View Full Version : TE Flap Control With Standby Power Only (B744)
Just wondering if you would have Trailing Edge Flap Control with Standby Power only (assuming you had hydraulics from windmilling or running engines to power the flap motors) on the Boeing 747-400.
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.
http://www.ozemail.com.au/~b744er/StbyEICAS.JPG
Thanks.
Rgds.
NSEU
My training notes also tell me that there are 3 position sensors (RVDTs) in the flap lever. The median position of these sensors is used for flap control. If two sensors have failed, flaps will only work in Alternate mode (according to my notes). With only Standby Power available, it seems that only one of these flap lever position sensors is powered.
Therefore, it seems, no flap movement during Standby Power ops.
Strikes me as odd, however, that aircraft logic is preventing you from using something which is essentially usable. :confused:
Don't have notes on me but I'd say shedding as much as possible is the idea on standby power so the position indicators are culled to a single I/B and O/B flap indication, odds of a flap asymmetry as well are slim, also there's no need for multiple FCUs, basically redundancy is a waste when on standby power. I'd say the flap would still work if hyd power was available.
spannersatKL
18th Jun 2009, 15:54
SMOC
Would need the FCUs (Flap Control Units) working to interpret the flap lever movement!! I would need to look at the busses that are powered in Standby Mode though....A good question in the first place though!
Jumbo Driver
18th Jun 2009, 16:04
My 744 TM say that the Main Standby Bus powers the following:
Avionics and warning system status assembly
Flight Control 1L and 2L AC
Left ADC
Left ATC transponder
Left EFIS control
Left EIU
Left FMS-CDU
Left ILS
Left MLS
Left VOR
Primary trailing edge flap control AC
RMI
Right ADF
Standby ignition 1-4
Standby instrument lights
Upper EICAS
So, on the basis that there is no point having primary (i.e.hydraulic) trailing edge flap control AC if you can't run the flaps, I would think they should run with Standby Power only.
If TE asymmetry is detected in these conditions (if it can be), I would expect EICAS caution message FLAPS DRIVE to be displayed and the associated FCU for that group to shut down.
Hope this helps ...
JD
:)
point8six
18th Jun 2009, 19:36
No longer current on B744, but........ T/E flaps assymetry is only available with normal (hydraulic) operation. If you are down to a minimum of 30 mins electrics available (standby power only), I would think that as the autopilot is not available, any assymetry would be detected early on in the flap deployment, through the control column, i.e. before Flaps 25 is selected.
Not a pleasant prospect, but in 40 years of operation, how many 747's have suffered a total electrics failure? Not even "Eric's glider" I believe!?
Hi spanners, I was pointing out you don't need multiple FCUs not you don't need any. :ok:
If the flaps didn't work on stanby power I think this would be common knowledge. (must have hydraulics).
Didn't QF land in BKK on standby power? Surely a discussion would have come up on a flapless landing if they had done one.
Spanner Turner
19th Jun 2009, 07:49
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.
:ok:
.
Excellent responses, guys.
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)
Thanks, Spanners. This seems to be a pivotal point.
I just wish I understood why my notes say:
If two (flap lever) sensors have failed, flaps will only work in Alternate mode
Didn't QF land in BKK on standby power? Surely a discussion would have come up on a flapless landing if they had done one.
During the QF Bangkok water-in-the-MEC incident, one person on the PPRuNe forum said that the leading edges were (already) deployed early for the approach into BKK, so it wouldn't have been a completely flapless landing. The same person said, however, that the flaps were deployed in Alternate mode (very unlikely if the main elec busses were dead!).
Many thanks....
Rgds.
NSEU
I found a semi-official report on the QF2/BKK incident. This said that the flaps and gear operated normally for landing.
Perhaps the single flap lever RVDT operates in the same manner as the flap position transmitters. The lever position signal may processed by the Left FCU and a signal is sent to the Flap Input Actuator, but the failure of the other flap lever RVDT's causes the Left FCU to also (attempt to) send a signal to the Flap Bypass Valve. However, the signal doesn't arrive because of Bus #3 failure.
Pure speculation, however.
Rgds.
NSEU