Align to nav the iru enters navigate mode from

ALIGN-TO-NAV – The IRU enters navigate mode from
alignment mode upon completion of alignment.

NAV-TO-ALIGN - The IRU enters the align down
mode from the navigate mode.

NAV-TO-ALIGN-TO-NAV – The IRU enters the align
downmode and after 30 seconds, automatically re-
enters the navigate mode


ALIGN-TO-ATT or NAV-TO-ATT – The IRU enters the
erect attitude submode for 20 seconds, during which
the MSU ALIGN annunciator illuminates. The IRU
then enters the attitude mode.
MSU Annunciators

ALIGN – Indicates that the IRU is in the alignment
mode. A flashing ALIGN annunciator indicates
incorrect LAT/LONG entry, excessive aircraft
movement during align.


NAV RDY – Indicates that the alignment is complete.

FAULT – Indicates an IRS fault.

ON BATT – Indicates that the back-up battery power is being
used.

BATT FAIL – Indicates that the back-up battery power is
inadequate to sustain IRS operation during back-up battery
operation (less than 21 volts).

NO AIR – Indicates that cooling airflow is inadequate to cool the
IRU.

INERTIAL SYSTEM DISPLAY UNIT (ISDU)

INERTIAL REFERENCE UNIT (IRU)

The IRU is the main electronic assembly of the IRS. The IRU
contains an inertial sensor assembly, microprocessors, and
power supplies and aircraft electronic interface.

Accelerometers and LRG in the inertial sensor assembly
measure acceleration and angular rates of the aircraft.

The IRU microprocessors performs computations required for:

Primary Attitude.

Present Position.

Inertial Velocity Vectors.

Magnetic and True North Reference.

Sensor Error Compensation

GYROCOMPASS PROCESS

The three gyros sense angular rate of the aircraft. Since the aircraft
is stationary during alignment, the angular rate is due to earth
rotation.

The IRU computer uses this angular rate to determine the direction
of true north.
INITIAL LATITUDE

During the alignment period, the IRU computer has determined
true north by sensing the direction of the earth’s rotation.

The magnitude of the earth’s rotation vector allows the IRU
computer to estimate latitude of the initial present position.

This calculated latitude is compared with the latitude entered by
the operator during initialization.

ALIGNMENT MODE

The systems software performs a vertical levelling and
determines aircraft true heading and latitude

The levelling operation brings pitch and roll attitudes to
within 1  accuracy (course levelling), followed by fine
levelling and heading determination.

Initial latitude and longitude data must be entered
manually, either via the IRS CDU or the Flight
Management System CDU.

The alignment time is approximately 10 minutes