mmmmm, NSEU - a great question as always, they definitely get me thinking.
For the purposes of my answer we shall assume the following:
1- All engine driven generators have failed in flight.
2-Therefore the only power available are the batteries and the aircraft is now reliant on the battery bus for DC and the Static Inverter for AC PWR.
3-No other defects exist with the aircraft. Therefore engines continue running and full hydraulic power is available.
The question is
Will the T/E flaps extend ?
Answer............
YES, now for the why!
I'll start at the flap drive units and work backwards.
For the Flap power Control Package to be positioned to operate hydraulically the Flap Input Actuator needs to operate.
The Input actuator is powered by 115v AC Standby Bus (which will be available) So the actuator can extend and retract. How does it get commanded to extend and retract?
Command comes from the Flap Cntr Units via relays. Will these be powered?
The control relays are powered via 28 volt DC which comes from the Main Battery Bus so the relays will be operative. Relay control comes from the FCU's but the only FCU that will be powered is the Left FCU. How does it get its power??
It is powered from the Left Flight Control Electronics Power Supply Module. The left PSM is in turn powered from the 115v AC Stby Bus (static inverter)
So far so good.
For the left FCU to "know" where to position the flap input actuator, it needs to get an input from the flap handle RVDT. This is a triple RVDT with one winding to each of the three FCU's. In our case only the left FCU will get information from its winding which is 26v AC provided via the FCU and FCE PSM. (both powered under standby power situation) As NSEU mentioned earlier, a MEDIAN is used but in this case there is only 1 reading instead of three so that then becomes the median.
As for assymetry protection your statement below is true,
With Standby Power only, you don't have TE flap position feedback from the flaps on the left hand wing. I assume the aircraft would see this as an asymmetry and shut down primary flap control.
The aircraft(FCU) would see it as asymmetry and attempt to shut down flap movement by closing the Flap Bypass valve. However the power for driving the valve to the close position comes from the 28volt DC BUS 3 which in the case above will be UNPOWERED so the valve cannot close therefore the flap drive unit will continue to drive to it's commanded position until mechanical followup nulls the PDU in the selected position (i.e whatever position the input actuator is in)
Gotta give it to Mr Boeing - a gem of an aircraft and well thought through.
Clear as mud as usual.
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