737 FCC

[KristianNorway]

Hello colleagues

I have a question regarding the 737 FCC. Here goes:

RHS FCC is master and A/P is connected. If HDG bug is turned, both F/D roll bars turn towards new heading. Fair enough.

Next, RHS FCC is master and A/P is connected. RHS VHF panel has a localizer frequency selected and crs selector set to an inbound course. VORLOC is selected on the MCP.
LHS VHF panel has a VOR frequency selected and CRS selector set to a 20 degree offset course from the localizer.

Ok, the A/P will follow the localizer track, but the LHS F/D will show an offset, right? Even though RHS FCC is still master.
The FCOM states:

Rotate Captain’s course selector – provides selected course information to:
• A FCC
• No. 1 VHF NAV receiver
• Captain’s course pointer and course deviation bar.
Note: In VOR LOC or APP mode, the A A/P and Captain’s F/D use selected
course and navigation data from the No. 1 VHF NAV receiver.
Rotate First Officer’s course selector – provides selected course information to:
• B FCC
• No. 2 VHF NAV receiver
• First Officer’s course pointer and course deviation bar.
Note: In VOR LOC or APP mode, B A/P and First Officer’s F/D use selected
course and navigation data from No. 2 VHF NAV receiver.

It also states:

F/D modes are controlled directly from the respective FCC under certain
conditions. This independent F/D operation occurs when neither A/P is engaged
in CMD, both F/D switches are ON and one of the following mode conditions
exists:
• APP mode engaged with LOC and G/S captured
• GA mode engaged and below 400 feet RA
• TO mode engaged and below 400 feet RA.
Independent F/D operation is indicated by illumination of both MA lights. When
independent operation terminates, the MA light extinguishes on the slaved side.

So my question is: Why at all have a master FCC in F/D - operation at all? In situations where it seems we may have differing inputs on the left or right hand side (VORLOC/APP mode), the master function is from what I understand not applicable.
I may be way off here, but any input is very welcome. Thanks :ok:

[flyvabb]

The FCOM states that 'This independent F/D operation occurs when neither A/P is engaged in CMD'.

In the example you posted, you mentioned that the A/P is engaged so the LHS F/D is a slave of the RHS F/D.

[STBYRUD]

....and if the AP is in CMD in your example the FOs display will not show the FD bars because the ILS is not set.

[KristianNorway]

Thanks for answering, but I'm afraid you're mistaken. Tested it in the simulator today. Only the RHS has MASTER-indication, but the LHS is NOT slaved. Read my first note from the FCOM:

Note: In VOR LOC or APP mode, the A A/P and Captain’s F/D use selected
course and navigation data from the No. 1 VHF NAV receiver.

So RHS show correct tracking, LHS shows an offset F/D. When A/P IS engaged I understand it's value, because you'd want to know what F/D-bars the A/P is following. On the other hand I'm not sure I get it if we were to have the same setup and the A/P's off.

[flyvabb]

I'm assuming you had the B A/P engaged for your test run in the sim?

[KristianNorway]

Both with the B A/P engaged-, and not engaged.