TM 1-1510-225-10 3B-21 heading  annunciator.    The  heading  is  labeled  HDG  in black letters on a solid red box.   (24)   Dead Reckoning and Receiver Autonomous  Integrity  Monitoring  Alert.    The  display annunciates     Dead     Reckoning     (DR)     or     Receiver Autonomous  Integrity  Monitoring  (RAIM)  in  yellow  to the  right  of  the  digital  heading  readout.    If  DR  and RAIM Alert are present at the same time, DR overrides the RAIM Alert. The FMS goes into DR when it loses navigation sources.    When  FMS  goes  into  DR,  it  calculates  its present  position  based  on  inertial  data.    The  display annunciates  DR  in  yellow  to  the  right  of  the  digital display. (25)   Offset.    The  crew  can  select  a  parallel offset    (OFST)    source    via    the    FMS.        The    lateral deviation scale is relative to the parallel offset course.   OFST is annunciated to the right of the digital heading readout in white. (26)   Wind Pointer. The wind indicator consists of  a  green  symbol  loc ated  on  the  outside  edge  of  the compass  and  graphically  layered  to  not  obscure  the heading  bug  symbol,  lubber  line,  or  heading  readout.   The  wind  indicator  rotates  about  the  aircraft  symbol such    that    the    arrow    is    always    pointing    into    the compass  at  the  wind  direction.    The  arrow  displays zero  to  five  green  fins  on  its  tail  to  represent  wind speed.    Long  fins  represent  10  knots  and  short  fins represent 5 knots.  If the wind velocity is 0 knots, the arrow is removed from the display.  If the wind velocity is  less  than  2.5  knots  the  arrow  is  displayed  with  no fins.  The wind indicator indicates wind from .1 knots to 45.5 knots.  If the wind exceeds 45.5 knots, the wind indicator  changes  to  a  maximum  wind  symbol  (wind indicator with a filled triangle fin at the  tail).  When the wind  speed  or  wind  direction  is  invalid  or  unavailable, the wind indicator is removed from the display. (27)   To/From Indicator.  A white triangle to or from indicator rotates around the aircraft symbol on the course  needle.    It  indicates  TO  the  station  when  the triangle  points  towards  the  head  of  the  course  needle and    FROM    the    station    when    the    triangle    points towards  the  tail  of  the  course  needle.    If  FMS  is  the selected course and FMS outputs a waypoint alert, the TO / FROM flag flashes. If the course source is not FMS, the indicator is set  to  TO  when  the  angular  difference  between  the relative  bearing  to  the  station  and  the  primary  course is less than 89.  When the angle is greater than 91 FROM  is  indicated.    The  indicator  is  OFF  when  the angle  is  between  89   and   91.    When  FMS  is  the primary   source   then   the   FMS   determines   the   TO  / FROM   indication.      If   the   TO  /   FROM  indication  is invalid  or  unavailable,  an  active  navaid  has  not  been selected,  or  the  indicator  is  OFF,  the  TO /  FROM flag will be removed from the display. (28)   Aircraft    Symbol.        The    white    aircraft symbol is located in the center of the compass rose. f.    EHSI Operations.  (1)   OBS Backup Mode.  When the heading is invalid   the   compass   rose   rotates   about   the   aircraft such that the selected course is always referenced at the  top  of  the  display.    When  the  selected  course  is invalid the compass freezes at the last known position. (2)   Course Source Flag. When the navigation    information    for    the    selected    course    is flagged  invalid,  the  course  source  label  is  removed from  the  display  and  replaced  with  the  course  source annunciation  drawn  in  black  on  a  solid  red  box.    The deviation bars and scale are removed from the display with   a   navigation   flag.      If   the   primary   course   goes invalid,  the  selected  course  readout   is  replaced  with red dashes.  If FMS-800 or the KLN 90 is selected and an FMS NAV Warn occurs (RAIM Alert), the  MS NAV  source  annunciator  turns  yellow  and  all  the  navigation information remains on the display. (3)   LOC/BC.   If an LOC frequency has been tuned and backcourse is selected on the flight director, BC is displayed in place of the LOC source.  Once BC has  been  set  it  will  remain  set  as  long  as  the  BC discrete is set. g.  HSI ARC Format.  The compass arc format, Figure   3B-9,   is   a   graphical   representation   of    the current   heading   of   the   aircraft.      It   is   similar   to   the compass rose except only an 80 slice is shown.  The arc format is an expanded compass with the compass segment   at   the   top   of   the   display   and   the   aircraft symbol centered laterally at the bottom.   The compass rotates about the center point of the aircraft such that the   current   heading   is   indicated   directly   below   the lubber line. The   HSI   format   with   the   360   compass   rose display  is  the  same  as  the  HSI  Arc  display  with  the following exceptions: (1)   Selected HDG Readout.  A digital readout of selected heading is displayed in the upper left-hand corner of the display.  When the heading bug is visible, it  will  appear  when  the  selected  heading  is  changed and   is   removed   10   seconds   after   the   heading   bug quits moving.  If the heading bug is off the screen, the readout is constantly displayed.