Hello again,

you´re right. Maybe the INS actually doesn´t drift so much.
A normal rate would be about 2 miles per hour of operation.
I am a rw flyer, but todays INSs are linked to a FMS and so there is an INS position which is a mix of all individual INS positions a GPS position (if thus equipped) and a FMS position which is also updated by radio beacons (if no GPS avail) so there also is a radio position  :D
The radios don´t update the INS itself but the FMS position which is then presented on the nav display.
What you would check after a flight is the offset of all individual INS positions from the mixed INS position and also the ground speed after parking brakes set. After some hours it is usually showing some knots (btw 0-5). If above 5 we usually realign the INSs.
I do the GS check after the flight in FSX too and it usually shows speeds around 3-5 knots so that seems OK to me.

But what about the accuracy index? When we start with 0 after alignment can it be 5 or worse after 20min of flight?
Or is that normal? Haven´t seen a real CIVA INS working yet...

Thanks for your replys  :)
Marc