TM  55-1510-221-10 (6.)   MB   Indicator.  Indicates  local  baro- metric   pressure   in   millibars.   Adjusted   by   use   of BARO knob. (7.)  Below  10,000  feet  symbol.  Presence indicates  aircraft  altitude  is  below  10,000  feet. (8.)  OFF  Flag.  Presence   indicates   loss   of power   to   instrument   and   unreliable   readings. NOTE If   the   OFF   flag   is   visible,   either   DC power  is  off,  the  fuse  has  blown,  or  there is  an  altimeter  encoder  failure.  Since  the OFF  flag  monitors  only  the  encoder  input to  the  altimeter  and  not  transponder  con- dition,   the   altitude   reporting   function may  be  inoperative  without  the  OFF  flag showing,  in  the  case  of  transponder  fail- ure  or  improper  control  settings.  It  is  also possible  to  get  a  good  Mode  C  test  on  the transponder  control  with  the  OFF  flag showing.  If  the  OFF  flag  remains  visible, radio  contract  should  be  made  with  a ground  radar  site  to  determine  if  the  alti- tude   reporting   function   is   operative. b. Pilot’s Altimeter - Normal Operation. (1.)  Turn-on  procedure:  Servoed  altimeter will   operate   when   transponder   is   operating   with M-C  switch  set  to  center  position. (2.)  Operating  procedure: 1. Barometric  set  knob  -  Set  desired altimeter  setting  in  IN.  HG.  win- dow. 2. OFF  flag  -  Check  not  visible. 3.  Needle  indicator  -  Check  opera- tion. NOTE If   the   altimeter   does   not   read   within   70 feet   of   field   elevation,   when   the   correct local  barometric  setting  is  used,  the  altim- eter  needs  calibration  or  internal  failure has  occurred.  An  error  of  greater  than  70 feet  also  nullities  use  of  the  altimeter  for IFR  flight. c.  Pilot’s  Altimeter  -  Emergency  Operation. Disregard   pilot’s   altimeter   and   utilize   copilot’s altimeter. 3-34.  COPILOT’S  ENCODING  ALTIMETER. Description.   The   copilot’s   altimeter   (fig. 3-29):’   provides   an   indication   of   present   aircraft pressure  altitude  above  sea  level.  It  also  supplies information  to  the  INS  and  GPS.  The  air  data  com- puter  supplies  altitude  information  to  the  transpon- der. b.  Controls/Indicators  and  Functions. (1.)  ALT  alert  indicator.  Not   used. (2.)   Needle   indicator.   Indicates   aircraft altitude  in  hundreds  of  feet  with  subdivisions  at  20- foot   intervals. (3.)   MILLIBARS   window.   Indicates   local barometric  pressure  in  millibars.  Adjusted  by  use  of set  knob. (4.)  IN  HG  window.  Indicates   local   baro- metric   pressure   in   inches   of   mercury.   Adjusted   by use  of  set  knob. (5.)  BARO  knob.  Used   to   manually   set barometric  pressure  displayed  in  the  MB  an  IN  HG windows. (6.) Drum  indicator.  Indicates  aircraft  alti- tude   in   ten-thousands,   thousands,   and   hundreds   of feet  above  sea  level. (7.)  Test  button.  Used   to   test   altimeter operation. c. Encoding Altimeter - Normal Operation. (1.)  Turn-on  procedure:  Encoding   altime- ter  will  operate  when  transponder  is  operating  with M-C  switch  set  to  center  position. (2.)  Operating  procedure: 1. Barometric  set  knob  -  Set  desired altimeter  setting  in  IN.  HG.  win- dow. 2. CODE  OFF  flag  -  Check  not  visi- ble. 3.  Needle  indicator  -  Check  opera- tion. 4.   TEST   button   - a. Push  -  Reading  decreases  by 500 feet. b. Release  -  Returns  to  original reading. 3-84