TM 1-1510-225-10 3B-29 BC  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 will be annunciated on the Mode Selector.  At the capture point,  BC CAP will be annunciated   with    BC   ARM   and    HDG  extinguished.   When BC is selected, the glideslope circuits are locked out. (f)   Approach Mode.          The     Approach (APR) Mode is used to make VOR, Localizer, and ILS approaches.      This   mode   is   more   sensitive   than  the NAV mode. (g)  Localizer Approach Mode. The approach  mode  is  used  to  make  an  ILS  approach. Pressing the  APR button with a LOC frequency tuned arms  both  the  localizer  and  glideslope  modes.    No alternate  NAV  source  can  be  selected  and  the  NAV receiver must be tuned to an ILS frequency.  When the APR  button  is  pressed  and  the  above  conditions  are met,   both   the   NAV   and   APR   modes   are   armed   to capture    the    localizer    and    glideslope,    respectively.   Operating LOC mode is the same as described above except,  if  the  radio  altimeter  is  invalid  in  APR  mode, gain  programming  is  a  function  of  glideslope  capture, time, and airspeed. 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 capt ure, if the NAV receiver,  GS  receiver,  compass  data  or  vertical  gyro becomes invalid, command cue will bias out of view.  If the   radio   altimeter   is   not   valid,   the   glideslope   gain programming  is  a  function  of  GS  capture,  time,  and airspeed. (h)  Pitch   Hold   Mode.     Whenever  a  roll mode is selected without a pitch mode, the 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 command  cue  will  be  biased  out  of  view  if  the  VG  is not valid. (i)    Altitude    Hold    Mode.        The    Altitude Hold   Mode   (ALT)   is   selected   by   pressing   the   ALT button on the mode selector.  When ALT is selected, it overrides the APR CAP, GA, IAS. VS. ALT SEL CAP. or  PITCH  HOLD  modes.    In  the  ALT  mode  the  pitch command  is  proportional  to  the  altitude error provided by  the  air  data  computer.    The  altitude  error  signal  is gain  programmed  as  a  function  of  airspeed.    Pressing and    holding    the     TCS    button    allows    the    pilot    to maneuver the aircraft to a new Altitude Hold reference without disengaging the mode.  Once ALT is engaged, the mode will be reset if the air data computer is not valid and the command cue will bias out of view if the VG is not valid. NOTE If    the    Baro    setting    on    the    altimeter    is changed,  a  command  is  generated  to  fly the   aircraft   back   to   the   original   altitude reference. (j)    Indicated  Airspeed  Hold  Mode.    The Indicated   Airspeed   (IAS)   Hold   mode   button   on   the mode selector.  When IAS is selected, it overrides the APR   CAP,   GA,   ALT,   VS,   ALTSEL   CAP,   or   PITCH HOLD 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  and  the command  cue  will  bias  out  of  view  if  the  VG  is  not valid. (k)  Vertical    Speed    Hold    Mode.        The Vertical Speed (VS) 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, IAS, or PITCH HOLD 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  VS  reference  without  disengaging  the  mode.   Once engaged in the VS mode, the mode will be reset if the air data computer is not valid and the command cue will bias out of view if the VG is not valid. (l)    Altitude Pre-select Mode. The Altitude   Pre-select   Mode   (ALTSEL)   is   selected   by