to   an   expected   output   signal.      If   the signals   do   not   match,   this   indicates   failure   of   the switching   relays,   connector   pins,   or   the   PC   board traces  for  the  LT  /  RT  or  UP  /  DN  deviation steering signals and the appropriate warning flags are displayed.    The  EFWS  cannot  determine  if  the  input signals  are  unreliable;  therefore,  the  EFWS  will  not detect     a     failure     of     the     VOR/LOC,     TACAN,     or ANTENNA systems.  It will not detect a failure of their associated wiring, connectors, or switching relays.  In addition,  the  EFWS  will  not  detect  a  failure  inside  the CDI. The failure warning system includes an individual and  separate  self-test  circuit  for  the  pilot  and  copilot.   Once activated, the self-test circuit will bias both VERT and  NAV  flags  in  view  when  the  deviation  exceeds +7.5 millivolts.  The flags will bias out of view when the deviation is less than +7.5 millivolts, or when the self- test  circuit  is  de-energized  (provided  a  valid  signal  is being received). A failure flag is displayed when the NAV signals monitored  by  the  HSI  failure  warning  system  differ  by more  than  7.5  millivolts  for  both  lateral  and  vertical deviation.      Filtering   is   provided   to   eliminate   false warnings    due    to    excessive    noise    and/or    spurious signals caused by switching or electromagnetic interference. NOTE 7.5  millivolts  correspond  to  approximately 0.1   dot   displacement   of   the   CDI   and   GS indicator bars.  3C-18.  AUTOMATIC FLIGHT CONTROL SYSTEM. a.     Description.    The  Automatic  Flight  Control system    is    a    completely    integrated    autopilot/flight