737 Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
11.20.4 D6-27370-866-EGP
Inertial System
The inertial system computes airplane position, ground speed, and attitude data for
the DUs, flight management system, autoflight system, and other systems. The
major components of the inertial system are the air data inertial reference units
(ADIRU), an inertial system display unit (ISDU), IRS mode select unit (MSU),
and an IRS transfer switch. For information about the air data part of the system,
see chapter 10. The ADIRUs provide inertial position and track data to the FMC,
and attitude, altitude, and airspeed data to the CDS. Each ADIRU has an IRS
section and an air data section.
Inertial Reference System
Two independent IRSs are installed. Each IRS has three sets of laser gyros and
accelerometers. The IRSs are the airplane’s sole source of attitude and heading
information, except for the standby attitude indicator and standby magnetic
compass.
In their normal navigation mode, the IRSs provide attitude, true and magnetic
heading, acceleration, vertical speed, ground speed, track, present position, and
wind data to appropriate airplane systems. IRS outputs are independent of external
navigation aids.
IRS Alignment
An IRS must be aligned and initialized with airplane present position before it can
enter the navigation mode. The present position is normally entered through the
FMC CDU. If the present position cannot be entered through the FMC CDU, it
may be entered through the ISDU keyboard. The airplane must remain stationary
during alignment.
Normal alignment between 78 degrees 15 minutes North or South is initiated by
rotating the MSU switch from OFF to NAV. The IRS performs a short power test,
during which the ON DC light illuminates. When the ON DC light extinguishes
and the ALIGN light illuminates, the alignment process begins. Airplane present
position should be entered at this time. Alignment time varies from five minutes
to seventeen minutes depending on airplane latitude.
Magnetic variation between 82 degrees north and 82 degrees south is stored in
each IRS memory. The data corresponding to the present position are combined
with the true heading to determine magnetic heading.
If the latitude/longitude position is not within 4 NM of the origin airport, the CDU
scratchpad message VERIFY POSITION is displayed. If the entered
latitude/longitude position does not pass the IRS internal comparison tests, the
scratchpad message ENTER IRS POSITION is displayed.
September 28, 2006
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