I think most, if not all IRUs are NEVER updated once initialized. The FMCs are updated via DME or GPS, but the IRS's simply provide pure inertial input to the FMS. I haven't used a GPS update before, so I don't know if the FMC allows a GPS update prior takeoff. I think it's best to be as accurate as practical. Also, you should always check to be sure the airplane symbol is in the center of the runway symbol prior to takeoff as verification that you've input a good initial position and that you're on the correct runway. That may not have helped the Singapore crew in TPE since the runways very close together -- but then again maybe it could have.
[This message has been edited by Roadtrip (edited 25 November 2000).]