Loss of Alignment
If an IRS loses both AC and DC power, the alignment is lost. Alignment can be lost if the
MSU switch is moved out of the NAV position. If alignment is lost in–flight, the navigation
mode (including present position and ground speed outputs) is inoperative for the
remainder of the flight. However, selecting ATT allows the attitude mode to be used to
relevel the system and provide an attitude reference. The attitude mode requires
approximately thirty seconds of straight and level unaccelerated flight to complete
releveling. Some attitude errors may occur during acceleration, but will be slowly removed
after acceleration stops.
The attitude mode can also provide heading information, but to establish compass
synchronization the crew must manually enter the initial magnetic heading. Drift of up to 15
degrees per hour can occur in the IRS heading. Therefore, when in attitude mode, an
operating compass system must be periodically cross–checked and an updated magnetic
heading entered in the IRS, as required.
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. Initial power–up requires battery bus power available and the
IRS mode selector to be in ALIGN, NAV, or ATT. If the IRS is turned off, it must complete a
full realignment cycle before the airplane can be moved. If AC electrical power is
subsequently removed from the airplane, the switched hot battery bus continues to supply
electrical power to the IRS. The ON DC light illuminates, and the ground-call horn in the
nose wheel well sounds to alert maintenance personnel that the IRS is on battery power.
