FMC FAILURE

[hellowk]

Hi,

I was trying to simulate a single FMC failure. For example, the master FMC is on left and I failed the left FMC. Won't I get a line across the LNAV and VNAV due to loss of FMC computed data?

Regards,

[Hardy Heinlin]

Hi,

the selection of the master FMC is automatic when just one FMC is available. In your situation, the right FMC is the master, even though the selector is set to Left.


Regards,

|-|ardy

[emerydc8]

Hi Hardy,

Am I misreading your answer or are you saying that in the event of the left FMC failing it automatically selects the right FMC?

It was my understanding that you had to physically move two switches to get the right FMC over to the left side -- both the FMC selector and nav source selector must be moved to the right FMC. Thanks.

Jon D

[Hardy Heinlin]

Hi Jon,

for LNAV/VNAV roll and pitch modes the available FMC is selected automatically (that was the original question).

For the A/T (for example) the available FMC must be selected manually with the FMC master switch.

For the captain's PFD/ND/CDU the available FMC source must be selected manually with the captains NAV source selector.

For the F/O's PFD/ND/CDU the available FMC source must be selected manually with the F/O's NAV source selector.


|-|ardy

[emerydc8]

Okay. I think I see what you are saying: When the checklist has you reselect LNAV/VNAV, it's because you selected the right FMC to get the autothrottle back and in doing so it knocked it out of LNAV/VNAV? The LNAV/VNAV function automatically switched over to the right FMC when the left FMC failed?

A single FMC is failed: ~ FMC selector ..................................... Select operable FMC ~ Reengage the autothrottle. ~ NAV SOURCE selector ...................... Select operable FMC ~ Reselect LNAV / VNAV as required.
--PROCEDURE COMPLETE -

[Hardy Heinlin]

In an earlier PSX version, when the master FMC failed, LNAV/VNAV disengaged.

Later on, Peter told me that this wouldn't happen on BA aircraft; the FMC selection should be automatic for the AFDS, so you need not fiddle with the master switch during critical flight phases. I modified that accordingly in one of the last updates.

As far as I know, switching the master FMC will disengage the A/T only, nothing else.

[emerydc8]

Thanks. Peter would know.

[hellowk]

I would think that you have programmed it to do automatic switching. However, the 44 that are on my company fleet sadly do not have this option. I would have to manually reselect the master FMC for a respective failed FMC. Is it possible that you have a option to do both?

[emerydc8]

As far as I know, switching the master FMC will disengage the A/T only, nothing else.

That is a true statement. But in the above scenario with a single, controlling FMC failure (captain's side), it looks like the A/T will disengage even before you switch the master FMC selector to the FO's side. This if for the CF6 engines so maybe there's a difference.

The autothrottle disconnects if:
 Autothrottle disconnect switch selected.
 Autothrottle arm switch off.
 Autothrottle faults.
 Two engines inoperative.
 Any EEC changes to the alternate mode.
 Reverse idle selected.
 FMC Master selector changed.
 The controlling FMC fails.
 Two FMCs inoperative.

Single FMC failure
 FMC inoperative requirements are shown in the FCOM limitations section and in the DDG.
 Symptoms of an FMC failure:
 EICAS: FMC LEFT/RIGHT.
 If the left FMC fails and the FMC master selector is on L:
 Autothrottle failure. EICAS: AUTOTHROTTLE.
 Autopilot operates in a degraded mode. Degraded autopilot mode (EICAS >AUTOPILOT) means roll is level and pitch is the pitch snapshot at the time of failure. Pilot with an operating ND becomes the PF. Select HDG hold and ALT hold on the MCP.
 The ND displays MAP and VTK (VNAV track). LNAV, and VNAV are not available. Heading select and FLCH
may be used if necessary.
 TIMEOUT-RESELECT and/or ND blank.
 Use the QRH and use the source selectors to choose the operating FMC. All operations may be restored after an operational FMC is in use.

When this degraded mode occurs, I wonder what FMA you get.

https://drive.google.com/file/d/0B-WRh0Hf7VdZaHVYWl9pcnNGN2s/view?usp=sharing

[Avi]

That is a true statement. But in the above scenario with a single, controlling FMC failure (captain's side), it looks like the A/T will disengage even before you switch the master FMC selector to the FO's side.

What is the problem here? PSX works exactly like that (I don't comment about the LNAV/VNAV issue).

Cheers,

[emerydc8]

There is no problem. I just wanted to point out that in the scenario where you lose the master FMC, the A/T disengages as soon as you lose it -- not when you switch the master FMC selector to the other side.

I am interested in knowing if the FMA continues to show LNAV and VNAV in this "degraded" AFDS mode mentioned in my post above or whether you see something else.

[Hardy Heinlin]

As far as I know, switching the master FMC will disengage the A/T only, nothing else.

That is a true statement. But in the above scenario with a single, controlling FMC failure (captain's side), it looks like the A/T will disengage even before you switch the master FMC selector to the FO's side.

Yes, it does this also before, -- also in PSX.

My statement was just a reply to this:

When the checklist has you reselect LNAV/VNAV, it's because you selected the right FMC to get the autothrottle back and in doing so it knocked it out of LNAV/VNAV?

The selection did not disengage LNAV/VNAV, it just disconnected the A/T.

In PSX, the A/T disconnects not only when moving the master FMC switch, but also in the event of a failure of that FMC which is driving the A/T.

[Hardy Heinlin]

I would think that you have programmed it to do automatic switching. However, the 44 that are on my company fleet sadly do not have this option. I would have to manually reselect the master FMC for a respective failed FMC. Is it possible that you have a option to do both?

I'd like to check whether it's really a company option.

Any further comments from pilots and engineers?

Also, you said you expected a (yellow) line through the LNAV/VNAV mode words. In PSX, there would be a mode change to HDG HOLD and V/S instead of a mode fault.

[emerydc8]

Hi Hardy,

If you look at the second page of the link I posted above under dual FMC failure, it says that in the degraded autopilot mode, "the FMA roll mode is HDG HOLD" (although I'm not sure if they mean it automatically goes to HDG HOLD or you should select HDG HOLD). As for the pitch FMA that corresponds to the "pitch snapshot" maybe Peter knows. At least on the Atlas aircraft, it doesn't appear that LNAV and VNAV will remain with a master FMC failure or dual FMC failure. Hellowk's position is that LNAV and VNAV both have lines through them. We could use more input here.

[Hardy Heinlin]

I see, it shouldn't automatically change to HDG HOLD | V/S. It should internally use wings level and pitch hold; these modes are not the same as ATT | V/S, so this certainly means that LNAV/VNAV remain annunciated but with yellow lines.

The remaining question is whether the FMC selection for LNAV/VNAV is really automatic in case of an FMC failure. Perhaps Peter tried to say that it shouldn't change to HDG HOLD | V/S but it should keep LNAV/VNAV with the yellow line and with those wings level and pitch hold submodes engaged?

[emerydc8]

The remaining question is whether the FMC selection for LNAV/VNAV is really automatic in case of an FMC failure. Perhaps Peter tried to say that it shouldn't change to HDG HOLD | V/S but it should keep LNAV/VNAV with the yellow line and with those wings level and pitch hold submodes engaged?

That sounds good to me. Maybe he can chime in.

[John H Watson]

My old engineering manuals don't specifically say what happens if there is a completely dead FMC. It just says if the Master FMC has failed an internal test, the non-Master will try to resync the Master. During this time, A/T mode shows blank, roll and pitch show mode fail [I assume yellow lines], with FD bars biased out of view, but a footnote says VNAV may change to altitude hold (ALT) if the airplane is near the MCP altitude. I assume LNAV remains with a yellow line, but the aircraft goes into some kind of roll submode to stop it flying erratically (wings level? track hold?). I'm not sure if the FD pitch bar remains in view if the A/P switches to ALT. The Upper EICAS shows >A/T DISC and >FMC MSG. The Master CDU display is frozen. The non-Master CDU shows SINGLE FMC OPERATION, then RESYNCING OTHER FMC with the MSG annunciator light illuminated.

If the resync fails (after 35 seconds), the Master CDU goes to the MENU page with the FAIL annunciator lighted. The EICAS message >FMC-L appears (not sure what happens to the A/T DISC or >FMC MSG). The pitch/roll indications remain the same.

If the non-Master becomes the Master (by manual switching), all the EICAS messages clear except the A/T DISC (cycle A/T the switch off/on). All the other indications are normal (including pitch/roll bars).

[Britjet]

We had a LOFT scenario where a failure of the L FMC with the FMC master at LEFT occurred just after take-off with the Captain flying.
In this event the autothrottle would disengage as you have discussed because of the selector position. If it was the right FMC that failed in this case the autothrottle would have remained engaged as the FMC selector was to the left.

In this flight the left side NAV display would be useless without switching. The easy option is to simply hand over control to the co-pilot as he had a full display to save fiddling about at low level.

My understanding and experience is that the flight system does not go to basic modes with a single FMC failure. You have to have a double failure for this to happen. Nor will you get amber lines through the LNAV and VNAV annunciations. They will be normal. Remember that these are not "picture" switches like the NAV source selectors - they show the state that the aircraft is in - it doesn't matter if one or other FMC side has failed.

Peter

[emerydc8]

Thanks for the input, John.

Peter, when you say the flight system does not go to basic modes with a single FMC failure nor will you get amber lines through the LNAV and VNAV annunciations, does this mean the LNAV and VNAV will continue to be fully-functional without moving the FMC master switch to the operable FMC? So, like Hardy mentioned earlier, there is some automatic switching to the operable FMC in case the master FMC fails?

I'm wondering if this is some sort of option that can be set by the individual operator, since the Atlas material has the autopilot EICAS and the autopilot operating in a degraded mode with the failure of just the master FMC.

[Britjet]

That is my understanding, Jon, yes.

The FMC selector decides which FMC gives guidance to the flight systems, as you know, so that both pilot displays show appropriate (and the same) guidance commands.

If there was no automatic changeover, then a failure of a single FMC (if it was the one selected) would cause failure of guidance on both sides. I am pretty sure that doesn't happen - it wouldn't seem safe to me to have a switching mechanism that caused that to happen?

I can't see how this could be an airline option myself...

I am in the big sim Friday and will try and fit in a single FMC failure and see what happens maybe?

Peter