A320 map shift
 

Dear all,
I noticed on A320 simulators that, when one sets a maps shift on the aft panel (either fast or slow), the reproduced scenario consists of the desired track line shifted sideways on the ND (Xtrack error).
I was expecting a discrepancy when crosschecking a/c position with raw data navigation but no change on the ND (a/c symbol on desired track).
What's the explanation?
Thanks!

Map shift is exactly that - the map is seen to shift under the aircraft symbol. It occurs when the aircraft obtains "new" information on it's position, and so it shifts the aircraft's position to the new spot.

As the pilot's display keeps the aircraft in the centre, and the map displayed in reference to that aircraft, the pilot see the map shift, not the aircraft symbol.

..ok, so the only scenario which could lead to a wrong position computation with no visible signs (a/c symbol on track line) is in IRS navigation only, after a significant flight time, right?

Yes - that's how map shift occurs, generally. You fly in IRS for a couple of hours, then come into range of a VOR/DME or paired DMEs or whatever - the aircraft recalculates it's position and shifts the map under you (as it moves the aircraft symbol to the new, updated, position).

It was quite common before GPS, flying in remote areas like Australia.