TM 1-1510-218-10 3B-56 1.  Altitude Alert (ALT) Annunciator 2.  Pointer Display 3.  Barometric Pressure and MILLIBARS Counter 4.  Baro Knob 5.  Altitude Pointer 6.  Counter Drum Display Figure 3B-19.  Pilot's Altimeter b.    Controls and Functions. (1)   Altitude   Alert   (ALT)   Annunciator.      The altitude alert annunciator illuminates to provide a visual indication  when  the  aircraft  is  within  1000  feet  of  the preselected altitude during the capture maneuver and extinguishes when the aircraft is within 250 feet of the preselected  altitude.    After  capture,  the  annunciator will illuminate if the aircraft departs more than 250 feet from  the  selected  altitude,  and  extinguish  when  the aircraft  has  departed  more  than  1000  feet  from  the selected altitude. (2)   Pointer Display. Displays altitude between 1000-foot levels with 20-foot graduations. In the event  of a total ac and dc electrical power loss, the warning flag will be in view, the  altimeter  will  be  inoperative,  and  the indicated altitude will remain as existed at the time of failure. (3)   Barometric    Pressure    and     MILLIBARS Counter.    The  barometric  pressure  and  MILLIBARS counter,  set  with  the  baro  knob,  displays  barometric pressure in inches of mercury and millibars. (4)   Baro Knob.  The baro knob is used to set the barometric pressure and MILLIBARS counter. (5)   Altitude   Pointer.        The    altitude    pointer points  to  the  altitude  on  the  pointer  display  between 1000-foot levels, in 20-foot increments. (6)   Counter    Drum    Display.        The    counter drum    displays    altitude    and    is    marked    in    20-foot increments. Altitudes below 10,000 feet are annunciated  by  a  black  and  white  crosshatch  on  the left-hand digit position of the counter display. NOTE A failure warning flag in view indicates the altitude information is unreliable; however, the Mode C information may be valid. c.     Operating Procedure. 1.   Barometric   set   knob   –   Set   desired altimeter setting in IN HG window. 2.   Warning flag – Check not visible. 3.   Needle indicator  – 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. WARNING