TM 1-1510-218-10 3B-26 3B-25.  AUTOMATIC FLIGHT CONTROL SYSTEM. a.    Description.    The  Automatic  Flight  Control System    is    a    completely    integrated    autopilot/flight director/air data system that has a full complement of horizontal  and  vertical  flight  guidance  modes.    These include all radio guidance modes and air data oriented vertical modes. When    engaged    and    coupled    to    the    Flight Director  (FD)  commands,  the  system  will  control  the aircraft  using  the  same  commands  displayed  on  the attitude    director    indicator.        When    engaged    and uncoupled  from  the  flight  director commands, manual pitch  and  roll  commands  may  be  inserted  using  the Touch  Control  Steering  (TCS ) or the pitch wheel and turn knob. When the autopilot is coupled, the flight director instruments act as a means to monitor the performance of the autopilot.  When the autopilot is not engaged,  the  same  modes  of  operation  are  available for flight director only.  The pilot maneuvers the aircraft to  satisfy  the  flight  director  commands,  as  does  the autopilot when it is engaged. b.    Air   Data   Computer.      A    digital    air    data computer, located in the forward avionics compartment, provides the altitude information for the pilot's  encoding  altimeter,  altitude  alerter,  flight  data recorder,    and    transponder.        The    computer    also provides altitude and airspeed hold function data to the flight    control    computers.        The    air    data    computer receives 28 Vdc power through, and is protected by, a 2-ampere    circuit    breaker,    placarded    AIR   DATA   – ENCDR,   located   in   the   AVIONICS   section   of   the overhead  circuit  breaker  panel,  Figure  2-16, Sheet 3.   All air data computer functions are automatic in nature and require no flight crew action. c.    Autopilot  Flight  Director  Transfer  Switch.   An alternate action autopilot and flight director transfer switch, placarded  AP FD 1 and  AP FD 2, is located on the  pilot's  instrument  panel  directly  below  the  glare shield.    Refer  to  Figure  2-17, Sheet 2.  This switch is used to select which autopilot flight director computer controls   the   aircraft   flight   servos.      If    AP   FD   2   is selected, the annunciator, placarded  AP FLT DIR NO. 2,  located  on  the  copilot's  instrument  panel  directly below the glare shield, will illuminate to alert the pilot that   the   No.   2   autopilot   flight   director   computer   is controlling the aircraft. NOTE The autopilot will disengage when transferring  between  the  pilot  and  copilot flight directors.  d.    Flight  Director/Mode  Selector.    The  flight director/mode selector located on the pedestal, Figure 2-9,   Sheet   1,   provides   for   selection   of   all   modes, except  go-around  that  is  initiated  by  remote  switches located on   the  left  power  lever  and  on  the  copilot's control wheel, for the flight director. The top row of split light   annunciated   push   buttons   contains   the   lateral modes    and    the    bottom    row    contains    the    vertical modes.        The    mode    buttons    will    illuminate    when manually  selected  or  automatically  selected  through other modes.  Refer to Figure 3B -16. The split light push button annunciators illuminate  amber  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.  Mode annunciation is also presented on remote annunciator blocks,  located  above  both  (pilot  and  copilot)  Attitude Director Indicators (ADI's), and on the pilot's ADI. e.    Autopilot Modes of Operation. (1)   Heading    Mode    Selector   (HDG).        The heading select mode is selected by pressing the  HDG button on the mode selector.  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 on the ADI is biased out of view. (2)   Navigation   Mode   Selector   (NAV).    The navigation   mode   represents   a   family   of   modes   for various  navigation  systems  including  VOR,  localizer, TACAN and VLF.