OPERATIONS MANUAL

Navigation Systems
Inertial Reference System:
IRS on battery light
Illuminates after a delay of 10 seconds when any IRU is powered by
the main or APU hot battery bus. (Each IRU is normally powered by a
115 V AC bus; the batteries are used for backup power only.)
IRS mode selector (IRU L, IRU C, IRU R)
To move the selector to NAV, or away from NAV, the selector must be pulled.
OFF Removes power from the IRU, and IRU loses alignment.
ALIGN If currently in ATT mode, the IRU remains in ATT mode. Otherwise,
removes any existing sensor errors, and—when the aircraft is parked—
performs the position alignment: this takes 30 seconds if the IRU was
in NAV mode before, else at least 10 minutes and will not stop before
the selector is set to the NAV position.
NAV If currently in ATT mode, the IRU remains in ATT mode. Otherwise,
performs a 10-minute position alignment if not already done and if the
aircraft is parked. When aligned, and when an IRS position is entered
in the FMC*, the IRU will operate in NAV mode and provide attitude
and navigational data to other aircraft systems.
ATT When the selector has been in the ATT position for more than
2 seconds, the IRU stops providing navigational data, loses its position
alignment, and performs the attitude alignment for 10 seconds—or for
30 seconds if the IRU was off. Thereafter, the IRU will provide attitude
and vertical speed only, and the magnetic heading must be initialized
(and updated circa every 5 minutes) by manual entries in the FMC*.
* Refer to POS INIT in chapter FMS.
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