TM  55-1510-221-10 (6.)  Digital  COURSE  counter.  Provides  a digital readout of selected magnetic course. (7.)  HDG  flag.  Indicates   loss   of   reliable heading   information. (8.)  Lubber  line  marks.  Fixed  heading marks  located  at  the  fore  (upper)  and  aft  (lower) position. (9.)  Course  pointer.  The  yellow  course pointer  is  positioned  on  the  heading  dial  by  the course knob to select a magnetic bearing that coin- cides  with  the  desired  VOR  or  TACAN  radial  or INS or localizer course. The course pointer rotates with the heading dial to provide a continuous read- out of course error to the computer. (10.) DIST  display.  Provides  digital  display of station distance. (Il.)  Heading  bug.  The   notched   orange heading  bug  is  positioned  on  the  rotating  heading dial by the heading knob, and displays preselected compass heading. The bug rotates with the heading dial. (12.)  Bearing  pointer.  The  bearing  pointer provides magnetic bearing to a selected TACAN or VOR station or INS waypoint. (13.)  Glideslope  pointer/scale.  The  glide slope  pointer  displays  glide  slope  deviation.  The pointer is in view only when tuned to a localizer fre- quency. If the aircraft is below glide slope path, the pointer is displayed upward on the scale. Each dot on  the  scale  represents  approximately  0.4  degree  dis- placement. (14.) NAV flag. Indicates  that  information derived from the selected navigational source (VOR, TACAN or INS) is invalid and should not be used. (15.)  To-from  pointers.  The  to-from  point- ers  aligned  on  the  course  pointer,  are  located  180 degrees apart. One always points in the direction of the  station,  along  the  selected  VOR  or  TACAN radial or toward INS waypoint. (16.)  Heading  knob.  Positions  the  heading bug to a preselected compass heading. (17.)  Course  deviation  dots.  In  VOR, TACAN or INS operation, each dot represents a 5 degree deviation from the centerline (° 10 degrees). In ILS operation, each dot represents 1 degree devi- ation  from  the  centerline.  In  INS  operation,  each dot represents a 3.75 nautical miles deviation from centerline. (18.)  Aircraft  symbol.  The  fixed  miniature aircraft symbol corresponds to the longitudinal axis 3-24 of  the  aircraft  and  lubber  line  markings.  The  symbol shows aircraft position and heading with respect to a radio course and the rotating heading (azimuth) dial. (19.)  Course  knob.  Positions  the  course indicator. (20.)   Copilot's COURSE   INDICATOR switch fig. 2-29). Selects desired source of data for display on copilot’s HSI. 1  system. (a.)  VOR  1.  Selects  data  from  VOR 2  system. (b.)  VOR  2.  Selects  data  from  VOR (c.)  TACAN.  Selects   data   from TACAN system. (d.) INS. Selects  data  from  INS. 3-19. PILOT’S ATTITUDE DIRECTOR INDICA- TOR. a.   Description.   The  pilot’s  attitude  director indicator  (ADI)  (fig.  3-12)  combines  the  attitude sphere display with computed steering information to provide the commands required to intercept and maintain a desired flight path. It also contains an eyelid display, expanded localizer, glide slope, radio altitude  display,  rate-of-turn  indicator,  mode  annun- ciators,  go-around  and  decision  height  annunciators, and  inclinometer.  Any  warning  flag  in  view  indi- cates that portion of information is unreliable. b. Controls/Indicators and Functions. (1.) Attitude  sphere.  Moves  with  respect  to the  symbolic  aircraft  reference  to  display  actual pitch  and  roll  attitude.  Pitch  attitude  marks  are  in  5 degree increments on a blue and brown sphere. (2.)  Roll  attitude  index.  Displays   actual roll  attitude  through  a  movable  index  and  fixed scale reference marks at 0, 10, 20, 30, 45, 60 and 90 degrees. (3.)  GA  (go-around)  annunciator.  Illumi- nates when go-around mode has been selected. (4.)  SPD  annunciator.  Illuminates  when airspeed is being held by the flight director, in the IAS mode. (5.)  ALT  annunciator.  Illuminates  when altitude is being held by the flight director. (6.)  HDG  annunciator.  Illuminates   when heading is being held by the flight director, in the NAV  ARM,  BC  ARM  mode.