If I'm already airborne, and discover that the IRS nav system was not aligned prior to take off (or, perhaps there was a power failure that's now been recovered from) and now want to align my IRS nav system, how do I do that?I saw something in the manual about waiting until the current position 'stabilizes', but, if I'm flying along while trying to do this, it's a bit of a problem.so.... how is it done, please?

It can't be done in the real world, and PMDG have modelled it this way. Perhaps you could select NAV and reload the aircraft in flight? (Not sure how much of your FMC stuff you will lose though).Cheers.Q> Ян

thank you for the reply.ok. lets say you're right, and there is no way to do it.what say the IRS all go off-line. what about the GPS units? is there a way to tell it to ignore IRS and use GPS instead?

The GPS (alone) will not drive your Map display. Real world pilots have made the same mistake thinking that!You can get your GPS lat/long position from your CDU, but you will have to figure out which way you are flying (the hard way). You can enter a HDG into the IRU's, periodically (from your whiskey compass), when they are in ATT mode, but still no Map. You will have to use dead reckoning and also fly manually (no A/P)Cheers.Q> イアン