NAVIGATION AND
COMMUNICATION
AIRPLANE
OPERATIONS
MANUAL
2-18-30
Page
4
Code
02
DECEMBER 20, 2002
INERTIAL REFERENCE SYSTEM COMPONENTS
Each IRS consists of an Inertial Reference Unit (IRU) and a Mode
Select Unit (MSU).
Each IRU uses two 28 VDC power inputs, one for normal power
(primary source) and the other for backup power (airplane batteries).
The IRU 1 primary power source is the Essential DC Bus 1 and its
backup power source is the Backup Essential Bus. The IRU 2 primary
power source is the DC Bus 2 and its backup power source is the
Backup Bus 2. If the IRU loses the primary power, it automatically
switches to backup power.
When the IRU operates solely on backup power, it will operate for 40
minutes and the ON BATT annunciator on the Mode Selector Unit will
illuminate. Airplanes equipped with EICAS 17 onwards will also present
the IRS 1 (2) ON BATT advisory message on the EICAS.
INERTIAL REFERENCE UNIT (IRU)
The Inertial Reference Unit contains three laser gyros and three
accelerometers that are mounted on each of the three axis inside the
IRU, which it uses to measure inertial motion.
The IRU requires initialization data from the Flight Management
System and Air Data Computer. From the inertial measurements,
initialization data, and air data inputs, the IRU performs the calculations
necessary to provide position, velocity, heading and attitude data to the
airplane.
The IRUs are located in the forward electronics compartment.
MODE SELECTOR UNIT (MSU)
The Mode Selector Unit is a cockpit-mounted control panel that
provides mode selection, status indication and test initialization for the
associated IRU. It has a four-position mode select switch and status
and fault annunciators.
To Next Page
