TM 1-1510-218-10 3A-34 (8)   IAS Indicator.  Illuminates when airspeed hold mode is selected. (9)   AP   DISC   Indicator.      Illuminates   when autopilot is disengaged. (10)   BACK  LOC  Indicator.    Illuminates  when back-course mode is selected. (11)   GS   CAP   Indicator.        Illuminates   when glideslope is captured. (12)   NAV   CAP   Indicator.      VOR-I   or   VOR-2 selected illuminates when selected radial is captured.   VLF  selected  illuminates  when  VLF  is  coupled  to  the flight director. h.  Autopilot Modes of Operation. (1)   Attitude   Mode.      The   autopilot   is   in   the attitude  mode  when  the   ENG  /  DIS  switch,  autopilot mode  selector  panel,  is  in  the   ENG  position  and  no mode selector switches (HDG,  NAV, etc.), have been selected.  Refer to Figure 3A-19.  The autopilot will fly the  aircraft  and  accept  pitch  and  roll  rate  commands from the autopilot pitch-turn panel, Figure 3A-20. (2)   Guidance Mode.  When the autopilot is in the  attitude  mode  and  a  mode  selector  switch,   HDG, NAV,  etc.,  is  pressed,  the  autopilot  is  coupled  to  the flight  director  and  accepts  steering  commands  from the   computer.      Depending   on   which   mode   selector switch on the autopilot mode selector control panel is pressed,  autopilot  operation  can  be  described  by  the following sub-guidance modes. (a)   Heading    Mode.        When    the    HDG mode is selected on the autopilot mode selector panel with  the  autopilot  engaged,  the  autopilot  will  fly  the aircraft to the heading, and then maintain the heading under   the   heading   marker   on   the   pilot's   horizontal situation indicator. (b)   Navigation   Mode.      When   the   NAV mode is selected  on the autopilot mode selector panel, the system initially switches to the NAV ARM heading- hold  submode,  as  shown  by  illumination  of  the   NAV  ARM    and   HDG   indicators   on   the   autopilot/   flight director   annunciator   panel.      The   autopilot   will   then command the aircraft  to  follow  the  heading  under  the heading    marker    on    the    pilot's    horizontal    situation indicator, with the heading marker set to produce the desired  VOR  or  localizer  intercept  angle.    The  flight computer   will   compute   a   capture   point   based   on deviation  from  desired  radio  beam,  the  rate  at  which the aircraft is approaching this beam, and the course intercept angle.  When beam capture occurs, the HDG and  NAV  ARM  indicator  lamps  on  the  autopilot/flight director annunciator panel will extinguish and the  NAV CAP lamp will illuminate.  The autopilot will then track the  selected  radio  course  with  automatic  crosswind correction. (c)   Navigation Mode (With Linear Deviation).        The    system    features    linearized    VOR deviation when a VORTAC is being used.  A LIN DEV  light   on   the   autopilot   annunciator   will   illuminate   to indicate  operation.    The  lateral  deviation  bar  indicates the distance in nautical miles from the selected radial regardless  of  how  close  the  aircraft  is  to  the  ground station.        Linear    deviation    measures    the    aircraft's displacement    in    nautical    miles    from    the    selected course rather than degrees of displacement associated    with    normal    VOR    navigation.        Linear deviation permits flying parallel to any selected course by  maintaining  the  appropriate  needle  deflection  on the horizontal situation indicator.  When the  LIN DEV  light   is   on,   the   flight   director   system   is   obtaining distance   data   from   the   DME   and   bearing   from   the VOR-1 receiver.  Linear deviation operates only when DME  is  on  NAV  1  receiver  and  is  displayed  on  pilot's indicator   only.      For   en   route   operation   in   the   NAV mode,  full-scale  deflection  of  the  lateral  deviation  bar equals  10  miles  from  the  selected  radial.    For  VOR approach    operation,    the    APPR    mode    should    be selected.      This   provides   linear   deviation   with   the sensitivity limits of the computer increased so that full- scale   deflection   of   the   lateral   deviation   bar   equals 1 mile from the selected radial.  APPR mode should be selected when within 10 miles of the final approach fix.   Capture is the same as in the NAV mode. NOTE Failure   to   select   APPR   for   VORTAC   or VOR/DME    instrument    approaches    when using   linear   deviation   may   result   in   not meeting obstruction clearance criteria. (d)   Backcourse    Mode.        When    BACK LOC mode is selected on the autopilot mode selector panel,   localizer   capture   is   the   same   as   in   a   front- course  approach  in  NAV  or  APPR  mode.    Glideslope is  inhibited  during  a  backcourse  approach.    The  HSI must be set to the front-course heading so that lateral deviation will be directional. (e)   Approach  Mode.  When APPR mode is   selected   on   the   autopilot   mode   selector   panel, localizer capture is the same as in the NAV mode but glideslope     arm     and     capture     functions     are     also provided.  When the APPR mode is selected, the  NAV ARM   annunciator  lamp  will  illuminate,  indicating  that the   system   is   armed   for   localizer   capture.      As   the aircraft approaches the localizer beam, the   NAV  CAP  annunciator lamp will illuminate.  Once the localizer is being  tracked,  the    GS   ARM  annunciator  lamp  will