Flap Control Units (FCUs)

[John H Watson]

Just looking back at some previous notes on some real world tests (with flaps UP)...

I did some tests today, but none of the schematics seemed to show what really goes on inside the FCE PS's. The DC CB's should influence what power comes out of the FCE's, but none of my tests today showed this. I pulled out all the FCE DC CBs and there were absolutely no flight deck effects (No messages, no flap indication changes)

Anyway, I had to pull all the AC FCE CBs to make the Alternate Flap Display appear. Unfortunately, because I pulled all of them, the flap display was covered with lots of amber "X"s (all the position transmitters were inop).

then (I reset) 2R AC.... 2R AC powers the Flap Lever's Right RVDT, the Left Inboard TE Flap position RVDT and the Left Outboard Flap position RVDT, so the "X"s were removed from these flaps. The Alternate Flap display disappeared and was replaced with the expanded display. The Flap Lever marker on all four flap tapes returned. If only ALTN control is available in the latter configuration, it would not be obvious to the pilots. However, all my tests were conducted with the flaps UP. Perhaps the displays change if the pilot selects 5 or more units (because this is when the TE flaps start moving).

I pulled 1R and 2R FCE CBs today, depowering 2 Flap Lever RVDTs. The alternate display did not appear.

Unfortunately, I didn't record the EICAS messages. Anyway, PSX seems to reflect the real world in relation to the ALTN FLAPS display (not necessarily what the books say).

[John H Watson]

Here's the only pic I have of the Inboard Flap Drive...

Inboard Flap Drive

The Flap Input Actuator seems relatively safe from tyres.

[emerydc8]

Thanks for the pic.

This is becoming difficult to reconcile with the checklist above. I would sure like to talk to the person who interfaced between the engineering side and the pilots' side at Boeing. The checklist is for all three FCUs failed. It asks you to choose whether the expanded display is in view after you move the flaps with the ALT FLAPS selector (See steps 5 and 6.)

So, is the checklist saying that you don't get the expanded view until you try to move the flaps? (e.g., not only do you have to arm the ALT FLAPS, you have to try to move them before you get the expanded display?). I don't know. This doesn't seem to comport with your actual experience on the real plane -- Unless there's a difference between pulling the breakers you pulled and an actual failure of the FCUs.

[EDIT]: I guess it does make sense that you won't see the expanded display until you move the flaps with the alternate selector. After all, in the normal mode you don't see the flap display until you select the first flap setting.

[Hardy Heinlin]

In the Thales 744 sim you have a fail button for each FCU; so you can fail them all together as well. The info text on the Thales sim's malfunction screen for each FCU failure reads:

=========================
CAUSE
Internal failure of FCU.
EFFECT
With one or two channels failed the flaps will move normally and position indication is available. Status messages are displayed stating which systems have failed.
With all three channels failed there is no flap control via the FCUs and a caution message is displayed. The alternate flap system remains available. There is no flap indication on EICAS.
QRH Reference: FLAPS CONTROL
MESSAGES
FLAPS CONTROL (caution)
FLAP CONTROL L (or C or R) (status)
=========================

[emerydc8]

With all three channels failed there is no flap control via the FCUs and a caution message is displayed. The alternate flap system remains available. There is no flap indication on EICAS.

Thanks for that piece of the puzzle, Hardy. It's looking like the complexities of the FCUs are too much even to replicate in the sim. So the Thales sim is failing every component of the FCU, which would include the position display component as well. I'm wondering if the title of this thread should be modified.

From the CAE sim List of Malfunctions:

With all FCUs failed:
-Only the alternate flaps are operational
EICAS: FLAPS CONTROL L, R. or C (L27623-625);
status
FLAPS CONTROL (L27109), caution

[emerydc8]

Hardy,

Do you have the QRH that the Thales sim refers to under FLAPS CONTROL? Is that a BAE sim or LH?

Jon

[John H Watson]

With all three channels failed there is no flap control via the FCUs and a caution message is displayed. The alternate flap system remains available. There is no flap indication on EICAS.

A very unlikely scenario, but this appears to be the same as the real aircraft (simulated when I pulled all the FCE AC CBs).
The EIUs seem to know when to show the alternate flap display. As far as I can see, the ALTN FLAP Switch only talks to the Flap Control Units, not the EIUs. So the EIUs are either told by the FCU to display the alternate flap display or, in the absence of a signal from 3 dead FCUs, the EIUs automatically show the alternate flap display.

So, is the checklist saying that you don't get the expanded view until you try to move the flaps?

Well, normally the expanded view appears when you have a disagreement between flap lever position and flap position. This usually happens when you have a flap drive problem (the flaps don't match the flap lever). However, to me, it also makes sense that you should get an expanded display if the flaps are not transmitting position data: The system knows it can't compute a disagreement (and shows you where the problem is with a yellow "X" on the expanded display).

I can understand a FLAPS CONTROL message appearing with 2 RVDT's failed. The FCU's process the flap lever RVDT signals and compute a median position.

https://www.mathsisfun.com/definitions/median.html

If the RVDT readings are 1.8, 1.1 and 0.9, the FCU picks the middle one (1.1). I assume if there are only two RVDT's operating, the FCU resorts to averaging. With only one RVDT operating, no median position can be computed.... Unfortunately, the real aircraft doesn't seem show the FLAPS CONTROL message or the alternate flaps display in that scenario.

[Hardy Heinlin]

Do you have the QRH that the Thales sim refers to under FLAPS CONTROL?

Re Thales I have no references at all.

[emerydc8]

However, to me, it also makes sense that you should get an expanded display if the flaps are not transmitting position data: The system knows it can't compute a disagreement (and shows you where the problem is with a yellow "X" on the expanded display).

So you will get an expanded view, but you won't get any flap position information on the expanded view because the data isn't available?

[John H Watson]

So you will get an expanded view, but you won't get any flap position information on the expanded view because the data isn't available?

Well, if you lose data for one flap group, I expect the expanded view. I don't expect it to un-expand if you start losing more groups (or all).

[Avi]

If we talk about the operation of the FCUs here are some more notes for the common good.

When the FCU switches to secondary mode (not alternate) due to lack of bleed air when moving the flaps from UP to 1 or 1 to 5 (or the other way around), the FCU switches back to primary mode once the LE flaps (slats) are in the commanded position (which means return to normal display).

When the FCU switches to secondary mode due to lack of hydraulic when moving the flaps between 1 and 30 (or the other way around), the FCU remains in secondary mode until the flaps are up (or hydraulic resumes) and then switches back to primary mode.

This is well modeled in PSX.

Cheers,

[jtsjc1]

It comes with practice, Joe... but the hard part is programming all this stuff into PSX. Hats off to Hardy 

I'm in complete agreement John. Hardy is an amazing programmer. Btw this is an interesting discussion.

[emerydc8]

I think I need to be more specific on my use of "expanded view." I'm referring to the alternate mode expanded view with the tick marks and 5 and 25 on the display -- not the secondary mode expanded view.

We have been all over the place regarding when the alternate expanded view appears. Initially I thought it was when you armed the alternate flaps, but the QRH suggests that the alternate expanded view would not appear until you not only armed the alternate flaps, but also tried to extend them with the alternate switch. I think another theory was that the alternate expanded mode would display immediately when all three FCUs were lost. But the Thales sim manual says that if all three FCUs are lost, you don't get any flap position indication. Maybe this is because in the Thales sim completely fails the FCUs -- including the flap position components.

It seems like this is getting down to the individual flap position component of each FCU. If all position components of all three FCUs are inop, you don't get any indication. If at least one position component is operative but all three FCUs are failed, you get the alternate expanded view, but there is some question as to when you get it.

So, when will the alternate expanded view appear on the EICAS if you've lost all three FCUs but still have at least one position component operative?

1. Immediately on loss of all three FCUs;
2. When you arm the alternate flaps; or
3. When you arm the alternate flaps and attempt to extend the flaps in alternate mode.

Or, is there another answer that I missed?

[John H Watson]

So, when will the alternate expanded view appear on the EICAS if you've lost all three FCUs but still have at least one position component operative?

1. Immediately on loss of all three FCUs;
2. When you arm the alternate flaps; or
3. When you arm the alternate flaps and attempt to extend the flaps in alternate mode.

1. is true because I have a photo of the alternate display in this condition. Why does it do this, since, as you say, there is no useful information displayed? It may just be a consequence of using a common signal to trigger the display change to the Altn Flap display. e.g. When you select Alt Flaps because the flap lever is broken, (as far as I can see) a signal is sent to the (functioning) FCUs from the Altn Flap switch. This may cause the FCUs to output a "0 (zero)" signal (instead of, say, a normal "1") to the EIUs to show the Altn Flap Display. A total failure of all 3 FCUs may coincidentally generate a "0" signal.  This "0" signal may also be the one which triggers the FLAPS CONTROL message. Speculation, of course.

The only thing I can't figure out is why my notes say that if two flap lever RVDTs fail, it should generate the FLAPS CONTROL message (and I assumed it would cause the Altn Flaps display to appear). It doesn't (or at least not by pulling the FCE CBs) on the real aircraft (or in PSX).

2 is true (I've selected this many times and have photos). Because 2 is true, I guess 3 becomes redundant.

The expanded display runs on different logic. Avi says that the expanded display will return to normal if all the flap groups reach their target flap (true). However, if a flap group has no position information, the system doesn't know if the flaps are in the right position, so the expanded display remains in view.
I would expect the expanded flap display to appear even with flap lever UP (because the system doesn't know if the faulty flap group is up). PSX operates this way and I probably have a few real world photos to show this.

I think another theory was that the alternate expanded mode would display immediately when all three FCUs were lost. But the Thales sim manual says that if all three FCUs are lost, you don't get any flap position indication. Maybe this is because in the Thales sim completely fails the FCUs -- including the flap position components.

I guess Thales assumed that a total failure of all 3 FCUs was more likely than an identical (control) section failure in each FCU.

[emerydc8]

I guess we also have to differentiate between having the alternate flap display appear and having an alternate flap display that shows actual flap position. Was your experience of seeing the alternate display appear when you armed the alternate flaps always done with the flaps retracted? (e.g., did you ever try to run them down electrically with the alternate flaps switch?).

I spoke with a friend who went through initial training at Atlas and he specifically remembers his instructor going through the above checklist, first with the alternate display available and again with the alternate display not available, so he had to time the extension of flaps. So, apparently there is a way to demonstrate this in the Atlas sim.

Avi says that the expanded display will return to normal if all the flap groups reach their target flap (true).

Is Avi referring to the "expanded display" of the secondary mode or the expanded display of the alternate mode? It sounds like he's just referring to when hydraulic power is lost in the primary mode and it switches to the secondary display scale -- not the alternate display scale.

[Avi]

The first (I did say secondary mode when bleed air or hydraulic is gone and not alternate mode which is where you switch the ARM switch to arm it).

[emerydc8]

Thanks, Avi. This is getting more complicated as it goes. Hopefully we will get some clarity to it, eventually.

[John H Watson]

Was your experience of seeing the alternate display appear when you armed the alternate flaps always done with the flaps retracted? (e.g., did you ever try to run them down electrically with the alternate flaps switch?).

Yes, I've run the flaps in Alternate Mode (manually selected). There was also an unexpected uncommanded movement during maintenance (after resetting some CBs), so we immediately selected Alternate Mode to freeze the movement until we could figure out what was going on. Oddly, the flap lever agreed with the flap position prior to resetting the CBs.

I have cycled the Alternate Switch with the flaps already 30 to see the effect. Upon deselecting the Altn Flap switch, the display changed to the Expanded display momentarily, then to the single tape. I don't know if the FCU is "warming up" or if it took the EIUs time to process the information. As you are aware, you can't drive the flaps to 30 in Alternate Mode, but you can retract them from 30 to up.

I don't recall the running the flaps in Alternate mode with an inoperative position transmitter.

I spoke with a friend who went through initial training at Atlas and he specifically remembers his instructor going through the above checklist, first with the alternate display available and again with the alternate display not available, so he had to time the extension of flaps. So, apparently there is a way to demonstrate this in the Atlas sim.

When you say the alternate display was not available, do you mean that there was the basic scale (outline), just no solid white moving tape...  i.e. yellow "X"s instead?

[emerydc8]

When you say the alternate display was not available, do you mean that there was the basic scale (outline), just no solid white moving tape...  i.e. yellow "X"s instead?

I'll check with him. Hopefully he remembers.

[John H Watson]

Here's a pic at 30units

Altn Flap 30 Indications