Boeing 737NG TE Flap Uncommanded Motion

[Cakov]

Can anyone out there enlighten me as to why the 737 NG FSEU compares data from the flap lever position sensor to the trailing edge SKEW sensor?

Why doesn't it just compare the flap lever position sensor to the signal from the trailing edge POSITION sensor?

Am I correct in assuming that this might be because the trailing edge position sensor could be the source of the fault that is causing the uncommanded motion in the first place?

Thanks very much for any ideas...

[avioniker]

From SDS 27-51-00-002
"The TE flap UCM detection function uses data from the flap skew sensors and the flap lever position sensor. If the TE flaps move from their commanded position, the FSEU moves the bypass valve to the bypass position to stop the TE flaps."

One of the more important reasons is because there's only one Position Sensor per wing (2 total) and they're at the most outboard transmission. A broken torque tube as well as a defective transmitter would cause the system to believe the flaps are in position at all times.
The skew sensors (8) are mounted on all transmissions so that a flap segment moving out of position would be immediately detected regardless of the condition of the rest of the system.

(And boy if that isn't super-simplified I don't know what is...)

[Cakov]

Thanks avioniker, but I'm still bemused:

27-51-00-062
"TE Flap Uncommanded Motion (UCM) Detection

The FSEU compares the data from the TE flap skew sensors 1 and 8 to the data from the flap lever position sensors"

From that, it seems that the system only uses two skew sensors - and they're the most outboard ones, too.

[B73567AMT]

Am I correct in assuming that this might be because the trailing edge position sensor could be the source of the fault that is causing the uncommanded motion in the first place?

Cakov

I would say that is a valid assumption...I will research further. But that makes the most sense. They are using the Flap Skew sensors (1 and 8) which are mounted on the same flap drive as the flap position transmitters.

[avioniker]

The system is looking at sensor #1 as X and #8 as Y. If X-Y = >13.25 Deg for more than 1 second with the airspeed >60kts, that is one of many things which will put the flap bypass valve into bypass. :8

The best answer I can get, that makes sense, is that the flap position sensors have an 800 to 1 mechanical turn ratio while the skew sensors are direct drive making the angular position difference easier to calculate in the FSEU.

(and yes, my oops for forgetting that only the outboard sensors are monitored for UCM)
:O

From SDS 27-59-00-002:
"Physical Description Each flap skew sensor has one resolver that uses 28v ac 400 Hz excitation. The table below shows the resolver degrees when the TE flaps are at up and at 40. For sensors 1 thru 4, TE flap extension causes the resolver degrees to increase. For sensors 5 thru 8, TE flap extension causes the resolver degrees to decrease.

Flap Skew Sensor Outputs in Resolver Degrees
Sens #1 2 3 4 5 6 7 8
Up: 303 338 6 340 20 354 22 57
40: 233 288 297 274 86 63 72 127

An input rod connects to the flap carriage and the skew sensor input assembly."