Flap Control Units (FCUs)

[John H Watson]

Latest theory:

With the aircraft (CF6) in the air with flaps not in landing range and NAI and CONT IGN off,  if you pull FCE CB "1L AC" (P7 C12) and "2L AC" (P7 C10) the Left FCU will output no earth signal to the idle control circuits causing engines 1 & 4 to go from low idle to high idle.

With the aircraft in the air with flaps not in landing range and NAI and CONT IGN off, if you pull FCE CB "1R AC" (P7 C14) and FCE CB "2R AC" (P7 C16) the Right FCU will output no earth signal to the idle control circuits causing engines 2 & 3 to go from low idle to high idle.

I don't know if big sims will show this (I would hope so, though). Peter?  If not, it's back to the drawing board.

Looking at the wiring schematic, you can also get high idle on an engine in the above situations (with the FCE CB's set) if you pull the respective engine IDLE CONT CB on P6 (F10, E10, D10 or C10). This is modelled in PSX. This doesn't trigger the magenta line to come down, so there must be other logic at play here.

[emerydc8]

Thanks, John. I don't know what is meant by then note, "S/B 747-34-2349 or production equivalent incorporated into entire fleet." Maybe something in that service bulletin would shed more light on the issue. Of course, those aren't made available to pilots.

[John H Watson]

No mention of it in my notes, but Google reveals:

https://publicapps.caa.co.uk/docs/33/20130306B747400CAAMMELSupplementRev8j.pdf

Date: 2013

NOTE
: If SB 747-34-2349 or production
equivalent has not been incorporated
and centre FCU is inoperative or
removed, MODE 4 of GPWS system is
inoperative (see 34-46-1).

Seems like another example of individual FCUs having specific functions.

Rgds
JHW

[EDIT: Checking old wiring schematics (2006), indeed there is a unique output from the Centre FCU going to the EGPWS system. I don't know if PSX has this SB modification  ]

[John H Watson]

P.S. Looking at more detailed wiring diagrams (of 2 different airlines), the Left and Centre FCUs output a "flaps in landing range" signal to the GPWS. I can't see an output from the Right FCU. Maybe the SB only added a signal from the Left FCU. The "flaps in landing range" wiring from the Left and Centre FCU hooks up with the FLAP OVRD inhibit switch. The output from the FCU and FLAP OVRD switch is identical.

P.P.S. There used to be a unique output from the Centre FCU to the TCAS system, but it is now no longer used.

[emerydc8]

One interesting thing I noticed about the FLAPS CONTROL QRH (all FCUs inop), under the headnote, "Expanded flap position indication is inoperative," is that it tells you to extend the gear after the flaps are extended to 25. My first thought was this would make a great oral question: When could you have more than flaps 20 with the gear still up and not get a config warning (other than inhibiting)? I suppose one of the FCUs is controlling that warning. Maybe the center?

[John H Watson]

"Expanded flap position indication is inoperative," is that it tells you to extend the gear after the flaps are extended to 25.

This is getting muddier and muddier...

Are they distinguishing between the "expanded flaps" display and the "alternate flaps" display or do they mean no flaps display at all? If no flaps display at all, how do you know you're at 25 units? 

I don't fully understand what is happening when you push the ALTN FLAPS switch. With a fully functioning aircraft, if you push the ALTN FLAP switch, you get the alternate flap display and the flap position is still shown. To get the moving white tape flap position displays, you need 26Vac for the RVDTs from the FCE (Flight Control Electronics) Power Supplies and, I assume some kind of DC power for the processors in the FCUs (e.g. analog to digital converters and ARINC transmitters) to get the flap position to EICAS. Clearly, pushing the ALTN switch doesn't disable all the electronics in the FCU, otherwise there would be no flap position of any kind. If the FLAPS CONTROL EICAS message pops up, you might assume that the FCUs are completely dead... but if the ALTN display works properly, then the FCUs are clearly not dead.

If the Centre FCUs (and Left FCU on modified aircraft) are outputting a flaps in landing range signal to the GPWS when the FCU processing circuits are completely unpowered (my earlier theory) then the GPWS warning will not occur. Why would you extend the flaps first though? Is this some kind of safety procedure or do the aircraft electronics require this for some reason?

[John H Watson]

P.S. Note that the CBs for the FCEs on the overhead CB panel have the following functions:

The (115 volt) AC circuit breakers provide 26 volts AC for the RVDTs and all the DC voltages for the FCUs (and other boxes)
The (28 volt) DC circuit breakers provide power for the processors in the FCE alone.

Pulling all the FCE AC circuit breakers removes all the power from the FCUs

[emerydc8]

I think this checklist should answer some of your questions. I checked PSX and in the alternate mode the timing is exactly in line with the times in the checklist. I didn't realize that the FCU generates the expanded view too. I thought it was just a crude indicator.

[EDIT]: FCOM 9.20.13

An alternate control mode which bypasses the FCUs can be manually selected. All flaps are extended or retracted by a simplified control system and electric motors.

Maybe this is just over-simplified for us less detail-oriented pilots.

[EDIT 2]:
744 Maintenance Manual 27-00-00 p.15 (Feb. 18, 2007)

The flap control units will disable their control and failure protection functions when the alternate system is engaged, but will continue to provide position information for flight deck displays and user systems.

So it is. But maybe if the FCUs are truly and completely inop, it won't even display the flap position.

[Hardy Heinlin]

I think the statement ...

"the flaps are extended"

... means the same as ...

"the flaps are out of up" or "the flaps are not retracted".

The flaps can be extended to any position between non-zero and the maximum.

I think not all of the to-do items in section 10 under "Choose one" are listed chronologically. Some of these items have to be considered at the same time; e.g. the item "Do not fly in stick shaker", which is written under the "flap 25" item, has to be considered not only after flap 25 is reached but also before, all the time.


|-|

[emerydc8]

As a crewmember, and since we are timing the flaps to get them to 25 (their max position in this case), I would extend the flaps to 25 first, then extend the gear, especially because this is more of a to-do list and the gear comes after already extending the flaps. I don't think there would be any reason to even mention the gear extension in the checklist unless they wanted you to do it after the flaps were at 25. It would be interesting, though, to try this in the sim to see if you would get the config warning if you did it this way.

[John H Watson]

I didn't realize that the FCU generates the expanded view too. I thought it was just a crude indicator.

The EIUs have no way of processing (directly) flap position RVDT signals to produce the moving tape/s on all 3 types of flap display (norm, expanded and alternate). Processing is done by the FCUs. The FCU's convert AC signals to digital (ARINC) data (1's and 0's). The EIUs convert these to moving tape graphics.

The EIU's figure out from digital and discrete data (or lack thereof) whether to display the normal, expanded or alternate flap scales (outlines) on which the moving tapes are superimposed.

[emerydc8]

Thanks, John. I have an email out to a check airman to see if there's even a feature in the real sim where you can fail all three FCUs (to get the FLAPS CONTROL EICAS) and have the option to make the expanded view not appear. I guess this would be so rare that it is never tested and maybe the sim designers didn't even address it.

It is strange, though, that Boeing would even install an alternate flap system for use when all three FCUs become inop (what are the chances of that?), yet they couldn't design an alternate indicator that didn't go through the same FCUs.

[John H Watson]

It is strange, though, that Boeing would even install an alternate flap system for use when all three FCUs become inop (what are the chances of that?),

It's simply not conceivable that all 3 should fail with so much redundancy in power supply. If you lost so many AC busses across the ship, you probably wouldn't get any EICAS displays to view the flap display on anyway.

However, according to my notes, FLAPS CONTROL can also be generated by two flap lever RVDTs failing [EDIT: See real world tests on next forum page]. All three RVDTs are contained in a single device under the pedestal. It's probably pretty safe from coffee spills though. Perhaps the most likely need to switch to ALTN FLAPS would be if there was a mechanical failure of the flap lever or its linkages to the RVDT assembly.  The flap lever is joined to the RVDT assembly by a single long rod (and assorted nuts and bolts). I can't remember if the nuts are regular nuts with Loctite on them, or if they are anchor nut (non-slip) type nuts.

[emerydc8]

Perhaps the most likely need to switch to ALTN FLAPS would be if there was a mechanical failure of the flap lever or its linkages to the RVDT assembly. 

That's in line with what the Atlas check airman just sent me. He fails the flap handle in the sim, but this doesn't get you to the part of their checklist that has you time the extension of flaps in the alternate mode when the indicator doesn't appear. I don't even know if you can get the sim to this point without pulling CBs like you discussed. I guess if losing all three FCUs is super rare, then losing the position indicator components of those FCUs as well would be even more unlikely. Some carriers don't even have this contingency in their QRH.

[Avi]

Another possible mechanical failure can be a broken linkage between the Flap Input Actuator and the two Flap Power Packages control valve. However you will not know about it until you try to lower the flaps.

[John H Watson]

Regarding the Flap Lever RVDTs causing the FLAPS CONTROL message.

Looking at this diagram, the redundancy of power supplies is not as good as I first thought.

FCE to FCU Power

I see that if you pull FLT CONT ELEC AC CBs 1R and 2R, you lose power to two flap lever RVDTs. In PSX, the FLAPS CONTROL message is not appearing. I don't know if I'm overlooking something here.

Does the ALTN FLAP display always appear when the FLAPS CONTROL message appears?

[emerydc8]

Another possible mechanical failure can be a broken linkage between the Flap Input Actuator and the two Flap Power Packages control valve. However you will not know about it until you try to lower the flaps.

Does only one input actuator control the two flap power packages? Where is the linkage located?

Does the ALTN FLAP display always appear when the FLAPS CONTROL message appears?

I thought the alternate (ticked) display appears when you arm the ALT FLAPS. According to the checklist, you can get a FLAPS CONTROL EICAS and still no alternate display. In any case, I think you have to arm the ALT FLAPS first to find out whether or not you will get the alternate display. Of course, every time we have a discussion like this I learn something I thought I knew -- but didn't.

[John H Watson]

Does only one input actuator control the two flap power packages?

Correct. There is only one FCU-controlled electrically driven actuator. 

Where is the linkage located?

It's in a corner of the left hand wing gear well (close to the keel beam and the centre wing fuel tank).(EDIT: I must have been thinking of something else. As you say, it's in the wing gear well). The actuator is connected to a pivoting lever. Two rods are connected to either end of the lever. The other end of one rod is connected to the inboard flap power package (in that wheel well). The end of the other rod is connected to the outboard flap power package (in the right hand wing gear well). The second rod goes through a hole in the keel beam. You can really only see the mechanism if you climb into the wheel well.

[John H Watson]

According to the checklist, you can get a FLAPS CONTROL EICAS and still no alternate display. In any case, I think you have to arm the ALT FLAPS first to find out whether or not you will get the alternate display.

We may have to look into this further. In PSX, it automatically appears if you pull all the FCE AC CBs. I can't remember if I checked this on the real aircraft [EDIT: See next page in this thread]

[emerydc8]

I see it now. It's in the body gear wheel well. I can see how a blown tire on takeoff could take something like that out.