IRS/2nd AUTOPILOT COUPLE FAIL.

[vagabond 47]

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.

[RAT 5]

NG a/c generally have 2 A/P's. If so then Fail passive. Please explain if different. Were there any warnings on the O/H IRS panel?

[vagabond 47]

Yeah, 2 A/PS, 2 IRSs,..no land 3,fail passive.......no warnings just the 2nd a/p will not couple.....but no IRS update on entire route . Single a/p either A or B will ILS couple.