TM 1-1510-218-10   3C-49 (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 EADI command bars from view.  While pressed,  SBY  acts  as  a  lamp  test  causing  all  mode annunciators to illuminate and  the  FD  warning  flag  on the   EADI   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 EADI command cue will bias out of view if the vertical gyro 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  vertical gyro is not valid. (8)   Altitude  Preselect  Mode  (ALTSEL).    The altitude  preselect  mode  is  selected  by  pressing  the ALTSEL  button  on  the  mode  selector.    The  desired altitude is selected on the altitude preselect controller.   Pitch hold, VS, or IAS may be selected as a mode to fly to the selected altitude.  When outside the altitude bracket trip point, the ALTSEL ARM annunciator along with   the   selected   pitch   mode   is   illuminated   on   the mode  selector.    When  reaching  the  bracket  altitude, the system automatically switches to the ALTSEL CAP mode   and   the   previously   selected   pitch   mode   is cancelled.  When the altitude is reached, the ALTSEL CAP  mode  is  automatically  cancelled  and  the  flight director switches to the ALT hold mode.  If the air data computer   is   not   valid,   the   altitude   preselect   mode cannot be selected.  The EADI command bars will bias out of view if the vertical gyro 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. (9)   Altitude  Hold  Mode  (ALT).    The  altitude hold mode is selected by pressing the   ALT  button  on the  mode  selector.    When  ALT  mode  is  selected,  it overrides  the  APR  CAP,  GA,  IAS,  VS,  and  ALTSEL CAP 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 engaged in the altitude hold   mode,   the   mode   will   be   reset   if   the   air   data computer is not valid  and the EADI command bars will bias out of view if the vertical gyro is not valid. (10)   Pitch  Hold  Mode.    Whenever a roll mode is selected without a pitch mode, the EADI 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 EADI  command  bars  will  be  biased  out  of  view  if  the vertical gyro is not valid. (11)   Go-around  Mode.    The  go-around mode is  selected  by  pressing  one  of  the  remote  go-around switches.  One go-around switch is located on the left power lever, Figure 2-9, and the other is located on the copilot's   control   wheel.      When   selected,   all   other modes  are  reset  and  the  remote  go-around (GA) and yaw damp (YD ENG) annunciators will be illuminated.   The   EADI   command   cue   receives   a   wings   level command   (zero  command  when  roll  is  zero).    The command  cue  also  receives  the  go-around command that    is    a    7°    visual    pitch    up    attitude    command.   Selecting  GA  disconnects  the  autopilot;  however,  the yaw damper remains on.