TM 1-1510-223-10 air data computer.  Altitude is displayed by a 10,000 foot counter, a 1000 foot counter, a 100 foot counter, and a single needle pointer (coupled with the 100 foot counter) which  indicates  hundreds  of  feet  on  a  circular  scale  in 20 foot increments.  Below an altitude of 10,000 feet, a diagonal  striped  symbol  will  appear  on  the  10,000  foot counter.    The  barometric  pressure  knob  allows  ground supplied altimeter setting to be manually set.  Altimeter setting is displayed in inches of mercury and millibars on barometric counters. a. Controls, Indicators and Functions. (1) Failure    warning    flag.         The    failure warning flag, placarded OFF, comes into view when the error   between   the   altitude   displayed   and   the   signal received is too great, the air data computer goes invalid, or the barometric altimeter loses primary power. (2) Altitude alert annunciator.   The altitude alert  annunciator,  placarded  ALT,  is  illuminated  when aircraft is within 1000 feet of preselected altitude during capture   maneuver   and   extinguishes   when   aircraft   is within  250  feet  of  preselected  altitude.    After  capture, annunciator  will  illuminate  if  aircraft  departs  more  than 250 feet from the selected altitude. (3) Altitude   scale.      Used   in   conjunction with altitude indicator needle to indicate aircraft altitude in hundreds of feet Subdivided into 20 foot increments. (4) Barometric pressure counter-drum indicator   (inches   of   mercury).       Indicates   barometric pressure in inches of mercury that has been set by the barometric pressure setting knob. (5) Barometric pressure counter-drum indicator  (millibars).     Indicates   barometric   pressure   in millibars  that  has  been  set  by  the  barometric  pressure setting knob. (6) Barometric pressure setting knob. Used  to  manually  set  barometric  pressure  displayed  in the   MB   pressure   displayed   in   the   IN   HG   and   MB windows. (7) Altitude    indicator    needle.       Used    in conjunction with altitude scale to display aircraft altitude in hundreds of feet. (8) Counter-drum altitude display. Indicates air-raft altitude in tens of thousands, thousands, and hundreds of feet above sea level. b. Altimeter Operating Procedure. 1. Barometric pressure setting knob - Set desired altimeter setting. 2. Failure     warning     flag     -     Check     not visible. 3. Altitude indicator needle - Check operation. NOTE If the altimeter does not read within 70 feet of   field   elevation,   when   the   correct   local barometric   setting   is   used,   the   altimeter needs    calibration    or    internal    failure    has occurred.    An  error  of  greater  than  70  feet also   nullifies   use   of   the   altimeter   for   IFR flight. 3-89