TM 1-1510-218-10   3C-37 (AGL).      Radio   altitude   does   not   display   when   the aircraft is over 2500 feet AGL. b.  The Radio Altitude (RA) flag comes into view, replacing the RA readout, if the RA input is invalid. c.   Pressing the  PUSH  TST  button  on  the  EFIS control panel initiates a self-test of the radio altimeter. d.  Decision   Height   (DH)   is   the   radio   altitude where  a  decision  needs  to  be  made  to  land  or  go- around.  The DH is set using the  DH/SET knob on the display  control  panel.    A   DH  annunciation  comes  on, flashing  for  5  seconds,  and  an  aural  annunciation is activated  when  the  aircraft  descends  to  the  DH.    The DH annunciator then stays on steady until reset.  It is reset    by    climbing    to    25    feet    above    the    DH    or descending to 20 feet radio altitude. The  DH annunciation will not come into view and is removed if  already  being  displayed  when  the  radio altitude input is invalid. The  DH  setting  is  removed  if  the  DH  or  display control panel input is invalid, or when the aircraft is in an extreme attitude. 3C-19.  ALTITUDE     ALERTER     /     PRE-SELECTOR (AL800). a.   Description.   The altitude alerter/pre-selector provides   a   means   for   setting   the   desired   altitude reference   for   the   altitude   alerting   and   altitude   pre- select   system.      The   altitude   alerter/pre-selector   is powered through a 1-ampere circuit breaker, placarded   ALT    ALERT,   located    on    the    overhead circuit breaker panel, Figure 2-16. b.  Altitude Alerter/Pre-Selector Control, Indicator, and Functions.  Refer to Figure 3C-16. (1)   Altitude   Display.      The   altitude   display indicates the selected altitude. (2)   Altitude Selector Knob. A knob, placarded   SET,  on  the  altitude  select  controller,  is used to set the altitude in the display. 1.  Altitude Display 2.  Altitude Selector Knob Figure 3C-16.  Altitude Alerter/Pre-Selector (AL-800) c.   Altitude Alerter/Pre-Selector Operation. (1)   Altitude Pre-Selection. 1.   Selector Knob – Set desired altitude in display window. 2.   Altitude Selector Switch (flight director mode  controller,  Figure  2-17)  – Push on. 3.   Pilot    must    now    initiate    the    altitude preselect  maneuver  by  flying  toward the preselected altitude. 4.   Flight Director Mode Controller (pedestal    extension,    Figure    2-9)    – Select   pitch   hold   mode   (pitch   hold, IAS, or VS) if desired. 5.   Autopilot controller (pedestal extension,   Figure   2 -9)   –   Engage   if desired. 6.   Upon capture of pre-selected  altitude, the previously selected pitch mode will be reset. (2)   Altitude Alert. (a)   Altitude  Selector  Control  Knob.  Set desired altitude in altitude display window.