IRS/2nd AUTOPILOT COUPLE FAIL.
Aircraft a NG.....SCENARIO is failed 2nd Autopilot to couple on a ILS approach...always.
CDU check of Navdata error shows failure of No2 IRS to update after 3 hrs even in the Terminal Area with VOR/DMEs. A/C is fail operational.
Got ideas but looking for the OB1 Theory.
Cheers.