Make absolutely certain that your a/c does not move AT ALL. Most flight manuels of a/c that use/used this INS insist that alignment is complete before a/c movement (which means before engine start-up). It depends on the model. some a/c will move during engine start up, others not.
vololiberista

However, the practice with INS then in the past and even now with modern inertial reference systems is for alignment to be fully complete before engine start up.

Depending on the sim model (and therefore authenticity) one can either allow the INS to align at it's own rate or if you are in a hurry there are always the secret fast align screws!!!
In the flight manuel for the VC10 "align and load" (meaning set to align and load position) is item no.34 out of 94 in the Pre-flight checklist. In the before starting check item no.4 ("Align Selected - Check")
Item no.40 in the same checklist ("Check and load") meaning enter waypoints. Item no.60 ("Loading Checked -NAV mode selected")
In the Starting check item no.1 is Brakes on and item no.27 ("Confirm NAV mode selected") All these items are marked CPE Meaning Captain,Pilot Officer and Engineering Officer. Every entry to the INS system has to be crossed checked by all three crew members!
So you can see that you should not be in the middle of the align process at all with the engines running.

vololiberista