TM 1-1510-225-10 3B-28 NOTE If  VOR  is  selected  on  both  the  pilot  and copilot’s  HSI,  the  DME  readout  and  Radar NAV  display  will  accompany  the  selected autopilot flight director. c.   Air    Data    Computer.        A    digital    air    data computer located in the forward avionics compartment provides the altitude information for the pilots’ servoed altimeter,    altitude    alerter,    flight    data    recorder,   and transponder.  The computer also provides altitude and airspeed    hold    function    data    to    the    flight    control computers,  generates  and  provides  true  airspeed  to the    data    field    on    the    EHSI    with    inputs    from    the airspeed/temperature  probe.    The  air  data  computer receives 28 Vdc power through, and is protected by, a 2-ampere    circuit    breaker,    placarded     AIR    DATA- ENCODER,   located   in   the   Avionics   section   of   the circuit  breaker  panel.    All  air  data  computer  functions are   automatic   in   nature   and   require   no   flight   crew action. d.  Flight Director/Mode Selector. This provides  all  mode  selection  (except  go-around that is initiated by a remote switch) for the flight director.  The top row of light annunciated push buttons contains the lateral modes and the bottom row contains the vertical modes.    The  split-light  push  buttons  illuminate  yellow for  armed  conditions  and  green  for  captured.    When more than one lateral or vertical mode is selected, the flight director system automatically arms and captures the submode. (1)   Heading   Select   Mode.        The    Heading Select  Mode  (HDG)  is  selected  by  pressing  the  HDG button  on  the  mode  selector.    The  HDG  annunciator illuminates.      In   the   HDG   mode,   the   flight   director computer   provides   inputs   to   the   command   cue   to command   a   turn   to   the   heading   indicated   by   the heading bug on the HSI.  The heading select signal is gain  programmed  as  a  function  of  airspeed.    When HDG  is  selected,  it  overrides  the  NAV,  BC  APR  and VOR APR modes.  In the event of a loss of valid signal from the VG or compass, the command  cue is biased out of view. (a)  Navigation    Mode.        The    Navigation Mode  (NAV)  represents  a  family  of  modes  for  various navigation   systems   including   VOR,   or   Localizer   as selected by the EFIS control panel. (b)  VOR    Mode.        The    VOR    Mode    is selected   by   pressing   the   NAV  button  on  the  mode selector  with  the  navigation  receiver  tuned  to  a  VOR frequency  and  DME  greater  than  20  miles  from  the station.      Prior   to   VOR   capture,   the   command   cue receives   a   heading   select   command   as   described above  and  the  HDG  mode  annunciator  on  the   mode selector    is    illuminated    along    with    the    NAV    ARM annunciator    on    the    mode    selector.        Upon    VOR capture, the system automatically switches to the VOR mode,  HDG  and  NAV  ARM  annunciators  extinguish, and  NAV  CAP  annunciator  on  the  mode  selector  will illuminate.    At  capture,  a  command  is  generated  to capture  and  track  the  VOR  beam.    VOR  deviation  is gain  programmed  as  a  function  of  distance  from  the station. This programming corrects for beam convergence   thus   optimizing   the   gain   through   the useful   VOR   range.      To  utilize  this  feature  the  DME must  be  tuned  to  the  same  VOR  station  as  the  NAV receiver that is feeding the flight director.  The course error   signal   is   gain   programmed   as   a   function   of airspeed.        Crosswind    washout    is    included    which maintains  the  aircraft on beam center in the presence of  crosswind.    The  intercept  angle  and  DME  distance are  used  in  determining  the  capture  point  to  ensure smooth and comfortable performance during bracketing. When  passing  over  the  station,  an  overstation sensor   detects   station   passage   removing   the   VOR deviation signal from the command until it is no longer erratic.    While  over  the  station,  course  changes  may be made by selecting a new course on the EHSI. If   the   NAV   receiver   is   not   valid   prior   to   the capture point, the lateral beam sensor will not trip and the   system   will   remain   in   the   HDG   mode.      After capture,  if  the  NAV  receiver,  compass  data  or  vertical gyro go invalid, the command cue will bias out of view.   The  NAV  CAP  annunciator  on  the  mode  selector  will extinguish if the NAV receiver becomes invalid. (c)  VOR   Approach    Mode.       The    VOR Approach   Mode   is   selected   by   pressing   the    NAV  button    on    the    mode    selector    with    the    navigation receiver   tuned   to   a   VOR   frequency   and   less   than 20 DME  miles  from  the  station.    The  mode  operates identically to the VOR mode with the gains optimized for a VOR approach. (d)  Localizer  Mode.  The Localizer Mode (LOC) is selected by pressing the   NAV button on the mode  selector  with  the  navigation  receiver  tuned  to  a LOC  frequency.    Mode  selection  and  annunciation in the   LOC   mode   is   similar   to   the   VOR   mode.      The localizer   deviation   signal   is   gain   programmed   as   a function  of  radio  altitude,  time,  and  airspeed.    If  the radio    altimeter    is    invalid,    gain    programming    is    a function   of   glide   slope   capture,   time   and   airspeed.   Other valid logic is the same as the VOR mode. (e)  Back    Course    Mode.          The     Back course   Mode   (BC)   is   selected   by   pressing   the    BC button on the mode selector.  The normal front course for  the  localizer  beam  is  set  for  the  selected  course.