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Flap Control Units (FCUs)

Started by John H Watson, Tue, 29 Nov 2016 23:58

John H Watson

Quote from: emerydc8During ground anti-ice operations, you have to manually run up the engines to 60% N1 for 30 seconds every 30 minutes.

Excellent tip, thanks, Jon.

Quote from: HardyIn PSX too anti-ice related high idle is inhibited while on the ground.

Thanks, Hardy.... Brain fade (on my part).

I just checked the following circuit and was wondering what effect disabling the Flap Control Units or flap position transmitters would have (in flight) with the flaps in landing range and the thrust levers at idle (anti-ice off).

High Idle Circuit (GE)

The circuit suggests that (unpowered) the FCU's would output a ground (earth) to the EEC, putting the the thrust back to low idle.  I couldn't remember if the idle system uses the inboard or outboard flaps, so I pulled all the FLT CONT ELEC CB's on the overhead panel. However, no change resulted in PSX.

Something to check on the big sims.... ;)

Rgds
JHW

John H Watson

QuoteI couldn't remember if the idle system uses the inboard or outboard flaps, so I pulled all the FLT CONT ELEC CB's on the overhead panel.

The schematics seem to indicate that the inboard TE flaps are responsible for the flaps in landing range high idle signal. However, only the Left and Right Flap Control Units output this signal to the EECs.

Does the QRH mention anything about high idle being affected by FCU failure? (or even Alternate Flap selection)?

emerydc8

There's nothing about it under the failure of all FCUs, but the FLAPS DRIVE checklist might shed some light on it.

QuoteFLAPS DRIVE
Condition: One or more flap groups have failed to drive in the secondary mode, or an asymmetry condition is
detected.
Do not use alternate flaps.
[Asymmetry and uncommanded motion protection is not provided in alternate mode.]
Do not use FMC fuel predictions with flaps extended.
LANDING PREPARATION:
GROUND PROXIMITY FLAP
OVERRIDE SWITCH. . . . . . . . . . . . . . . . . . . . . . . . . . OVRD
Use flaps 25 and VREF30 + 25 for landing (using flaps primary or secondary). If the amber minimum maneuvering speed band is above the flap maneuvering speed:
AUTOTHROTTLE. . . . . . . . . . . . . . . . . . . . DISCONNECT
Use the displayed flap maneuvering speeds.
Do not accomplish the following checklist:
AIRSPEED LOW
If the inboard trailing edge flaps are affected:
CONTINUOUS IGNITION SWITCH. . . . . . . . . . . . . . . .ON
[Automatic continuous ignition may may be inoperative. Provides automatic approach idle.]

Hardy Heinlin

#3
I just split this thread from there: http://aerowinx.com/board/index.php?topic=3948.msg41740#msg41740

I've made a modification in PSX 10.0.7 http://aerowinx.com/board/index.php?topic=3388.0 item 0.7.0049. Since this modification the amount of operative RVDTs have no influence anymore on the flap setting data that is sent to other systems.

To refresh my memory: Is RVDT L powered by FCU L, and RVDT C by FCU C, and RVDT R by FCU R?

In previous PSX versions, the last detected flap lever position was frozen (and used) when all three RVDTs failed. Should it now be frozen (and used) when FCU L+R are inoperative?

For the high idle logic, PSX uses the flap lever position (flap 25 or 30).

(Not to confuse with the continuous ignition logic for which PSX uses FCU L+R and the actual flap surface position.)


Cheers,

|-|ardy


John H Watson

#4
Quote from: JonIf the inboard trailing edge flaps are affected:
CONTINUOUS IGNITION SWITCH. . . . . . . . . . . . . . . .ON
[Automatic continuous ignition may may be inoperative. Provides automatic approach idle.]

So this seems to be saying that the CONTINUOUS IGNITION overrides the (now inoperative) flap position logic...  and therefore gives approach (high) idle.

Quote from: HardyTo refresh my memory: Is RVDT L powered by FCU L, and RVDT C by FCU C, and RVDT R by FCU R?

All 3 RVDT positions are fed to all FCUs. However, the RVDTs are powered directly by the FCE Power Supplies (each of which have multiple power supplies (CBs)). I recall everything in this area was modelled in PSX) .

Quote from: HardyIn previous PSX versions, the last detected flap lever position was frozen (and used) when all three RVDTs failed. Should it now be frozen (and used) when FCU L+R are inoperative?

For the high idle logic, PSX uses the flap lever position (flap 25 or 30).

I'm starting to think that the approach/high idle also uses similar logic to the continuous ignition. I'll join a few diagrams together to see if I can follow the path. Back shortly....

Cheers
JHW






emerydc8

Quote[Automatic continuous ignition may be inoperative. Provides automatic approach idle.]

Wonder what they mean by "may."

John H Watson

QuoteWonder what they mean by "may."

There are probably a number of unusual faults which may not cause high idle problems. E.g. flaps stopping at 24 units  ;D (enough to trigger the high idle).

Here's a compilation of diagrams... The signal path from the inboard TE flap position sensors is not quite clear. There is also a labelling anomaly on one diagram.

High Idle Circuitry (GE)

John H Watson

P.S. Looking at more diagrams, it looks like the Left Flap Control Unit controls 1 & 4 engine idle and the Right Flap Control Unit controls 2 & 3 engine idle. I don't know if there is any FCU switching for these functions (if FCU's fail). FCU flap control functions have switching (due to control failures), but position information could be handled differently.

emerydc8

Quote from: Hardy Heinlin on Wed, 30 Nov 2016 04:49

For the high idle logic, PSX uses the flap lever position (flap 25 or 30).

(Not to confuse with the continuous ignition logic for which PSX uses FCU L+R and the actual flap surface position.)

So, with a failure of all three FCUs, our manual doesn't have us match the flap lever to the actual flap position, which would have been set using alternate flaps. If approach idle is controlled by flap lever position, it's strange that the checklist doesn't require that we either match the flap handle to the flaps or select CON IGN at flaps 25 to force it into approach idle.

Quote
I don't know if there is any FCU switching for these functions (if FCU's fail). FCU flap control functions have switching (due to control failures), but position information could be handled differently.

It sounds like there is. Thanks for the chart.




Avi

Quote from: Hardy Heinlin on Wed, 30 Nov 2016 04:49
To refresh my memory: Is RVDT L powered by FCU L, and RVDT C by FCU C, and RVDT R by FCU R?

There are RVDTs all over the place but if you mean the 3 RVDTs that transmit the flaps lever position, the answer is yes.
Avi Adin
LLBG

John H Watson

Quote from: AviThere are RVDTs all over the place but if you mean the 3 RVDTs that transmit the flaps lever position, the answer is yes.

Are you sure, Avi?

Do you have wiring schematic 27-51-01 Sheet 1? RVDT excitation is provided by 26Vac (M7358(L)), M7360(C) and M7361(R)).

M7358 = FCE Power Supply 1L
M7360 = FCE Power Supply 1R
M7361 = FCE Power Supply 2R

This 26vac power also goes to the FCU's (but I can't recall, offhand, what it's used for).

Rgds
JHW

John H Watson

Quote from: emerydc8So, with a failure of all three FCUs, our manual doesn't have us match the flap lever to the actual flap position, which would have been set using alternate flaps. If approach idle is controlled by flap lever position, it's strange that the checklist doesn't require that we either match the flap handle to the flaps or select CON IGN at flaps 25 to force it into approach idle.

It may depend on the failure. It would be extremely rare to get a complete failure of all 3 FCUs (both control and indication circuits). Even if you force the flaps into ALTN mode (with the button), the flap position is usually available on the Upper EICAS (the flap position transmitter signals are processed by the FCUs).

Only if all FCUs were completely dead would you not get flap data. You can see this if you pull all FCE Power Supply CBs on P7. During Standby Power ops, some of the TE flap position transmitters will be unpowered, but at least one FCU remains powered to provide flap position (albeit only on one wing).

I'll keep looking for a position data switching function, but I don't think I'll find it.

Rgds
JHW

John H Watson

Here's an extract from some engineering manuals regarding switching of functions...

FCU Functions


Even though it says all FCUs have indication capability, the wiring schematics and the wiring diagrams still show that the Left Flap Unit only outputs to engines 1 & 4 for idle control and the Right FCU only outputs to engines 2 & 3 for idle control.

I'm still looking for the source of the 15vdc required for the internal switch in the FCU (and figuring out what the default position is if there is no power)

emerydc8

#13
As far as I know, the only time you would use the ALT FLAPS is if all three FCUs fail or if the flap lever fails. I was taught that the ALT FLAPS switch completely bypasses the FCUs. Are you saying that the expanded/ticked flap position display in ALT mode is generated by an FCU?

[EDIT]:
Looking at my QRH for failure of all three FCUs (FLAPS CONTROL EICAS), there is a point where you decide if the expanded flap position indicator is inoperative. If it is inop, the autopilots are inop, and the outboard ailerons are unlocked. During extension, they have you start out at clean maneuvering speed, select the EXT position and slow to flaps 5 speed after 3 minutes and 45 seconds. Then slow to flaps 25 approach speed after 5 minutes total.

Avi

John,

I do have 27-51-01 but I looked at 27-09-00. I should have said it differently: the same power source (PSM) of a FCU (L/C/R) is powering the flap lever RVDT (L/C/R). You took one out, you took the other out (for example: FCU R is unpowered, flap lever RVDT R is unpowered and vice versa).

Cheers,
Avi Adin
LLBG

jtsjc1

Quote from: John H Watson on Wed, 30 Nov 2016 07:43
QuoteWonder what they mean by "may."

There are probably a number of unusual faults which may not cause high idle problems. E.g. flaps stopping at 24 units  ;D (enough to trigger the high idle).

Here's a compilation of diagrams... The signal path from the inboard TE flap position sensors is not quite clear. There is also a labelling anomaly on one diagram.

High Idle Circuitry (GE)
John I give you all the credit in the world, when I look at that wiring diagram my head spins. I got my hands full wiring Christmas lights!
Joe

John H Watson

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

John H Watson

QuoteThe circuit suggests that (unpowered) the FCU's would output a ground (earth) to the EEC, putting the the thrust back to low idle.

Back to the drawing board...

In some Boeing diagrams, the switch symbol in the FCU is shown as a transistor circuit. This appears to change the operation of the circuit when the FCU is unpowered.

FCU Switch

If I'm reading this circuit correctly, no earth will be generated when the FCU is unpowered (unlike the original diagrams).

Theory: Without power, the transistor will not conduct and therefore present an open circuit. This means there will be no earth presented to the idle control circuit. No earth = high idle.

I stand to be corrected  ;D

More PSX testing required to see if PSX corresponds to this new logic....

(EDIT) I also see that different flap units are given for circuit triggering.

emerydc8

QuoteIt may depend on the failure. It would be extremely rare to get a complete failure of all 3 FCUs (both control and indication circuits).

I think I see what you are saying. The FLAP CONTROL is the control circuit and the FLAP SYS MONITOR is the indication circuit? So, the FCU can be inop but the indication circuit can still work?



John H Watson

QuoteSo, the FCU can be inop but the indication circuit can still work?

I would say it's possible. Individual computer chips can fail in any black box. Also, each FCU is supplied with a number of voltages by the Flight Control Electrical (FCE) Power Supplies rather than the FCU just having just one power supply, so it would be relatively immune to having one power failure taking out all the circuits.

I'd have to do a lot more reading to make sense of the MEL you posted. There are dozens of defects which can generate the FLAP SYS MONITOR message.

Your MEL does show clearly, however, that a particular FCU may have unique functions (e.g. hydraulic pump control). The FCUs process stabiliser position, too... not for the stab trim indicators on the pedestal, but for autopilot use. The Left FCU supplies stab trim position to the Left Flight Control Computer, etc.

Although there is nothing specific in this FCU MEL for checking that engine idle is ok, I just discovered that the Fault Isolation Manual "fault tree" (fault flow chart) for IDLE DISAGREE EICAS messages does include a step for replacing, say, the Right FCU if there is a common fault on engine 2 & 3 (assuming the fault tree doesn't take you in a different direction after carrying out the various tests).