(b)
The alignment mode can be entered either by turning the mode select switch from OFF to ALIGN or from OFF to ALIGN to NAV. 1) OFF to ALIGN. If left in the ALIGN position, the system
will continue to zero the velocity errors and update the heading and latitude calculations.
2) OFF to ALIGN to NAV. Once the alignment process is complete, the IRS ALIGN MODE message(s) will be removed and the IRU will enter the navigation mode.
(c) Initial position must be entered via the CDU to succesfully complete the alignment process and enter navigation mode. 1) The IRU compares entered latitude and longitude with the
last known latitude and longitude stored in memory. If the difference is greater than one degree, the ENTER IRS POSITION message appears on the CDU. If a second, identical entry is made it is taken as correct.
NOTE: To prevent nuisance messages, the entered position
____ reasonableness tests are inhibited after selection of ATT mode, after a shop visit until first entry into NAV mode and after an abrupt power loss.
NOTE: Position can be reentered even if a position is
____ already present on the SET IRS POS line.
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(d)
(e)
(f)
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747-400 MAINTENANCE MANUAL
Entered latitude is also compared with an IRU estimated latitude to verify the IRU correctly measures the earth's rotation (latitude sin/cosine test). If the cosine of the entered latitude and the cosine of the estimated latitude differ by more than 0.01234 or, if the sine of the enterd latitude and the sine of the estimated latitude differ by more than 0.15, the ENTER IRS POSITION message appears on the CDU. 1) If a second latitude entry is made that is not identical to
the first entry and, if the latitude sine/cosine test fails, the ENTER IRS POSTION message will continue to be displayed.
2) If a second latitude entry is made that is not identical to the first entry and, if the latitude sine/cosine test passes, the ENTER IRS POSITION message will be removed.
3) If a second latitude entry is made that is identical to the first entry, the IRU assumes it has failed to measure Earth rotation correctly due to a hardware failure and the IRS LEFT, IRS CENTER or IRS RIGHT message appears on the main EICAS display. The fault ball is also set. If a subsequent latitude entry is made which passes the latitude sine/cosine test, the IRS LEFT, IRS CENTER or IRS RIGHT message will be removed and the fault ball reset.
NOTE: The latitude sine/cosine test is performed each time
____ a latitude entry is made and once just prior to entering NAV mode.
A quality of alignment index is generated by BITE to monitor gyro performance. This index is checked just prior to entering NAV. If the index exceeds a threshold value the IRS LEFT, IRS CENTER or IRS RIGHT message appears on the main EICAS display and the fault ball is set indicating a hardware failure.The airplane must not be moved during the alignment period. If excess motion is detected by the IRU, the IRS MOTION message appears on the main EICAS display for a minimum of 30 seconds and until the motion stops. Alignment automatically restarts after the motion ceases. If initial position has already been entered when motion is detected, it is not necessary to reenter the position when alignment restarts.
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747-400 MAINTENANCE MANUAL
(g)
Ten minutes are required to complete the alignment for any latitude between 70.2 degrees North latitude and 70.2 degrees South latitude. This is called a regular alignment.
(h)
17 minutes are required to complete the alignment for any latitude between 70.2 degrees and 78.2 degrees. Since the IRU will automatically enter the navigation mode after 10 minutes if the mode select switch is in the NAV position, a high latitude alignment requires that the mode select switch be left in the ALIGN position for 17 minutes before being selected to the NAV position.
(2) IRS Rapid re-alignment mode
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