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switch. Spring-loaded
from TEST.
CW Enables the beat frequency
oscillator (BFO)
to permit tuning to continuous
wave stations.
VOICE Enables the set to operate
as an AM receiver.
TEST Rotates the bearing indicator
approximately
180 to check operation
of the set. The TEST
switch functions when
the function switch is at
COMP only.
3-3-7. Normal Operation — Direction Finder
Set. The following paragraphs discuss ADF set operation.
a. Starting.
(1) Interphone control panel - Receivers NAV
switch — ON.
(2) Mode switch — ON.
(3) CW-VOICE-TEST — CW or VOICE.
(4) Tune controls — Set frequency. Tune for
maximum signal strength on the tuning meter.
(5) AUDIO control — Adjust.
b. ADF Operation.
(1) VOR ADF switch on HSI MODE SELECT
panel — Press, if ADF segment is not lit.
(2) Check for the correct bearing indication, on
HSI No. 2 bearing pointer.
c. Radio Receiver Operation.
(1) Function switch — ANT
(2) Tune control — Set frequency; then tune for
maximum signal strength on the tuning meter.
d. Aural Null.
(1) CW-VOICE-TEST switch — CW.
(2) Function switch — LOOP.
(3) Loop switch — Rotate for audio and tune
indicator null. Release switch. Observe bearing to station
on indicators. Two null positions, 180 apart, may be
obtained.
TM 1-1520-240-10
3-3-4
3-3-8. Gyromagnetic Compass Set (AN/ASN-43).
The gyromagnetic compass set (fig. 3-3-3) is a direction
sensing system which provides a visual indication of the
heading of the helicopter with respect to the earth’s magnetic
field or referenced to a free directional gyro. Heading
information is displayed on both HSIs. The display is
used in a navigation to maintain flight path direction.
Also, any heading selected with the heading bug on either
HSI can be automatically maintained by the AFCS.
Power to operate the system is supplied by the No. 1 AC
bus and the AC instrument bus through the NAV CMPS
circuit breakers on the No. 1 PDP.
3-3-9. Controls and Function, Gyromagnetic Compass
Set (AN/ASN-43). (fig 3-3-3 and 3-3-4)
CONTROLS/
INDICATOR
FUNCTION
LATITUDE Switch Two-position switch on
the directional gyro located
on the avionics
shelf. This switch is
used to correct for gyro
precession for the hemisphere
of operation: N
for northern hemisphere
and S for southern
hemisphere.
LATITUDE Control Switch Rotary control on directional
gyro labeled 0 to
90. It is used to set the
local latitude into the directional
gyro for free
gyro operation.
SLAVED-FREE Switch On compass control
panel. When switch is at
SLAVED, the compass
cards on both HSIs are
slaved to the output of
the remote compass
and-stabilized by the directional
gyro. At FREE,
the cards are referenced
to the directional gyro
only and act as turn indicators.
FREE is used
primarily in polar regions
where magnetic indications
may be unreliable.
PUSH TO SET synchronizing
knob
Rotated in () or (+) direction
as indicated by
annunciator pointer to
align compass cards
with gyro-compass.
CONTROLS/
INDICATOR
FUNCTION
HDG Flag A red flag on both HSIs
indicates failure of the
gyromagnetic compass
system when displayed.
3-3-10. Operating Procedures — Gyromagnetic
Compass Set. (fig. 3-3-3)
a. Slaved Gyro Operation. With compass slaving
switch at SLAVED, the system operates in the slaved
mode and the directional gyro precesses to align the
compass cards on the HSIs with the magnetic heading
of the helicopter. During the first 2 minutes after power is
applied, the system operates in a fast slave mode while
the gyro attains its speed. After this initial alignment period
is complete, the gyromagnetic compass will return to
the normal slaved mode. During this mode of operation,
the compass cards will remain aligned with the magnetic
heading of the helicopter. The annunciator pointer will
occasionally point to a dot () or a plus sign (+) indicating
that corrections are automatically being made.
b. Free Directional Gyro Operation. If the compass
slaving switch is at FREE, the system operates in the free
directional mode. In this mode, the compass cards can
be set to any heading by pressing the PUSH TO SET
knob and turning it until the cards reach the selected
setting. Normally, the free directional gyro mode is
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