Auto pilot/ FCC

[samaryaj]

Hello ppl,
My question is that during our simulators, our instructors have told us to switch off the opposite side F/D in case an engine fails on an ILS approach after VOR/LOC and G/S captures so that the FD which is lost due to eng failure comes up as a result of the other side FD being put off!
Could anybody elaborate on this as to why this has to be done. NO QRH, FCOM talks about this procedure .
Points to be noted,
1. IF an eng fails, does the respective FCC fails, or does it get powered from the opposite side because of the BTB.
2. If FCC fails does the FD also fail or is it independent of the FD. ( my ans as per FCom is it depends on the FCC)
3. and mostly if a loss of a sing gene, does the FCC fail, or only the auto pilot? and of the BTB closes, can the autopilot be re engaged on the failed engine side or no...

Very normal doubts, bordering silly, but would appreciate if anybody can clear these doubts.:confused::confused:
Thanks v. much

[de facto]

What aircraft do you fly?

[samaryaj]

I fly the boeing 737 NG

[de facto]

1) your instructor is talking crap.
2)if you lose an engine ,both fds are working.
3)if you lose one gen,the autopilot can still be engaged on the failed side until you select the apu gen on,it will disconnect and you can then reconnect it...messy though.
4)if an fcc fail,its fd will fail.
5)if fcc A fails,you can not engage A/P A ,if FCC B fails,A/P cant be engaged.

[samaryaj]

hmmm you seem to make sense. but still a little confused, as I have personally witnessed in my sims, the FD bars going away with an engine failure on ILS app.
Now I am not sure if it has to be the master FCC side eng failure or the opp side.

[de facto]

The fds may come out of view for many reasons but i dont think after an engine failure.
In any case switching the other side fd off makes no sense at all.
The only time the PM fd would be out of view in normal ops would be during an ILS with DME located on a VOR.
IF an eng fails, does the respective FCC fails, or does it get powered from the opposite side because of the BTB.
So if you lose an engine,both FCC still have power but the opposite engine FCC is normally used.(ie eng1 failed,A/P B engaged to avoid possible redisengagement.)
The FCC are electrically powered and as long as your transfer bus works,both will still be powered and available.
If FCC fails does the FD also fail or is it independent of the FD.
If an FCC fails,its Fd will go out of view and the FD flag will annunciate on the PFD.
and mostly if a loss of a sing gene, does the FCC fail, or only the auto pilot? and of the BTB closes, can the autopilot be re engaged on the failed engine side or no...
The Auto pilot will disconnect and the associated FCC will not send signal to the flight controls actuators but STILL available after power is restored on the associated gen.

[samaryaj]

thank you very much. you have clarified the theory aspect. Based on this ofcourse the switching off FD doesnt make sense.

[samaryaj]

Just a clarification though, if the FCC fails for some reason, then what would you do? and its possible that with a certain type of eng failure like accessory gear box failure where the N2 is < 15%, hence making the EEC inop, would intunr also make the associated FCC inop.
In such a scenario, is it possible to put the other side FD off to get your side FD? If so ,How and why?
P.S Some sort of transferring FCC???

[de facto]

NO.

If otherwise let me know,im interested.
Will check in the sim my self,but i think it might be related to your company SIM.

[samaryaj]

As per one of the check pilots its possible.. i too have a sim today..will be checking it out myself.. will get back

[de facto]

please ask him the technical reference behind it and check if the speed trim fail comes on.
Check with and without the start switches ON.