malfunctions flap disagree bug?

[John H Watson]

Outboard flap drive



https://www.youtube.com/watch?v=-_OM6kSCJNU

[John H Watson]

In my extreme PSX scenario, the opposite side gets no hydraulic and no electric actuation. Depending on where the connection is, I could just say the clutch doesn't provide any friction anymore :-) (Instead of saying the link is broken into pieces.)

Any kind of torque tube jam would lock both sides (no asymmetry and no electric/hydraulic operation)

In the first picture, the hydraulic motor seems to be attached to a big cog which drives the torque tube directly. I can't see how the electric motor (behind the power control module) is attached

[Hardy Heinlin]

No, there is a single combined hydraulic/electric power control module in one of the wheel wells for inboard flaps (and in the other wheel well, the same setup for the outboard flaps).

Of course, you're right. Hua, I need to wake up :-) ... and relearn my own software model. My brain was sleeping too long :-)

OK, back to my original idea: So the only way to get a continuous, asymmetric TE "run-away" is, indeed, a broken link and a failed asymmetry detection? (Clutch is irrelevant.)


|-|ardy

[John H Watson]

So the only way to get a continuous, asymmetric TE "run-away" is, indeed, a broken link and a failed asymmetry detection?

I'm just wondering which Flap Control Unit decides there is an asymmetry. Does the monitoring task follow the FCU in command?

Hydraulic: LEFT >> CENTRE >> RIGHT
ELECTRIC: RIGHT >> CENTRE >> LEFT

Or is there averaging?

The notes say there does need to be an agreement in the "command and monitor" circuits, but both of these seem to be in the FCUs.

[Jeroen Hoppenbrouwers]

The FCUs are probably too crude but do the position sensors at the far ends play a role in asymmetry detection maybe?

[John H Watson]

The FCUs are probably too crude but do the position sensors at the far ends play a role in asymmetry detection maybe?

The position sensors send data to all three FCUs. I just don't know how it is processed by the 3.

[Hardy Heinlin]

Another suggestion: The sensors are OK, the stop commands are OK, but the electric contact to the drive will not open. So the drive will run away like a stab trim runaway malfunction. How about that? :-)

Stab trim runaway is a closed-contact problem, isn't it? That wouldn't be too Hollywood.

[John H Watson]

The sensors are OK, the stop commands are OK, but the electric contact to the drive will not open.

The electric relays are normally open. If there is an asymmetry during hydraulic operation, the FCUs won't allow the electric drive relays to close (in the first place)

The circuit is very complicated. If the extend relay jammed, I'm wondering if the runaway can be stopped by selecting retract (on the Alternate Flap switch) with the Altn system selected...or if the asymmetry system stops this.




I can't figure out how the FCUs operate those relays normally. Where is the power for the coils coming from (in primary and alternate electric?

[John H Watson]

I can't figure out how the FCUs operate those relays normally. Where is the power for the coils coming from (in primary and alternate electric?

After much headscratching... I think I've figured out at least alternate flap operation (with no faults present).

The power side of the extend relay R113 is "X1". Either the FCUs or the Alternate Flap rotary switch provide an earth (ground) to the relay's "X2" contact in the EXTEND position.

Alternate flap operation needs the hydraulic bypass valve to be activated. Power is provided to the bypass motor from CB C644 via the B1 and B2 contacts of the (armed) Alternate Flap Arm switch. Once the bypass motor has reached its fully open position, the bypass valve position limit switch is activated and power is provided to relay R113 (X1 contact) via the de-energised contacts of relay R8114 (this is the no fault situation). The position of R8114 seems to be determined by the FCUs. I'm not sure what logic is used as this has the potential to even lock out alternate flaps.

I can't see how the extend relay contacts can be stuck in the extend position unless one form of electric operation has been active.

It seems there are too many protections to get a runaway asymmetric electric situation with the FCUs operating normally. No flap operation is far more feasable. Even a short to ground on the X2 contact of relay R113 wouldn't activate the relay. The relay still needs power from the hydraulic bypass valve limit switch (which wouldn't be present unless the crew or the FCU activated the hydraulic bypass valve)

[Hardy Heinlin]

I'm wondering if the runaway can be stopped by selecting retract (on the Alternate Flap switch) with the Altn system selected...or if the asymmetry system stops this.

By the way, in PSX the runaway can only run when the ALTN position is in RET or EXT.

OFF stops it.

[John H Watson]

By the way, in PSX the runaway can only run when the ALTN position is in RET or EXT.

Sounds more feasable. I think that, say, a short (to ground) on the X2 (coil) contact of the relays R113 and R114 might produce runaway in the applicable direction. I guess in ALTN mode, the FCU would not interfere and activate the Inboard (or Outboard) Elec Disable relay (R8114/Rxxxx). Note that in PRI Hyd or PRI Elec modes, the same short circuit would be defeated by the safety circuits in the FCUs (uncommanded movement protection). Deselection of ALTN mode would stop the runaway (as would pulling the CB on P6)

Does the QRH allow the pilots to select ALTN control for runaway/uncommanded movement or is this just a hypothetic situation? 

[John H Watson]

[Hardy Heinlin]

Does the QRH allow the pilots to select ALTN control for runaway/uncommanded movement or is this just a hypothetic situation?

QRH says for "FLAPS DRIVE":
"Do not use alternate flaps. Asymmetry and uncommanded motion protection is not provided in alternate mode."

As PSX will only make it asymmetric in ALTN mode, the selection of ALTN would then be a demonstration of a pilot error.

The pilot can, at any time, stop the runaway by deselecting the ALTN mode entirely or by switching the motion off. So this particular scenario is really a mix of machine error and human error.

[dhob]

The flap lever is linked to a triple Rotary Variable Differential Transformer (RVDT) assembly. Each RVDT converts the flap lever position to an electrical signal sent to the flap control units in the main electrical equipment center. The flap control units monitor flap lever position and other airplane systems to command the flap actuator to change flap position.

RVDT flap position transmitters are installed in 7 places throughout the TE flap drive mechanisms. An RVDT transmitter is installed in each power package and in the outboard transmission of each flap. The flap actuator has an integral RVDT transmitter. Each RVDT transmitter is wired to all 3 flap control units, which process the position signal for flap control, position indication, and asymmetry and failure detection.

The asymmetry system protects against a broken or disconnected flap drive torque tube. The Flap Control Units monitor signals from the flap position RVDTs. The left inboard compares to the right inboard flap position, and the left outboard flap position compares to the right outboard. If the difference between left and right signals is more than 7 percent of full travel, the related system is shut down after 0.1 seconds. To do the shutdown, the FCU signals the bypass valve to close and stop flap movement. An asymmetry fault also inhibits primary electric mode operation for the related
system but does not affect alternate operation.

From MX manual. The last paragraph is from 747-8 AMM; 744 AMM does not give the exact percentage of flap travel before the FCU shutdown, but it is likely the same.

Most of the FLAP DRIVE Non-normals in the full motion simulator have this 7% difference in flap travel shown on the expanded EICAS flap indication. Depending on when the NNC is inserted and the adjusted VREF, crews are always surprised at the significant rolling moment when the A/P is disengaged.

[Jeroen Hoppenbrouwers]

If the Captain considers the remaining rolling moment to be too much of a danger to safe operation, would they dare to enable alternate control to at least get the moving half aligned with the stuck half?

Assuming that the moving half extends a bit further than the breaking half, or retracts a bit further, it "should" be possible to reverse that moving half so it agrees again with the stuck half.

Would this be so dangerous that the Captain who tries it would get punished even if they saved the plane?


Hoppie

[Hardy Heinlin]

Dhob, in PSX each malfunction has a "severe" and a "non-severe" mode (for additional variety).

For example:

TE outboard L, non-severe mode:
Hydraulic drive fails, elec drive will operate normally.

TE outboard L, severe mode:
Both hydraulic and elec drives fail in primary mode. There's no motion at all, so there's no asymmetry so far. Extended EICAS symbology and amber bands will occur after a certain time-out (depending on gear position) and certain time delays. In PSX the scenario is designed so that the asymmetric damage will only occur when the ALTN mode is selected by the pilot. An unknown cause will activate the elec drive when in ALTN mode, but not when in primary mode. So when the system keeps running in ALTN mode, the link will break and the asymmetry will increase as the ALTN mode provides no protection. The pilot can stop the motion by deselecting the ALTN mode or by rotating the selector to OFF.