(c)  
A similar situation exists for ground speed except that ground speed error will oscillate sinusoidally with a period of 42 minutes (half the "Schuler cycle") as it grows. This shows up as a residual ground speed at the end of a flight that slowly grows then lessens but always growing more than it lessened the last time.

 (4)  
IRS attitude mode

 (a)  
In the attitude mode, the IRU provides a rapid attitude/heading realignment capability. The ATT mode is used if the IRU has lost both its ac and dc backup power for more than 0.010 seconds or if certain types of failures are detected by BITE. During ATT mode, only pitch, roll, rotation rates, accelerations, inertial altitude, vertical speed and magnetic heading (if initialized through the CDU) are valid.

 (b)  
The attitude mode is selected by switching the mode select switch to the ATT position. The IRS ALIGN MODE message will appear on the main EICAS display and the IRU will enter a 30 second alignment period while the reference axis is aligned to the local vertical. The IRU software assumes the airplane is level and not accelerating. If the IRU determines that the airplane is turning, it will stop the alignment and wait for the airplane to stop turning.

 (c)  
Once the alignment is complete, the IRS ALIGN MODE message will be removed and it becomes possible to enter a set heading using the CDU POS INIT page. Once entered, the IRU will output a valid magnetic heading which will track airplane turns. The magnetic heading output will not compensate for the Earth's rotation and will need to be updated periodically. Set heading inputs from the CDU are only accepted by the IRU while it is in ATT mode.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL

 (5)  
Power Distribution and control

 (a)  
Normal system power is 115v ac from circuit breakers on P6-1 with 28v dc from the APU hot battery bus providing a backup power source. This backup power automatically switches to the main hot battery bus during APU starts. Switching to DC backup power is accomplished automatically by the IRUs when loss of 115v ac is sensed.

 (b)  
During standby power operation the center IRU is disconnected from its dc backup to reduce the load on the battery. Whenever power is not being provided by the 28v dc No. 3 bus, the Center IRS Disconnect Relay is deenergized which provides 28v dc from the hot battery bus through the center mode select switch to the IRS DC Power Disconnect Relay. The circuit runs through the center mode select switch to prevent the IRS DC Power Disconnect Relay from being energized when all power is removed from the airplane at the end of the day thus draining the battery. After a five minute delay, the IRS DC Power disconnect Relay energizes, removing dc backup power from the center IRU. The right and left IRUs remain powered from the airplane battery until the voltage drops below 18 volts (at least 30 minutes).

 (c)  
If all power is removed from the airplane at the end of the day and the IRUs are not turned off, the ground crew call horn will sound and the IRS ON BAT annunciator on the P461 panel will be turned on. With no AC power, the IRU's 28v ON DC discrete will power the IRS On Battery Relay through the Air/Ground Relay and the On DC diode and a loading resistor. After 10 seconds the relay will energize. If the ground service bus is also not powered the IRS Ground Service Relay will be deenergized thus connecting the hot battery bus to the ground crew call horn and giving a ground to the annunciator.

 (6)  
System Interfaces

 (a)  
The MSU supplies system mode discretes from the mode select switch. One set of switch contacts is used by the power disconnect circuitry. If there is an ac power loss while the center IRU is in the OFF mode, the dc power will not be disconnected. The other set of switch contacts manipulate the operational modes of the IRUs.

 (b)  
Each IRU has two low speed ARINC 429 ports dedicated to air data input. If an ADC fails, the IRU will automatically switch to the alternate ADC input. The IRU receives pressure, altitude, true airspeed and, during ATT mode, altitude rate from the ADCs. The ADCs are connected to the IRUs through the ADC Bus Switching Relays. The ADC instrument source select switches control these relays. They also control which air data port the left and right IRUs are using. The F/O's IRS instrument source select switch controls which air data port the center IRU is using. 1) The left and right IRU use whichever ADC the captain's and
　
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