ELAPSED TIME T = 10 MINUTES. 
NO LATITUDE AND/OR LONGITUDE HAS BEEN ACCEPTED OR ENTERED.  FLASHING  OFF  ALIGN 
LATITUDE DATA WITHIN ONE DEGREE, ENTERED FOR FIRST TIME, MISCOMPARES WITH COMPUTED LATITUDE.  FLASHING  OFF  ALIGN 
ACCEPTED LATITUDE DATA MISCOMPARES WITH COMPUTED LATITUDE.  FLASHING  OFF  ALIGN 
IDENTICAL LATITUDE DATA ENTERED SECOND CON-SECUTIVE TIME.  O N  O N  ALIGN 
LONGITUDE DATA INVALID, NOT WITHIN ONE DEGREE OF STORED VALUE OR MISCOMPARES WITH PREVIOUS ENTRY.  FLASHING  OFF  ALIGN (DATA STORED) 
IDENTICAL LATITUDE DATA ENTERED SECOND TIME.  ON  OFF  ALIGN (DATA ACCEPTED) 
LATITUDE AND LONGITUDE DATA HAVE BEEN ACCEPTED, NO EXCESSIVE MOTION, NO OTHER PROBLEMS PREVENT-ING ALIGNMENT, MODE SELECT SWITCH IS IN NAV POSITION.  OFF  OFF  NAV 
MODE SELECT NAV-ALIGN-NAV ZEROES OUT RESIDUAL VELOCITY ERRORS (MINIMUM OF 30 SECONDS).  OFF  OFF  ALIGN 

IRS Alignment Conditions
Figure 18

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737-300/400/500MAINTENANCE MANUAL
 (4)  If the airplane is moved with the IRUs not aligned in NAV mode, the change in present position may be sufficient to cause the ALIGN annunciator to flash during initialization. Present position must be re-entered to extinguish the ALIGN annunciator.
 NOTE: For IRU P/N S242T101-105 and on (user friendly features),
____ the IRUs will automatically restart the alignment process if airplane motion is detected. The ALIGN light will flash for 30 seconds when excess motion is detected.
 C. System Operation
 (1)  The inertial reference system (IRS) activates the ground crew call horn whenever one of the following conditions exist.
 (a)  
Equipment Cooling (all of the following conditions must exist): 1) Airplane on the ground 2) No cooling on the IRU's 3) IRS AC circuit breaker is closed 4) IRS is on (mode selector not in the OFF position)

 (b)  
IRS on DC power (all of the following conditions must exist): 1) Airplane on the ground 2) IRS AC circuit breaker open or loss of IRS AC power, and DC

 circuit breaker is closed
 3) IRS is on

 (2)  
The ground crew call horn is deactivated by setting the mode selector switch to OFF on the IRS mode selector unit (MSU) on the aft overhead panel P5.

 (3)  
Figure 10 shows the internals of the IRU and identifies all power, control and data bus interfaces.

 (a)  
The internal IRU power supply furnishes the control circuits and voltages for the IRU, MSU, and ISDU.

 (b)  
The power supply receives 115v, 400 Hz power and 28v dc back-up power from the airplane.

 (c)  
If 115v ac is not available from the airplane, the IRU automatically switches to 28v dc battery power. It also provides a signal to light the ON DC annunciator on the MSU. If the battery voltage drops below 18v, the IRU will generate a DC FAIL discrete which is displayed on the MSU.

 (d)  
When all airplane ac power fails, the IRU automatically switches to battery power. IRU-2 stays on battery power for a maximum of 340 |40 seconds.

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 (e)

(f)

(g)

(h)

(i)

(j)

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After 340 |40 seconds on battery power, the DC power disconnect relay in the flight instrument accessory unit opens to remove battery power from IRU-2. IRU-1 stays on battery power until the voltage drops below 18v.When the mode select switch on the MSU is moved from OFF to ALIGN or NAV, a self-test BITE signal disables the 115v ac input and inserts the 28v dc battery as the power source when both sources are available and valid. This tests the battery and associated power switching circuits. The ON DC annunciator lights momentarily during this test.The ISDU or the FMC supply initialization data and the ADC's furnish required air data inputs to the system. The MSU also supplies mode selection discretes to the IRU. A remote test discrete directs IRS to output fixed value test signals for checks of the system.Gyro electronics - Each laser sensor assembly contains a laser gyro fringe pattern detector, and control circuits. The main function of each laser gyro assembly is to produce pulses indicating CW/CCW rotation.The laser gyros detect pitch, roll, and yaw rates and provides this data to the IRU computer for computation and navigation. A temperature sensor provides a signal that is used for correction of thermal drifts. Each gyro has correction values stored in a PROM which details the gyro's unique bias, thermal, and alignment characteristics. The PROM data is routed to the CPU at turn-on. It is used to provide error correction of laser gyro outputs.The laser gyro is started by -950v dc and +3500v dc levels from the HV power supply. Once laser action starts, the +3500v dc is turned off and operation is sustained on the -950v dc supply. Control signals standardize the path length for each gyro such that gyro operation efficiency is maximized. A dither drive circuit controls the operation of the dither motor. The dither motor develops a vibration to keep the gyro out of the lock-in region during low rotation rates.
　
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