Originally Posted by
fox niner
Well, the Boeing answer is in the FCOM:
ADIRU
ADIRU alignment must not be attempted at latitudes greater than 78 degrees, 14.75 minutes.
This is the latitude of Svalbard/Longyear ENSB.
Which Boeing? The 744 FCOM says:
IRS HIGH LATITUDE ALIGNMENT
A high latitude alignment must be accomplished when the latitude of the origin airport is greater than 70°12.0’ and less than 78°15.0’.
IRS Mode selectors ............................................................ ...... OFF, then ALIGN
Note: The IRS Mode selectors must remain in ALIGN for a minimum of 17 minutes.
CDU ............................................................ ................................................... Set
Enter present position on SET IRS POSITION line of position initialization page.
IRS mode selectors ............................................................ ...........................NAV
There is no limitation on operating the IMUs. FAA OpSpec B050 requires all 3 INS systems to be fully operational for flights through the Area of Magnetic Unreliability ("system" is not defined explicitly, but FMCs may be included in that definition). Regardless, common sense would dictate that GPS and FMCs are operating properly for Polar operations.