TM 1-1510-225-10 3C-46 (2)   Displays.  The  servoed altimeter provides the following displays: 1.   Counter  drum  display  of  altitude,  in 20-foot increments. 2.   Pointer    display    of    altitude    between 1000-foot levels in 20-foot graduations. 3.   Altitudes below 10,000 feet are annunciated    by    a    black    and    white crosshatch on the left digit position of the counter display. (3)   Barometric Pressure Counter. A barometric   pressure   counter,   manually   set   by   the BARO knob, displays barometric pressure in inches of mercury  (Hg.),  and  millibars  (Mb.),  and  provides  this information  to  the  air  data  computer.    A  red  warning flag    in    view    indicates    the    altitude    information    is unreliable,   however   the   Mode   C   (altitude   reporting) information may be valid. In  the  event  of  a  total  aircraft  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.    A  dc  power loss only (retains ac power) usually results in   the   altimeter   spooling   down   toward   a lower    altitude    instead    of    retaining    the altitude at which the power failure occurred. (4)   Altitude  Alert  Annunicator.    The  Altitude Alert  Annunciator  (ALT),  located  on  the  upper  bezel, illuminates  to  provide  a  visual  indication  of  when  the aircraft  is  within  1000  feet  of  the  pre-selected  altitude during  the  capture  maneuver  and  extinguishes  as  the aircraft   is   within   300   ±   50   feet   of   the   pre-selected altitude.  After capture, the light will re-illuminate if the aircraft   departs   more   than   300   ±   50   feet   from   the selected altitude, and extinguish  when the aircraft has departed   more   than   1000   feet   from   the   selected altitude. b.    Copilot’s Pneumatic Altimeter. The copilot’s altimeter is a pneumatic instrument equipped with a 28 Vdc internal vibrator.  The vibrator is required to  overcome  friction  and  assure instrument accuracy.   Altitude is displayed on the altimeter by 10,000, 1,000 and   100-foot   counter   drums,   and   a   single-needle pointer   that   indicates   hundreds   of   feet   in   20-foot increments.    The  barometric  pressure  setting  knob  is provided  to  simultaneously  adjust  the  baro  counters display in inches of mercury (Hg.), and millibars (Mb.).   Below an altitude of 10,000 feet, a diagonal symbol will appear  in  the  10,000-foot  counter.    A  circuit  breaker, placarded  COPILOT ALTM, located in the flight group on   the   right   side   circuit   breaker   panel   protects   the vibrator circuit. 3C-28.  RADIO ALTIMETER INDICATOR. a.     General.  Displays radio altitude information from  2500  feet  to  touchdown  with  an  expanded  linear scale under 500 feet. b.    Controls, Indicators, and Functions.   Refer to Figure 3C-20. (1)   Decision  Height  Annunciator  –  Lights  to alert that aircraft is at or below selected DH. (2)   Decision Height Bug – Triangular decision height bug is manually set by knob. 1.  DH (Decision Height) Annunciator 2.  Decision Height Bug 3.  Failure Warning Flag 4.  DH (Decision Height) SET Knob 5.  TEST Push Button Figure 3C-20.  Radio Altimeter (3)   Failure Warning Flag  – A failure warning flag  in  view  indicates  the  system  information  may  be unreliable. (4)   Decision   Height   Set   Knob   –   Sets   the radio  altimeter  decision  height  marker  to  a  decision height altitude. WARNING