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747-400 MAINTENANCE MANUAL
 INERTIAL REFERENCE SYSTEM - DESCRIPTION AND OPERATION
_____________________________________________________
 1.  General (Fig. 1)_______
 A.  The inertial reference system (IRS) provides basic heading and attitude reference to the flight instrument displays and other user systems.
 B.  The IRS consists of three inertial reference units (IRUs), a single mode select unit (MSU) and a heading reference switch. Power is supplied to the system through the IRS AC, IRS DC and C IRS DISCONNECT circuit breakers on the P6 Main Power Distribution panel, and IRS ON BAT circuit breaker on the P180 DC Power Distribution panel.
 C.  Several systems input data to the IRS. The IRS accepts test initiation data from the central maintenance computers (CMC); altitude, altitude rate and true airspeed data from the air data computers (ADC); and initialization data from the flight management computers (FMC).
 D.  The IRS outputs data to the following systems:
 (1)
Flight Management Computer

 (2)
Autopilot/Flight Director

 (3)
Integrated Display

 (4)
Weather Radar

 (5)
Ground Proximity Warning

 (6)
Autobrake

 (7)
Yaw Damper

 (8)
Airplane Condition Monitoring System Data Management Unit

 (9)
MAWEA card file (stall warning)

 E.  Digital data is output on three ARINC 429 high speed busses. The output data is identical on each bus.
 F.  The IRS provides navigational data as follows to the user systems:
 -Accelerations
 -Angular rate
 -Attitude (pitch, roll, yaw)
 -Ground speed
 -Heading (true, magnetic)
 -
Inertial altitude

 -
Inertial vertical speed

 -Track (true, magnetic)
 -Position
 -Vertical speed
 -Wind speed, Wind direction, Drift angle
 -Flight path vector, Drift angle
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747-400
MAINTENANCE MANUAL

SELECT MODULE
IRS ON BAT LIGHT
C

B
Inertial Reference System - Component Location Figure 1 (Sheet 1)
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747-400
MAINTENANCE MANUAL

EICAS
EIU SEL
R
EVENT

HDG

REF SWITCH
SEE

EICAS UNITS
D EICAS LEVEL B IRS CAUTIONS
E
CDU ATTITUDE EICAS LEVEL C IRS ADVISORIES
HEADING
TRACK > IRS AC LEFT > IRS DC LEFT
IRS LEFT IRS MOTION SOURCE SEL IRS

F
Inertial Reference System - Component Location Figure 1 (Sheet 2)
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747-400
MAINTENANCE MANUAL

E1 RACK
LEFT IRU SEE G
RIGHT IRU
SEE G

CENTER IRU SEE G
MAIN EQUIPMENT CENTER
TOTAL TIME FAULT INDICATOR INDICATOR (REF) (REF)
TEST SWITCH
1
(REF)
INERTIAL REFERENCE UNIT
G
AIRPLANES WITH -110 IRUs AND SUBSEQUENT;
NO FAULT BALL INDICATOR IS INSTALLED. THE
CHASSIS INDICATOR HOLE CONTAINS A PLUG.

1
Inertial Reference System - Component Location Figure 1 (Sheet 3)
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