TM  55-1510-220-10 coupled to the flight director and accepts steering commands from   the   computer.      Depending   on   which   selector   switch (autopilot mode selector control panel, fig.  3-20) is pressed, autopilot    operation    can    be    described    by    the    following subguidance modes. (a) Heading  mode.    When  HDG  mode  is selected  (autopilot  mode  selector  control  panel,  fig.    3-20), with  the  autopilot  engaged,  the  autopilot  will  fly  the  aircraft to, and then maintain, the heading under the heading marker on the pilot's HSI. (b) Navigation mode.  When NAV Mode is selected  (fig.    3-20),.    the  system  initially  switches  to  the NAV    ARM    heading    hold    submode,    as    shown    by    the illumination of NAV ARM and HDG indicators (fig.  3-22). The  autopilot  will  then  command  the  aircraft  to  follow  the heading  under  the  pilot's  HSI  (when  the  heading  marker  set to   produce   the   desired   VOR,   TACAN,   INS   or   localizer intercept angle).  The flight computer will compute a capture point  based  on  deviation  from  the  desired  radio  course,  the rate at which the aircraft is approaching this course, and the course   intercept   angle.      When   course   capture   occurs,   the HDG    and    NAV    ARM    annunciators    (fig.        3-22)    will extinguish  and  the  NAV  CAP  annunciator  will  illuminate. The autopilot will then track 'the selected radio or INS course with automatic crosswind correction. (c) Back-course mode.  When B/C mode is selected on the autopilot mode selector control panel (fig.  3- 20),   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 frontcourse   heading   so   that   front   course   sensing   will   be experienced. (d) Approach    mode.    When    APPR  mode is selected (autopilot mode selector control panel, fig.  :-20), 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 light will illuminate, indicating that the system is armed  for  localizer  capture.    As  the  aircraft  approaches  the localizer    beam,    the    NAV    CAP    annunciator    light    will illuminate.  Once the localizer is being tracked, the G(S ARM annunciator   light   will   illuminate.      Glideslope   capture   is dependent on localizer capture and must occur after localizer capture.  The localizer is always captured from a selected heading, but the glideslope may be captured with the autopilot operating in any vertical mode (pitch hold, altitude hold,   or   indicated   airspeed   hold),   and   from   above   (not recommended),   or   below   the   glideslope.      At   the   point   of glideslope   intercept,   the   GS   CAP   annunciator   light   will illuminate and all preselected vertical modes will be cleared. For  localizer  capture  above  the  glideslope,  NAV  should  be engaged  until  the  aircraft  has  descended  to  the  glideslope. After  descending  to  the  glideslope,  APPR  mode  should  be engaged for a coupled approach. (3) Go-around mode.  Pressing the GO AROUND button on the outboard side of the left power lever (fig.  2-8) selects   the   go-around   mode.      Go-around   mode   may   be selected from any lateral mode (HDG, NAV, APPR, or B/C). When    go-around    mode    is    selected:    (1)    the    autopilot    is disengaged, (2) the GA annunciator light will illuminate, and (3)  a  command  presentation  for  wings  level  and  7°   nose  up pitch    attitude    will    appear    on    the    pilot's    flight    director indicator. NOTE The heading marker may be preset to the     go-around     heading     after     the localizer is captured.  After go-around airspeed     and     power     settings     are established,  select  the  HI)G  mode  to clear    the    go-around    mode.        Pitch attitude  will  remain  at  that  used  for go-around    until    changed    with    the PITCH  SYNC  &  CWS  button  or  the selection of a vertical mode. (4) Pitch   hold   mode.      The   pitch   hold   mode   is selected  by  (1)  selecting  one  of  the  vertical  mode  selector switches,  or  (2)  actuating  the  pitch  synchronize  and  control wheel  steering  switch  (PITCH  SYNC  &  CWS),  located  on each control wheel (fig.  2-17). (5) Control wheel steering mode.  Pressing one of the  PITCH  SYNC  &  CWS  switches  located  on  each  control wheel  (fig.    2-17)  disconnects  the  autopilot  servos  from  the control  surfaces,  and  allows  the  pilot  to  fly  the  aircraft  to  a new pitch attitude, and synchronizes the horizontal command bar (pilot's F1)1) to aircraft attitude.  The ALT or IAS mode will disengage (if selected) when the PITCH SYNC & CWS button  is  depressed.    When  the  autopilot  is  coupled  to  the HDG,   NAV,   APPR,   or   B/C   modes,   releasing   the   PITCH SYNC & CWS switch will cause the autopilot to couple to the previously selected mode. Change 2      3-41