e. Shutdown Procedure. Both compass sys-
tems are shut down when the INVERTER No.1 or
INVERTER No.2 switch is turned off. (If either
inverter is on, both compass sets will be energized.)
3-23. ALTITUDE SELECT CONTROLLER
The Altitude Select Controller (fig. 3-15) pro-
vides a means for setting the desired altitude refer-
ence for the altitude alerting and altitude preselect
system.
(1.) Altitude Alert. As the aircraft reaches
a point 1000 feet from the selected altitude, a signal
is generated to light the warning light on the altime-
ter. This light remains on until the aircraft is 250
feet from the selected altitude. If the aircraft now
deviates by 250 feet or more from the selected alti-
tude, the light is again energized. The light remains
on until the aircraft returns to within 250 feet or
deviates more than 1000 feet from the selected alti-
tude.
(2.) Altitude preselect The altitude is
selected by turning the selector knob until the alti-
tude display reads the desired value. No further
action is taken on the controller. To initiate altitude
preselect, the ALTSEL button is selected on the
flight director controller. The pilot must initiate a
TM 55-1510-221-10
maneuver to fly toward the preselected altitude. Any
of the following pitch modes may be engaged: Pitch
Hold, Airspeed Hold or Vertical Speed Hold. Upon
initiation of altitude preselect capture, the previ-
ously selected pitch mode is automatically reset.
3-24. RADIO ALTIMETER INDICATOR.
Description. The radio altimeter indicator
(fig. 3-16) displays radio altitude information from
2500 feet to touchdown with an expanded linear
scale under 500 feet.
b. Controls/Indicators and Functions.
(1.) DH annunciator. Light illuminates to
alert that aircraft is at or below selected DH.
(2.) Decision height bug. Manually set by
knob to establish DH.
(3.) Failure warning flag (not shown).
When visible, indicates that system information may
be unreliable.
(4.) Altitude pointer. Points to dial reading
for current radio altitude from 0 to 2500 feet.
(5.) Decision height set knob. Used to
manually set DH.
Figure 3-15. Altitude Select Controller (AL-800).
3-29
