FMC FAILURE

[Hardy Heinlin]

Thank you, Peter!

What looks strange to me is your observation of LNAV | VNAV being armed and shown in white on the FMA while the FDs are OFF. Are you sure they didn't blank completely in this moment? Perhaps you re-armed them manually when HDG HOLD (or ATT) and V/S modes were re-engaged?

On the ground, before take-off, with both FDs off, can you arm LNAV and VNAV?


Cheers,

|-|ardy

[hellowk]

Thanks Peter for the confirmation! Have been waiting eagerly for this.

[Britjet]

Hi Hardy,

That is with the FDs back ON again..the LNAV and VNAV appear white armed..
I have suddenly thought that my training notes might have been dated back to my 777 days, which of course did have automatic switching. Doh!

Peter

[Hardy Heinlin]

Hi Peter,

and, when the FDs were on again, you pushed the LNAV/VNAV buttons, right? They didn't arm automatically. (Why, after a full FD reset, should the system arm them automatically?)


Cheers,

|-|ardy

Selecting FDs OFF then ON gives HDG (or ATT, depending on bank angle) and V/S modes on the FMA, with LNAV and VNAV showing ARMED (buttons still pressed on the MCP).

[emerydc8]

Thank you, Peter!

[John H Watson]

(Why, after a full FD reset, should the system arm them automatically?)

Selecting FDs OFF then ON gives HDG (or ATT, depending on bank angle) and V/S modes on the FMA, with LNAV and VNAV showing ARMED (buttons still pressed on the MCP).

Selecting "FMC R" causes LNAV and VNAV to reengage GREEN. Autothrottle can be re-engaged.

Trying to understand what Peter is doing/seeing in the sim.

If the A/P loses FMC LNAV/VNAV, and the modes have lines through them on the PFD FMAs, do the LNAV/VNAV buttons on the MCP remain illuminated? (when in the default A/P stabilisation submode).

Normally, if you switch off the FDs in flight with no abnormalities, the A/P remains in the mode it was last in. i.e. no A/P disengagement (yes?)
If you cycle the FD switches with A/P already disengaged (in flight), the FD's go into HDG HOLD and V/S(yes?)

Selecting FDs OFF then ON gives HDG (or ATT, depending on bank angle) and V/S modes on the FMA, with LNAV and VNAV showing ARMED (buttons still pressed on the MCP).

So the FMA goes from LNAV/VNAV with yellow lines to what with the FD's off?

I thought the table I posted earlier was saying that everything returned to normal (GREEN LNAV/VNAV) with the other FMC selected (ok, something has to be done to re-engage the A/T).

Thanks
Rgds
JHW

[Hardy Heinlin]

When I was writing my comment, I was intuitively assuming the A/P was disengaged while the FD switch cycling engaged V/S etc.

[Britjet]

I didn't touch the LNAV VNAV buttons on the MCP. They were either yellow strikeout on the green background, or white ARM, depending on the FMC master switch position. I moved it left and right several times to see the annunciation change.

I think the best thing is to see if I can get a video of it on Tuesday.

Peter

[Britjet]

Hi Peter,

and, when the FDs were on again, you pushed the LNAV/VNAV buttons, right? They didn't arm automatically. (Why, after a full FD reset, should the system arm them automatically?)

The autopilot was engaged throughout, so it wasn't a full reset....

[Britjet]

Here's a video of last night's efforts on the big sim. Apologies for the poor quality, but I think it should clear things up.

Peter

https://youtu.be/Z54Z3P5WG04

[Hardy Heinlin]

Thank you, Peter. Very helpful.

The reason why you didn't get the LNAV/VNAV modes armed by cycling the FD switches: This was probably because you cycled FD R off-on, then FD L off-on (if I understood your video comments correctly). So there was no moment where both FDs were off at the same time.


Cheers,

|-|ardy

[Britjet]

Hi Hardy

I think both FDs were off together at one stage..?
Peter

[Hardy Heinlin]

Aha, so it behaved different this time? I'll take this one as a reference :-)


|-|ardy

[Britjet]

Yes, not sure what happened last time - maybe finger trouble..

[emerydc8]

Thanks again, Peter. So, you do get the AUTOPILOT EICAS in addition to AUTOTHROTTLE.

I guess we can add the lines through LNAV and VNAV in the list below, along with some other interesting indications, thanks to your video.

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.

[John H Watson]

Interesting to see TERR POS on the Upper EICAS.  Is this because the Captain's Map was not available? I noticed that when the R FMC was selected on the Captain's ISSM, the message didn't disappear (or is that because the message takes a few seconds to disappear?)

I see the A/P didn't go into ALT (green), despite the altitude agreeing with the MCP altitude.

I wonder what the Status message/s were.

[Britjet]

Hi John

The status messages were there before the start of the experiment. I don't recall what they were..
Peter

[Hardy Heinlin]

If the resync fails (after 35 seconds), the Master CDU goes to the MENU page with the FAIL annunciator lighted.

I tried to model this delayed appearance of the MENU and FAIL light. It's too complicated; it has too many risks and side effects in my model design. I leave this detail as it is: In PSX, the MENU and FAIL light appear during the resync already. Access to an FMC page is denied in any case, so it shouldn't confuse any student.


|-|

[Hardy Heinlin]

Is it true that the FAIL light illuminates on all three CDUs whenever the master FMC is inoperative?

Even if a CDU is linked with an operative FMC, the FAIL light illuminates on this CDU?


|-|

[Hardy Heinlin]

... a footnote says VNAV may change to altitude hold (ALT) if the airplane is near the MCP altitude.

I assume this is true not only in the moment when the FMC failure occurs, but also during an already running "mode-fault-pitch-hold" climb or descent ...
• when the MCP ALT is captured
• or when you move the MCP ALT bug to the current altitude.

My reference for "near" is the +/- 50 feet width of the PFD tape bug.


|-|