Yesterday I measured the drift on ground in "ATTIUDE".
It was a brand new Airbus and on this there is no display on the IRS mode selector panel, but only on FMGS MCDU. On MCDU it is indicated "True heading" and "M heading". After changing to "ATT" the "M heading" indication is changing to "heading".
The lateral was 40 27.9
The duration of the observation was 20 minutes.
On MCDU the "True heading" was not changed but "heading" decreased 3.3 degrees. Means 9.9 degrees per hour.
360/24 X sin 40 27.9 is 9.7347
Maybe a longer period would give more accurate result.
I still can't undersand that on MCDU the "true heading" was not changed, maybe just a bloody French idea again.
Next I'll check it on airplane with conventional IRS panel.
Lotetu