TM 1-1520-240-10
2-14-3
When the helicopter descends below the low index set-
ting or rises above the high index setting, the correspond-
ing HI or LO caution light will illuminate. If helicopter
altitude exceeds 1,500 feet, pitch or roll angle exceeds
45_, or the system is unreliable, the following will occur.
The OFF flag will appear, the pointer will move through
1,500 feet behind the dial mask, and the digital display
and LO and HI caution lights will extinguish.
If power to the system is lost, the following will occur. The
OFF flag will appear, the digital display and caution lights
will go out, and the pointer will remain at the last valid
indication when power was lost. The altimeters have a
self-test feature. Pressing the PUSH-TO-TEST knob will
cause the pointer and digital display to indicate between
900 and1,100 feet. If LO set and HI set are indexed
below 900 feet, the LO caution light goes out, and the HI
caution light comes on. Power to operate the radar altim-
eter is supplied by the No. 2 DC bus through the NAV
RAD ALT circuit breaker on the No. 2 PDP.
Figure 2-14-3. Radar Altimeter (AN/APN-209)
2-14-10. Controls and Functions, Radar Altimeter
(AN/ APN-209A).
CONTROLS/
INDICATOR
FUNCTION
LO SET knob
Either pilot’s LO SET knob
applies power to altimeter
system. LO set index on
both altimeters can be set
independently. Both LO set
indices must be masked to
turn the set off.
HI SET knob
Sets position of HI set index
and tests altimeter system
when pressed.
CONTROLS/
INDICATOR
FUNCTION
LO set index
Indicates altitude trip point
for LO caution light.
HI set index
Indicates altitude trip point
for HI caution light.
Indicator pointer
Indicates absolute altitude
from 0 to 1,500 feet.
Digital indicator
Provides direct reading four
digit indication of absolute
altitude from 0 to 1,500.
LO caution light
Comes on when helicopter
descends below altitude on
LO set index.
