Fig. 32-3  Alignment
[4] In the navigation mode the IRUs provide outputs for attitude, heading, accelerations, ground speed, position and drift angle. These outputs are all computed using inputs from the gyros and accelerometers. The initial navigation outputs are continuously modified during operation by the computer using inputs from the accelerometer and gyros, to maintain real time present parameters. Inputs from the ADCs are used to compute inertial altitude, inertial vertical speed, and wind speed/direction. Altitude referenced to 29.92 inHg is used for computing altitude. The altitude data along with altitude rate is used for computing vertical speed. True airspeed is used to compute wind data.

                 Fig. 32-4   Navigation mode

[5] The mode select unit (MSU) provides mode selection for two IRUs through two mode select switches and monitors the operation of the IRUs through two sets of alert lights mounted on the unit. The MSU contains four position mode select switches; one for the left and one for the right IRU. The position are OFF, ALIGN, NAV and ATT. To minimize inadvertent mode selection, the switches gave barriers when switching from “ALIGN”to “OFF”and when switching between the “NAV”and “ATT”modes. To switch between these modes you must pull the mode select switch and turn.
 ALIGN
   When ALIGN is selected, power is supplied to the IRUs and the IRUs progress through a alignment period of approximately ten minutes.

 NAV
   The NAV position of the switches enables the navigation mode, provided that the alignment has been completed.

               Fig. 32-5   IRS  mode select unit

 ATT
   When ATT position of the switches places the IRUs into the attitude mode, which is a degraded mode of operation. The attitude mode provides only attitude information.

 OFF
   When the mode selector switches are in the OFF position, power is removed from the IRUs. When turned OFF, the IRUs go into a shutdown procedure and shutdown approximately thirty seconds later. In addition, Two set of four annunciators are installed on the unit, one annunciator (ALIGN) for an alert purpose and three (ON DC, FAULT, DC FAIL )for warning purposes. 
　
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