Inertial Reference System
Two independent IRS's are installed. Each IRS has three sets of laser gyros and
accelerometers. The IRS's 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 IRS's 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.
