IRS Alignment (Cont'd.)
If the latitude/longitude position is not within 4 NM of the origin airport, the CDU scratchpad
message VERIFY POSITION is displayed. If the entered latitude/longitude position does not
pass the IRS internal comparison tests, the scratchpad message ENTER IRS POSITION is
displayed.
The flashing ALIGN light alerts the crew that the position entered does not pass one of the
two internal comparison tests and should be checked for accuracy. If the entered position
does not agree with the last stored position, the first internal test is failed, and the ALIGN
light will flash. If the same position is reentered, the IRS will accept the position and
continue the alignment process. A second internal position test compares the entered
latitude with the system-computed latitude. If this test is failed, the ALIGN light will again
flash. If two consecutive entries of the same position do not pass the second internal
position test, the FAULT light will illuminate. If the test is passed, the IRS will proceed to
complete the alignment process and enter NAV mode.
During transit or through–flight stops with brief ground times, a thirty second fast
realignment and zeroing of ground speed error may be performed by selecting ALIGN while
the airplane is parked. Present position should be simultaneously updated by manually
entering latitude and longitude prior to selecting NAV.
Note: If the airplane is moved during alignment or fast realignment, the IRS automatically
begins the full alignment process.
