TM 55-1510-219-10 (c)    Navigation mode.  When NAV mode is  selected  (AP  mode  select  panel),  the  system  initially switches  to  the  NAV  ARM  heading  hold  submode,  as shown by illumination of NAV ARM and HDG indicators (AP/flight director annunciator panel).  The autopilot will then  command  the  aircraft  to  follow  the  heading  under the heading marker on the pilot's HSI (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 the 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 (AP/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. (d)    Back-course   mode.      When   BACK LOC  mode  is  selected  on  the  autopilot  mode  selector panel, localizer capture is the same as in a frontcourse approach    in    NAV    or    APPR    mode.        Glideslope    is inhibited during a back-course 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   (AP   mode   select   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    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  lamp  will  illuminate and all preselected vertical modes will be cleared. (f)    Go-around mode.  Pressing the GO AROUND button on the outboard side of the left power lever  selects  the  go-around  mode.    Go-around  mode may  be  selected  from  any  lateral  mode  (HDG,  NAV, APPR,   or   BACK   LOC).      When   go-around   mode   is selected:   (1)the   autopilot   is   disengaged,   (2)   the   GA annunciator  lamp  will  illuminate,  and  (3)  a  command presentation for wings level and    3-38 7°  nose up pitch attitude   will   appear   on   the   pilot's   horizon   reference 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  HDG  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. (g)    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. (h)    Control wheel steering mode. Pressing   one   of   the   PITCH   SYNC   &   CWS   switches located on each control wheel disconnects the autopilot servos  from  the  control  surfaces,  allows  the  pilot  to  fly the aircraft to a new pitch attitude, and synchronizes the vertical command bar (pilot's horizon ref ind) 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 BACK LOC modes, releasing the PITCH SYNC & CWS switch will cause the autopilot to couple to the previously selected mode. (i)    Altitude   hold   mode.      Pressing   the ALT    switch    (AP    mode    select    panel)    when    desired altitude  has  been  reached  (with  autopilot  engaged)  will (1) cause the autopilot to fly the aircraft to maintain the barometric altitude at which the aircraft was flying when ALT    switch    was    pressed,    (2)    illuminate    the    ALT annunciator lamp (AP/flight director annunciator panel), and   (3)   display   the   altitude   hold   commands   on   the vertical command bar (pilot's horizon reference indicator) (j)    Indicated airspeed hold mode. Pressing  the  IAS  switch  (AP  mode  select  panel)  when desired    airspeed    has    been    reached    (with    autopilot engaged) will (1) cause the autopilot to fly the aircraft to maintain the indicated airspeed at which the aircraft was flying  when  IAS  switch  was  pressed,  (2)  illuminate  the IAS   annunciator   lamp   (AP/flight   director   annunciator panel),   and   (3)   display   IAS   hold   commands   on   the vertical  command  bar  of  the  pilot's  horizon  reference indicator. 3-38