TM 1-1510-225-10 2-115 Altitudes below 10,000 feet are annunciated by a black and white crosshatch on the left digit position of the counter display. 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   re main   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. The  altitude  alert  annunciator  (ALT), located on the    upper    bezel,    illuminates    to    provide    a    visual indication when the aircraft is within 1,000 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 1,000 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 count drums, and a single-needle pointer which indicates hundreds of feet in 20-foot increments.   The  barometric  pressure  setting  knob  is  provided  to simultaneously   adjust   the   bare   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  T3  F3 COPILOT ALTM and R COPLT VS & ALTM, located in the Flight group on the right side circuit    breaker    panel,    protects    the    vibrator    circuit.   Refer to Figure 2-6, Sheets 1 through 5. c.   Radio   Altimeter   Indicator   F3   .       Displays radio altitude information from 2,500 feet to touchdown with an expanded linear scale under 500 feet. (1)   The radio altimeter provides the following displays.  Refer to Figure 2-37. 1.   Radio    Alt    Pointer    display    of    radio altitude from 0 to 2,500 feet. 2.   Triangular decision height bug, manually set by knob. 3.   Annunciator light to alert that aircraft is at or below selected DH. 4.   A failure warning flag in view indicates the system information may be unreliable 5.   Test button to check indicator R/T unit and flag operation. Operating   the   test   button   causes   the   flag   to come    into    view    and    altitude    pointer    to    indicate approximately   100   feet.      Release   of   button   causes pointer to return to existing altitude and flag to retract. The   radio   altimeter   also   provides   altitude   and decision   height   information   to   the   ground   collision avoidance system (GCAS).  Setting the radio altimeter decision  height  marker  to  a  decision  height  altitude with    the    DH   SET    knob    will    cause    the    GCAS    to announce  the  "Minimums,  Minimums"  aural  warning when  the  aircraft  transitions  through  the  selected  DH altitude. 1.  Decision Height Annunciator 2.  Decision Height Bug 3.  Decision Height Flag 4.  Decision Height Set Knob 5.  Test Push Button Figure 2-37.  Radio Altimeter Indicator  F3 WARNING