NAVIGATION AND
COMMUNICATION
AIRPLANE
OPERATIONS
MANUAL
2-18-30
Page
12
Code
01
REVISION 25
If the present position is not entered during the normal alignment time,
the AHRS 1 (2) ALN FAULT caution message will be displayed on the
EICAS. For airplanes equipped with EICAS 18.5 and on, the AHRS 1
(2) NO PPOS or ARHS 1-2 NO PPOS advisory messages will be
displayed on it. The AHRU will not enter the NAV mode until it receives
a valid position input.
The AHRU accepts multiple entries of latitude and longitude, which
means that various positions may be stored. This feature allows the pilot
to select the current position previously stored, instead of enter it again.
A new position entry writes over the previous entry. More than one
entry may be necessary to confirm, update or correct the position. This
occurs because the AHRU does not accept new position inputs until 2
seconds after the previous input or new position input that has more
than 1 degree of disagreement from the stored latitude/longitude from
the last power down from the NAV mode.
The AHRU conducts a position-compare test on latitude and longitude
immediately after each data has been entered. The AHRU uses only
the latest entry for its test calculations. To pass the test, the entered
data must compare within 1 degree of the stored latitude/longitude
from the last power down from the NAV mode. If the test fails, the
AHRS 1 (2) ALN FAULT caution message is presented on the EICAS.
For airplanes equipped with EICAS 18.5 and on, whenever the airplane
is on the ground and the AH-900 is in align mode, the MAP/PLAN label
on MFD 1 main menu changes to PPOS INIT. By selecting PPOS INIT,
the operator will access the Present Position Initialization menu, and
will be able to set the present- position coordinates with the data set
knob or confirm the stored one. The coordinates are sent to the AH-
900 computer when the ENT bezel button is pressed.
No attitude and heading data is displayed during align mode.
NAVIGATION MODE
The AHRU enters the NAV mode from the align mode. In the NAV
mode, the AHRU uses the last valid position data entered during the
align mode as its initial present position, and updates the present
position based only on inertial data while it remains in the NAV mode.
The AHRU algebraically adds computed magnetic variation from a
magnetic variation topographical map (MAGVAR) to true heading and
true track to produce magnetic heading and track magnetic angle. The
magnetic heading and magnetic tracking angle outputs are set to no
computed data (NCD) inside a northern and southern latitude cutout
area.
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