TM 1-1510-218-10 3B-28 With the APR mode armed, the pitch axis can be in any one of the other pitch modes except go-around.   When reaching the vertical beam sensor trip point, the system automatically switches to the glideslope mode.   The pitch mode and APR     ARM     annunciators extinguish and   APR  CAP  annunciator  illuminates  on the controller.  At capture, a command is generated to asymptotically approach the glideslope beam.   Capture can be made from above or below the beam.   The  glideslope  gain  is  programmed  as  a  function  of radio   altitude,   time   and   airspeed.      The   APR   CAP  annunciator on the mode selector will extinguish if the GS receiver becomes invalid after capture. Glideslope   capture   is   interlocked   so   that   the localizer must be captured prior to glideslope capture.   If  the  glideslope  receiver  is  not  valid  prior  to  capture, the  vertical  beam  sensor  will  not  trip  and  the  system will remain in the pitch mode.  After capture, if the NAV receiver,  GS  receiver,  compass  data  or  vertical  gyro becomes  invalid,  ADI  command  cue  will  bias  out  of view.  If the radio altimeter is not valid, the glideslope gain  programming  will  be  a  function  of  GS  capture, time and airspeed. While in the localizer approach mode, the pilot is able    to    use    a    Flight    Management    System    (FMS) preprogrammed    runway/outer    marker    waypoint    for reference, by pressing the OMEGA DIST/BGR switch, located  on  the  pilot's  instrument  panel,  Figure  2-17, Sheet 2.  Pressing the switch while in the APR (NAV) mode   with   the   NAV  CTL  mode  selector  switch  also pressed, will cause the waypoint distance and bearing to   display   on   the   pilot's   HSI.      This   function   will   be disabled if the FMS switch and/or the TACAN mode of operation are selected. (a)   Pitch   Hold   Mode.    Whenever  a  roll mode   is   selected   without   a   pitch   mode,   the   ADI command    cue    will    display    a    pitch    attitude    hold command.      The   pitch   attitude   can   be   changed   by pressing   the   TCS   button   on   the   control   wheel   and maneuvering  the  aircraft.    The  command  cue  will  be synchronized   to   zero   while   the   button   is   pressed.   Upon release of the button, the pitch command will be such as to maintain the new pitch attitude.  In the pitch hold mode, the ADI command cue will be biased out of view if the VG is not valid. (b)   TACAN  Mode.    The  TACAN  mode  is selected  by  pressing  the   DME/TAC  alternate  source selector   switch   located   on   the   pilot's   radio   control panel.    Annunciators,  placarded   TACAN/ADF  1,  and located on the instrument panel (both pilot and copilot side) will illuminate.  TACAN navigation information will display   on   the   pilot's   HSI,   and   on   the   DME   slave indicator. NOTE The NAV/TAC receiver must be tuned to a valid TACAN frequency. TACAN    functions    are    identical    to    VOR using TACAN information rather than VOR signals.  The ARM/CAP annunciation is the same as in VOR mode. (4)   Back    Course    Mode    (BC).        The    back course mode is selected by pressing the  BC button on the mode selector.  Back course operates the same as the  LOC  mode  with  the  deviation  and  course  signals reversed   to   make   a   back   course   approach   on   the localizer.  When BC is selected, and outside the lateral beam sensor trip point, BC ARM and HDG annunciators will illuminate.  At the capture point,  BC CAP   will   be   annunciated   with   BC   ARM   and   HDG annunciators extinguished.  When BC is selected, the glideslope circuits are locked out. (5)   Standby     Mode    (SBY ).        The    standby mode  is  selected  by  pressing  the   SBY  button  on  the mode  selector.    This  resets  all  the other flight director modes  and  biases  the  ADI  command  cue  from  view.   While  pressed,  SBY  acts  as  a  lamp  test  causing  all mode annunciators to illuminate and the flight director warning  flag  on  the  ADI  to  come  in  view.    When  the button    is    released,    the    mode    annunciator    lights extinguish  and  the  flight  director  warning  flag  retracts from view. (6)   Indicated     Airspeed     Hold     Mode    (IAS).   The   indicated   airspeed   hold   mode   is   selected   by pressing the  IAS button on the mode selector.  When IAS  is  selected,  it  overrides  the  APR  CAP,  GA,  ALT, VS,  and  ALTSEL  CAP  modes.    In  the  IAS  mode  the pitch    command    is    proportional    to    airspeed    error provided   by   the   air   data   computer.      Pressing   and holding the   TCS  button  allows  the  pilot  to  maneuver the  aircraft  to  a  new  airspeed  hold   reference  without disengaging   the   mode.      Once   engaged   in   the   IAS mode, the mode will be reset if the air data computer is not valid.  The ADI command cue will bias out of view if the VG is not valid. (7)   Vertical   Speed   Hold   Mode   (VS).      The vertical speed hold   mode  is  selected  by  pressing  the VS   button    on    the    mode    selector.        When    VS    is selected, it overrides the APR CAP, GA, ALT, ALTSEL CAP,  and  IAS  modes.    In  the  VS  mode,  the  pitch command  is  proportional  to  VS  error  provided  by  the air   data   computer.      Pressing   and   holding   the   TCS  button  allows  the  pilot  to  maneuver  the  aircraft  to  a new vertical speed hold reference without disengaging the mode.  Once engaged in the VS mode, the mode will be reset if the air data computer is not valid.  The ADI command cue will bias out of view if the VG is not valid.