IRU Operation
Captain Jeeves is, again, correct. Please refer to Airbus FCOM 1.22.20 P5 (Auto Flight).
Now, this applies to systems equipped with the GPS system...where GPS Primary exists.
"Note: The FM position update at takeoff is inhibited when GPS PRIMARY is active."
With GPS PRIMARY NAV ACCURACY HIGH, the FM always knows where the aircraft is, regardless of what you input into the MCDU. There is no runway update necessary, since the aircraft already knows where it is.
As long as the aircraft knows where it is, and it knows where it is supposed to be going, there is no navigation error...no need to do a runway update upon application of takeoff thrust, etc., no "Map Shift" after takeoff...none of this.
You can see all of this on the "POSITION MONITOR" page.
Regards,
Tom