TM 1-1510-225-10 3A-23 (c)   Navigation Mode Push-Button Selector Switch.  Pressing the navigation mode push- button   selector   switch,   placarded    NAV,  selects  the navigation  mode.    When  the   NAV  mode  is  engaged, the flight director commands roll attitudes necessary to track    the    course    selected    on    the    EHSI.        Upon selection, the  NAV mode engages either NAV ARM or NAV  capture,  depending  upon  the  aircraft's  proximity to the selected course and its closure rate.  While any horizontal tracking mode may be engaged in conjunction    with    NAV    ARM    to    provide    intercept guidance, initiation of NAV capture and track sequences    cancels    the    coexisting    mode.        If    the aircraft's deviation from the selected course centerline is  sufficiently  small,  or  if  the  rate  of  closure  with  the new    course    is    sufficiently    high,    the    flight    director initiates the NAV capture sequence immediat ely. (d)   Navigation   Mode   Arm   Annunciator.   The navigation mode arm annunciator, placarded  NAV  ARM, illuminates to indicate that the navigation mode has been selected and is armed. (e)   Approach Mode Push-Button Selector  Switch.    Pressing  the  approach  mode  push- button   selector   switch,   placarded    APR,  selects  the approach flight director mode.  The approach mode is similar   to   the   navigation   mode   with   regard   to   arm, capture, and track operations. Upon initial selection the approach mode engages  in  either  the  approach  arm or capture mode, depending     upon     the     aircraft's     closure     rate     and proximity    to    the    selected    course.        Due    to    the heightened  sensitivity  of  the  deviation  display  in  the approach   mode,   the   flight   director   may   initiate   turn commands    before    the    course    deviation    indicator displays less than a full scale deflection.  Selecting the approach  mode  after  the  aircraft  has  already  passed the  point  at  which  approach  capture  normally  would begin  may  result  initially  in  course  overshoot  due  to the flight director's roll command limits. The autopilot automatically discriminates between  front  and  back  course  approaches.    Front course/back  course  selections  are  determined  by  the relative  angle  between  the  aircraft's  heading  and  the course   selected   on   the   EHSI.      Intercept   angles  of between   0   and   105   will   cause   the   flight   control system   to   select   the   ILS   front   course.      Angles   of between  106  and  180 will cause the flight director to command back course interception and tracking. NOTE It   is   essential   that   the   course   selection arrow  on  the  EHSI  always  be  aligned  with the  ILS  front  course.    Failure  to  align  the course     arrow     properly     may     result     in erroneous front course/back course selection by the flight control system. The   flight   director   automatically   engages   the glideslope  arm,  capture,  and  track  sequences  during ILS  front  course  approaches.    Glideslope  coupling  is inhibited during back course procedures. Any horizontal tracking mode may be employed during approach arm phases, but will cancel automatically  upon  initiation  of  approach  capture  and track. (f)    Approach    Mode    Arm    Annunciator.   Illumination  of  the  approach  arm  mode  annunciator, placarded   APR   ARM,   indicates   that   the   approach mode has been selected and is armed. (g)   Flight    Director    Mode    Push-Button Selector   Switch.      Pressing   the   flight  director  mode push-button   selector   switch,   placarded   FD,  initiates flight  director  functions  independently  of  the  autopilot or  yaw  damper.    The  flight  director  will  engage  roll attitude hold and pitch attitude hold and the command bar   will   come   into  view  synchronized  to  the  current aircraft   attitude.      Roll   attitudes   of   less   than  2   will revert to wings level flight. NOTE It   is   not   necessary   to   press   the   flight director    key    prior    to    selecting    another flight  director  mode.    Selecting  any  flight director mode initiates flight director commands in that mode. Pressing the flight director key a second time will disengage the flight director. NOTE The flight director will not disengage if the autopilot is in use. (h)   Flight    Director    Mode    Annunciator.   Illumination   of   the   flight   director   mode   annunciator, placarded  FD,  indicates  that  the  flight  director  mode has been selected and is engaged. (i)    Not used. (j)    Not used. (k)   Not Used.