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from the No. 1 essential ac bus through the HARS AC
circuit breaker; both circuit breakers are on the pilot
overhead circuit breaker panel.
b. Controls and Functions. Control of the HARS is
provided by the HARS mode selector switch. Control
and indicator functions of the HARS mode selector
switch are described in table 3-11.
The HARS control panel (fig 3-10) is located on the pilot
lower right instrument panel. The control panel has a
mode selector switch with four positions: OFF, OPR
and two ALIGN positions: FAST and NORM. Signals
are sent to the MUX and doppler for use by the fire control
computer and other systems such as DASE, navigation,
stabilator, and symbology. The VDU (fig 4-2) and
the HDU (fig 4-9) display this information to the
pilot.
Figure 3-10. HARS Control Panel
NOTE
*. Loss of the heading tape during HARS alignment
indicates a fault occurred with the HARS during
alignment. Check the FD/LS.
*. If any helicopter movement occurs (heading or
position) with the HARS switch in either of the
ALIGN positions, the HARS shall be turned off
and realigned without moving the helicopter.
*. Whenever the helicopter is on the ground and
will not be moved for longer than one minute, set
the HARS switch to NORM. This will prevent the
HARS inertial velocities from drifting. The HARS
switch shall be set to OPR prior to moving the
helicopter.
Change 3 3-35
TM 1-1520-238-10
Table 3-11. HARS Control Functions and Alignment Methods
Method/Control How to Initiate Requirements/Function Effects On Accuracy
Off Turns HARS off
Normal Switch to NORM PPOS, MV and spheroid Best accuracy attained by permitting
data entered through completion of HARS alignment prior to
SP1 on data entry starting the engines.
keyboard (DEK) prior to
initiating alignment Some degradation to alignment accuracy
will occur if the engines are started prior
to the HARS completing its alignment.
This degradation will not occur if one
engine is started and the rotor speed
established at 100% within one minute
and forty-five seconds after placing the
HARS switch to NORM.
Stored Heading Switch to FAST Helicopter shall not have Uses data from previous HARS alignment
been moved since HARS and flight. Accuracy is dependent on that
was shut down data. Alignment time is the shortest of all
methods (approximately 90 to 120
seconds). Accuracy is not affected by
engine starts.
FAST Switch to FAST Same as normal Least accurate method. Fastest method
alignment when stored heading alignment is not
possible.
Inflight Restart Switch to OFF, then AN/ASN-128 doppler If the HARS loses alignment in flight, the
to OPR MAL light must be OFF DASE will disengage. Switch the HARS
Recycle doppler if OFF, then to OPR. After approximately 90
necessary seconds, the HSI HDG flag will disappear
and the DASE may be reengaged. The
heading will appear to oscillate as the
HARS attempts to align. The heading
accuracy should be within 3 degrees after
five minutes. Navigation performance will
be degraded significantly.
OPR Switch to OPR Operating mode. Switch
shall be in OPR prior to
moving the helicopter.
3.14.2 Required Navigation Data. The non integrated
navigation system uses two components of data for
proper operation: magnetic variation (MAGVAR) and
spheroid (SPH). The HARS aligns with inertial North.
MAGVAR is used to correct the inertial heading to one
referenced to magnetic North. The symbolic heading
tape, HSI, and RMI will all indicate magnetic heading.
MAGVAR is entered in two places. Using the DEK, the
present position MAGVAR on page 2 of Spare Position
3-36 Change 3
1 (SP1) shall be checked and entered, if necessary, during
the AFTER STARTING APU-CPG checklist. The
second place that MAGVAR is input is in the DNS
whenever coordinate data is entered. The spheroid is
used to define the geographic reference frame that is
used for the coordinate data. Spheroid is entered in 2
places. Using the DEK, the spheroid on page 2 of SP1
shall be checked and entered, if necessary during the
AFTER STARTING APU-CPG checklist. The second
TM 1-1520-238-10
place that spheroid is input is the DNS. Only 1 spheroid
can be utilized by the DNS. The DNS PPOS and destination
spheroids must be the same. The DEK SP1 and
DNS PPOS spheroid must be the same.
3.14.3 Doppler Navigation Sets.
NOTE
The non integrated navigation system uses one of
two Doppler Navigation Sets (DNS): AN/ASN-128 or
AN/ASN-137 for navigation. The DNS which is
installed may be determined by which Computer
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