747-400 Autopilot Localiser Capture Logic

[NSEU]

A question was asked of me recently...

What would happen if a 747-400 was on an intercept heading to an ILS with the Left Autopilot in command, APProach armed, aircraft height above 1500' and the Left MMR (ILS receiver) suddenly failed? Would the aircraft capture the localiser?

On the ground, with the LOC/GS armed (F/D-only ops), if you pull an MMR circuit breaker, the onside PFD FMA "LOC/GS" arm annunciations disappear, but the ok side annunciations remain in view.

Normally, the autopilot FCCs use their respective MMR signals, but after LOC capture, each FCC starts using data from all three MMRs on inter-FCC databusses (and the FCCs compute a "mid-value" for guidance). However, does this mid-value selection process only begin if the first-in-command FCC captures the LOC? In this case, I can imagine the aircraft flying through the LOC beam, with the PFD FMA on the ok side showing capture (green LOC/GS), but the aircraft continuing in a straight line until an amber line appears through the LOC annunciation (not a desirable situation).

Thanks for any insight.

[Shawn Coyle]

Logic for the failure aside, is there any limitation in the AFM for the maximum intercept angle?

[MarkerInbound]

Haven't seen it as a limitation but the Boeing Flight Crew Training Manual says the maximum intercept angle is 120 degrees. It says, "For large intercept angles some overshoot can be expected."