TM 1-1510-218-10 3A-18 3A-15.  PILOT'S HORIZONTAL SITUATION INDICATOR. a.   Description.    The  pilot's  Horizontal  Situation Indicator   (HSI)   provides   heading,   course   deviation, and glideslope deviation information.  Information from this instrument is also fed to the flight control system. b.  Associated Controls/Indicators and Function.  Refer to Figure 2-17, Sheet 1. (1)   Pilot's  VOR 1 / VOR 2  Switch.   Controls course select and display circuits of the HSI. (a)  VOR  1.      Circuits   are   connected   to VOR No. 1 receiver. (b)  VOR  2.      Circuits   are   connected   to VOR No. 2 receiver. (2)   Pilot's    VOR    /    GPS    Switch.        Selects information  for  pilot's  HSI  display  and,  when  pressed, illuminates the HSI display. (3)   Pilot's    COMPASS    #    1    /    #   2    Switch.   Selects desired source for magnetic heading information for display on compass card of indicator. (a)  # 1.  Selects compass system No. 1 for display. (b)  # 2.  Selects compass system No. 2 for display. c.   Controls/Indicators  and  Functions.    Refer to Figure 3A-8. (1)   Compass Card. Indicates aircraft magnetic   heading   supplied   by   system   selected   on pilot's COMPASS # 1 / # 2 switch. (2)   Heading    Marker.      Positioned   by   HDG knob to selected heading. (3)   Lubber     Line.          Indicates     heading     of aircraft. (4)   Course  Arrow.    Positioned by   COURSE knob to selected VOR radial. (5)   HEADING Flag.  Indicates loss of reliable heading information. (6)   COURSE   Indicator.      Presents   a   digital readout of course selected by the  COURSE knob. (7)   NAV  Flag.    VOR  1  or  VOR  2  selected indicates  loss  of  or  unreliable  navigation  signal  VLF selected.    The  CMA-734 drives the normal VOR/LOC warning flag when VLF information is displayed on the pilot's  HSI.    The  warning  flag  will  be  in  view  any  time the  VLF  information  is  invalid  or  suspected,  or  when any   of   the   system   status   annunciators   (SYS,   SYN, AMB, or  DR) are illuminated.  If the flag is in view with the Ambiguity (AMB) annunciator illuminated, the flag may be cancelled by pressing the BK key on the CDU. (8)   COURSE  Control  Knob.    Used  to  select desired VOR course. (9)   Course  Deviation  Bar.     Indicates  lateral course deviation selected by the pilot's VOR  # 1 /  # 2,  or VLF switch, if installed. (10)   To-From     Arrow. Indicates     direction toward  the  VOR  station  or  VLF  waypoint  (if  installed and  if  VLF  is  selected)  along  the  course  selected  by the  COURSE  knob.    The  arrow  is  not  visible  when  a localizer frequency is selected. (11)   HDG    Control    Knob.        Used    to    select desired heading. (12)   GS Flag.  Covers glideslope pointer when not receiving glideslope information. (13)   Glideslope   Pointer.      Displays   deviation from correct glideslope during ILS approach. 3A-16.  COPILOT'S HORIZONTAL SITUATION INDICATOR. a.   Description.        The    copilot's    HSI    provides heading,   course   deviation,   and   glideslope   deviation information.  It  also  switches  to  back  localizer  sensing whenever   a   localizer   frequency   is   tuned,   and   the selected  course  and  aircraft  heading  differ  by  more than 105. b.  Associated Controls and Functions.   Refer to Figure 2-17, Sheet 1. NOTE If    the    pilot's    and    copilot's    VOR    1    /    2 switches are in the same position, the pilot has  control  of  the  course  select  circuits  of the  selected  receiver  and  the  copilot  can only   monitor   deviation   information   from the   selected   receiver.      A   PILOT   SELECT indicator will illuminate to notify the copilot that  he  has  selected  the  same  receive r as the pilot. When    the    pilot    has    GPS    selected,    the copilot  has  the  option  of  selecting  VOR  1 or VOR 2 on the copilot's HSI.