Mark asked how did we align the IRS's on the 757?

This goes for all the IRS's, not just 757

One of the first things after the safety check and powering the plane, was to start the alignment. Alignment would take around 10 minutes or less depending on the last position of the plane when the IRS was shutdown. If the plane was moved from the last shutdown position without first aligning the IRS's that could cause some problems as the IRS brain remembers its last position and will balk at taking the new position.

Example: The plane is moved from one gate to another without being aligned, or one of the IRS units is changed and the new unit still thinks it's at the workbench back in repair. Sometimes it was a real chore to convince the unit to take the new position.

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With all buses powered, set the IRS's to NAV and enter the gate position. (You can place the unit in the align position and later switch to NAV, but going directly to NAV by-passes that step) Do this before anything else with the computer as most entries will be lost if you do alignment later. That does not include a fast align which was sometimes done right before takeoff using the runway end as a position. Fast align could only be done after the full align procedure. This was more a procedure for the older INS and early IRS units that did not have GPS to massage the position.

Lou