TM 55-1510-222-10 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, airspeed, and the middle marker. (8)  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 CWS button on the  control  wheel  and  maneuvering  the  aircraft.    The command  cue  will  be  synchronized  to  zero  while  the button  is  depressed.    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. (9) TACAN Mode.  The TACAN mode is selected by  positioning  the  selector  switch,  located  in  the  pilot's instrument  panel  to  TACAN.    A  TACAN  annunciator, placarded TAC, located in the pilot's HSI, will illuminate. TACAN navigation information will display on the pilot's HSI and RMI. NOTE The TACAN receiver must be tuned to a valid TACAN frequency. TACAN   functions   are   identical   to VOR using TACAN information rather    then    VOR    signals.        The ARM/CAP annunciation is the same as in VOR mode. (10) Altitude Hold Mode (ALT).  The Altitude Hold Mode  is  selected  by  depressing  the  ALT  button  in  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.  Depressing and holding the CWS 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  ADI  command  cue  will  bias  out  of  view  if  the VG is not valid. NOTE If    the    barometric    setting    of    the altimeter is changed, a command is generated to fly the aircraft back to the original altitude reference. (11)  Indicated  Airspeed  Hold  Mode  (IAS).    The Indicated Airspeed Hold Mode is selected by depressing the  IAS  button  in  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.      Depressing   and   holding   the CWS 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. (12)    Vertical    Speed    Hold    Mode    (VS).        The Vertical Speed Hold Mode is selected by depressing the VS button in 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.      Depressing   and   holding   the   CWS   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. (13)   Altitude   Preselect   Mode   (ALTSEL).      The Altitude   Preselect   Mode   is   selected   by   pressing   the ALTSEL   button   in   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  ADI  command  cue  will  bias out of view if the VG is not valid. (14) Standby Mode (SBY).  The Standby Mode is selected  by  depressing  the  SBY  button  in  the  mode selector.  This resets all the other flight director modes and  biases  the  ADI  command  cue  from  view.    While depressed,  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   annunciators   extinguish   and   the flight director warning flag retracts from view. 3-38