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Some of Hardys work in use with a fixed base sim (twitch)

Started by dts, Mon, 1 Dec 2014 19:40

Hardy Heinlin

Britjet, try the inactive route when you want to enter a company route in flight :-)

Gary Oliver

The flight planning department and the captain are ready for a fight it seems :)

Who's job was it to check the flight plan against the paper copy :)?

Hardy Heinlin


Hardy Heinlin

The FMC master switch is above the upper EICAS :-)

Hardy Heinlin

What are you doing with the FMC CBs? :-)

There is nothing wrong with PSX.

The A/T disconnects when the FMC master switch is on L and FMC L fails, or when the FMC master switch is on R and FMC R fails :-)

Britjet

Hi Hardy

Thanks. Re the autothrottle and the FMC master switches - thanks, we know that - in fact that was meant to be part of the demo - but on GAZUK sim not all the forward switches work yet - so we had to match the PSX instance up with the GAZUK instance, hence some confusion.

We tried failing the left FMC CB to produce a single FMC left failure, but it failed both FMCs. Similarly with the right CB. I will look into that and see if I can reproduce it.

Thanks for the advice about the inactive route. I did try that but it didn't work - will test that again later..

Thanks for looking in!

Regards

Britjet

Britjet

Hi Hardy,

I tried the FMC CBs again on my rig which is pure PSX with no hardware involved.

There does seem to be something unexpected here...
If an FMC fails (ie CB pulled) when the FMC selector is selected to the SAME side, BOTH FMC NAV displays displays fail (ie show MAP display), and both CDUs blank without an FMC prompt, so it is unusable, unless the FMC selector is moved to the other side.

The FMC selector will of course govern which FMC controls the autothrottle and Flight system, but I don't think it should blank the unaffected CDU or NAV display.

For such a failure in rw sims, it was recommended that the pilot whose "picture" was not degraded should take over the flying while the other pilot "fixed the picture" on his side, but in PSX both "pictures" and CDUs blank initially..

if the FMC fails and the selector is placed to the other side, the displays behave as I would expect..

What are your thoughts, Hardy?

Regards,

Britjet

PS I tried the airborne RTE 2 route downlink again - it worked fine this time - must have been finger trouble!

Hardy Heinlin

#27
Hi Britjet,

according to our ground tests, the FMS is not available when the master FMC fails. E.g. when FMC L is the master and FMC L fails, you cannot work with FMC R. FMC R is just a slave.

When FMC R is the single operative FMC on the aircraft, you can use FMC R only when you make FMC R the master FMC.

Also consider the NAV source selectors. When FMC R fails and FMC L is operative, the F/O won't see  map data on ND L until the F/O's NAV source is set to FMC L.

Is that different to BA aircraft?


Cheers,

|-|ardy

Britjet

Hi Hardy,

The master/slave concept idea is a new one on me, but that doesn't mean it isn't the case.
I think both sides can, and do, "resynch" each other in flight if necessary..

On BA aircraft at any rate, I am pretty sure that the loss of the other FMC panel and NAV display doesn't happen. If you imagine a "dark and dirty night" departure - if both sides lost their FMC and NAV function (due to the FMC failure and the master switch position being coincident - albeit that you can change switch positions to eliminate it) this would be potentially catastrophic from a situation awareness and navigation point of view, so I am pretty sure in my own mind that this shouldn't happen.

I remember in BA that we had just such a sim scenario, happening on the Captain's side while he was flying the departure, which initially meant that the F/O took over the flying without having to change anything, and the crew then fixed the problem when the SID was completed..

If my logic is correct, I would have thought that this would not be an airline-specific option.

Regards,


Brit

Jeroen Hoppenbrouwers

#29
Potentially useless input...

The MCDUs receive wired input from both FMCs. This is the only subsystem for which the MCDUs have dual inputs, i.e., there is a direct independent line from each FMC to all three MCDUs. Therefore the MCDUs can, uniquely, decide for themselves which FMC to display as
To be precise, the 429 wiring is a single transmit pair from the left FMC which forks electrically to feed the three MCDUs. Another single transmit pair from the right FMC feeds the three MCDUs as well. Hence each FMC sends exactly the same data to all MCDUs in electrical parallel, but the MCDUs will only listen to 429 words that contain their specific system address label. So each FMC sends every display command three times, once for each (active) MCDU, in rapid succession, over one single transmit pair.

Then each MCDU has one single transmit pair that is electrically forked to feed all subsystems connected to that MCDU. So the same transmissions from each MCDU go to the left FMC, the right FMC, the CMC, the SATCOM, the CMU, ... Each FMC thus gets three independent receive pairs, one from each MCDU.

This whole constellation technically allows a MCDU to gracefully and transparently switch over from a failed FMC to the working one. It 'just' needs to know which one is the master in case of two working ones.

Although I cannot test it on a real A/C right now, I expect a failing FMC to cause all active MCDUs that were using it (on an FMC page) to back out to MENU with the TIMEOUT RESELECT message, but immediately offering the
Note: this is typical engineering chatter without a real A/C at hand. WARNING. This is theory only.


Hoppie

Hardy Heinlin

#30
Hi Britjet,

you mean the FMC master switch has no control when a single FMC fails? The relay would automatically select the operative FMC as a master FMC, and when the failed FMC comes back, the FMC master switch has the control again?

How long was your F/O flying with a single FMC? Thanks to a memory buffer, it takes 30 seconds until the map data finally disappears from the ND (this delay is also modelled in PSX).

I hope in your sim scenario at least your A/T disconnected even though FMC R took over? Changing the master FMC should disconnect the A/T, no matter if it is changed manually or automatically.


Cheers,

|-|ardy


Quote from: Jeroen HoppenbrouwersThis whole constellation technically allows a MCDU to gracefully and transparently switch over from a failed FMC to the working one. It 'just' needs to know which one is the master in case of two working ones.
Since this would affect the CDU, not the A/T, the associated "FMC switch" would not be the "master switch" on the center panel but the "source selector" on the respective pilot's side. Can the NAV source selection be automatic too when a single FMC fails? I think this would disagree with all our empiric tests.


|-|


P.S.: What's the point of having a FMC master switch or FMC NAV source selectors if the selection was automatic anyway? Some aircraft have AUTO on the ADC source selection, or IRS selection -- that's optional -- but none have AUTO on NAV and FCC source selectors, for example. Well, the IRS source selection is indeed automatic (in the background) when the selected IRS fails (also modelled in PSX). The autoswitching is visible by a momentary dropout (fraction of a second).

Britjet

Hi Hardy

I take your point about the FMC master switch taking over again - I would have thought that if the system "recovers" then all would revert to normal?

Re the sim scenario - different pilots would handle it in different ways but the usual technique would be to be "if it ain't totally broke, don't try to fix it until we are not busy" so the F/O would typically have the picture for the whole of the SID. I note that PSX delays the map disappearance, just like the real a/c, for the event of a failure on that side, but I don't think his map should actually disappear at any stage. I'm not sure about the TIMEOUT message that Jeroen mentions - this is a possibility..

(I doubt that it would be possible to certify an electronic cockpit if a single failure could cause total loss of both picture and CDUs at any stage, even if it was feasible to recover the picture by checklist action?)

With regard to the autothrottle disconnect for an FMC Master switch changeover - yes - PSX models that correctly, of course.

Cheers,

Britjet

Hardy Heinlin

#32
I see your point, Britjet. I would like to read a second opinion on this before I change it :-)

I understand the instruments are important during critical flight phases. But is the FMC map on the ND an essential feature? Is LNAV/VNAV guidance essential? You still have raw navaid data, all IRS data, all air data ...


Cheers,

|-|ardy

Britjet

I'm not sure about that Hardy. My gut feeling is that it would be required, since there wouldn't be time to hard-wire essential navaids etc...

Now who has access to a full-flight or CBT?

Brit.

John H Watson

I have the feeling that the FMC prompts disappear on both CDUs (with the Master FMC CB pulled) only if the IRUs are not aligned.

Checking through some old notes.... Standby One.

Hardy Heinlin

The IRUs not being aligned is probably the key criterion. That explains Britjet's points -- and also the current PSX version.

I think my old memories are coming back now ... I once made a mod in this area. Well, I guess I can reintroduce this criterion.


|-|

Hardy Heinlin

#36
The QRH (Lufthansa's QRH, I haven't checked others yet) says, if one FMC fails:

FMC Master Sel . . . . . . L or R
A/T . . . . . . . . . . . . . . Re-engage
Nav Source Sel . . . . . . FMC L or FMC R

I think the first and the third item might be just for additional safety; i.e. the L or R selection is probably automatic (in the background) if the IRS is aligned.


|-|

John H Watson

QuoteI think the first and the third item might be just for additional safety;

The FMC Master Switch controls (among other things), A/T and autotuning. The ND maps and CDU prompts may appear automatically (without changing the position of the FMC Master switch to the operative FMC), but the switch is required for A/T and autotuning. For example, power for the A/T servo goes through the master relay.

This is why step one precedes step two.

Perhaps the master FMC has to see an onside IRU operating before taking over ? Unfortunately, some systems behave differently in the air (than they do on the ground) regarding sensor requirement. In the air, it may be a completely different ballgame.

Rgds
JHW

Hardy Heinlin

#38
I thought (a few hours ago) that the automatic master selection is actually moving the master relay, so that not only the FMC prompt of the good FMC appears on the bad side's CDU, but also so that the A/T and everything else is automatically linked with the good FMC. (A/T disconnection just because of the interrupting switch event.)

If this auto switching does not move the master relay, the auto switching is just a function of the CDU and just helps the CDU (and through that the ND). That's probably what Hoppie wrote.


Cheers,

|-|ardy

Hardy Heinlin

#39
Summary 1.0:

When the master FMC fails:

a) If IRU L or R is aligned, the remaining operative FMC automatically becomes the master FMC, regardless of the master relay position. The master relay may even be unpowered.

b) The master relay does not move automatically. The relay can only be controlled when power to the relay is available and only by turning the selector manually.

c) The NAV source selector may stay in vertical position; the affected CDU (and ND) will automatically be linked with the good FMC.

d) A/T, autotuning, auto mode of landing altitude will fail until the master relay selects the good FMC.


|-|