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.
IRS Power
The IRS's can operate on either AC or DC power. The left IRS is normally powered from the
AC standby bus, and the right IRS from the AC transfer bus 2. If AC power is not normal,
either or both systems automatically switch to backup DC power from the switched hot
battery bus. Backup DC power to the right IRS is
automatically terminated if AC power is not restored within five minutes.