TM 1-1510-225-10 3A-17 upon    detecting    a    fault.        The    system    can    either disengage  affected  autopilot  control  axes  individually or, if necessary, shut down the entire flight director/autopilot  system.    The  flight  computers  also generate  audio  alerts  if  the  autopilot  disengages  or trim fails. c.   Air  Data  Computers  (KDC  481).    The  air data computer processes pitot and static pressure and air   temperature   inputs,   and  s upplies  the  processed information to the flight computer and the altitude/vertical  speed  indicator.    The  flight  computers also   provide   the   air   data   necessary   for   the   flight management system to provide manual VNAV guidance and automatic three dimensional navigation.   The  air  data  computers  use  the  altitude/vertical  speed indicators as part of the basic system. d.  Altitude/Vertical    Speed    Indicators    (KAV 485).      The   air   data   computers   drive   the   altitude, vertical   speed,   and   density   altitude   displays   on   the altitude/vertical speed indicators. The altitude indicator    is    a    digital    counter    drum    pointer    style indicator and is combined in the same instrument with a    vertical    speed    indicator.        The    instrument    also provides   altitude   pre-selection,   alerting,   and   vertical speed pre-selection. (1)   Altitude/vertical Speed Indicator Controls, Indicators, and Functions.  Refer to Figure 3A -7. (a)   Inches of Mercury/Millibar Push- button Selector Switch.  The inches of mercury/millibar push-button  selector  switch  alternates  calibration  of the    altimeter    setting    display    between    inches    of mercury (IN HG) and millibars (MB), as annunciated. (b)   Altimeter    Scale    (hundreds    of    feet) and    Indicator    Needle.         The    altimeter    scale    and indicator needle display altitude information derived by the   air   data   computer.      The   digital   portion   of   the display  provides  altitude  resolution  to  within  20  feet when the aircraft's vertical speed is less than 1000 feet per minute, and to within 100 feet when vertical speed is greater than 1000 feet per minute. (c)   Altitude   ALERT   Annunciator.       The altitude alert annunciator illuminates when the aircraft's  current  altitude  is  within  300  to  1000  feet  of the  value  specified  in  the  altitude  pre-select display.   Upon    reaching    the    selected    altitude,    the    altitude annunciator  illuminates  again,  briefly.    An  aural  alert sounds    upon    illumination    of    the    annunciator    at 1000 feet   before   and   300   feet   outside   the   selected altitude. (d)   Altitude Select/Density Altitude Display.      The  altitude  select/density  altitude  display provides a continuous display of selected altitudes for altitude     alerting     and     flight     director     capture     and tracking,   and   momentary   display   of   current   density altitude derived by the air data computer.  Pre-selected altitudes  are  displayed  in  100-foot  increments  when selected     by     the     altitude/vertical     speed     indicator controls,   and   in   10-foot   increments   when   selected through  the  flight  management  system.    The  current density   altitude   will   be   displayed   for   approximately 5 seconds  at  the  end  of  the  air  data  systems  preflight test function.  Pressing the  PUSH  BARO  TEST push button  twice  in  rapid  succession  changes  the  display to density altitude at any time. (e)   Altitude Select/Density Altitude Annunciator.      The   altitude   select   annunciator,   ALT SEL,  or  density  altitude  annunciator,   DEN   ALT,  will illuminate to show which information is being displayed. (f)    Altitude/Vertical Speed Pre-select Control.   The  altitude/vertical  speed  pre-select control uses   dual   concentric   knobs   to   control   altitude   and vertical speed pre-selection.  To pre-select altitude, the smaller knob should be pressed to ensure that it is in the  inner  position.    Rotating  the  smaller  knob  adjusts the  pre-selected  altitude  in  100-foot  increments,  with automatic  rollover  to  higher  values.    The  outer  knob adjusts altitude in 1000-foot increments.  Altitudes may also   be   pre-selected  through  the  flight  management system. Pulling   the   smaller   knob   to   its   outer   position initializes  the  vertical  speed  marker,  synchronizing  it with  the  last  vertical  speed  selected  and  references the  control  knob  to  vertical  speed.    The  smaller  knob adjusts  selections  in  100  foot  per  minute  increments with   automatic   rollover.      The   larger   knob   adjusts selections in 1000 foot per minute increments. (g)   Altitude    Select    Key.      The   altitude select key, placarded SEL, is the same as the altitude select key on the flight director mode selector.  When pressed,   the   altitude   select   key   engages   the   flight director's altitude arm function in coordination with the digital altitude pre-select display on the altitude/vertical speed indicator. (h)   Vertical   Speed   Engage   Key.       The vertical   speed   engage   key,   placarded   ENG,  is  the same  as  the  vertical  speed  key  on  the  flight  director mode   selector.      When   pressed,   the   vertical   speed engage key causes the flight director to command the aircraft to climb or descend at the rate indicated by the vertical speed marker on the vertical speed indicator.