AUX HYD pump inflight

[ASCTU744]

Is there a way to operate the AUX hyd pump inflight? I powered the GH bus with the APU (inflight), but the AUX pump still doesn't operate.

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GenEXT1: 000.0V          GenAPU1: 115.1V          GenAPU2: 115.3V          GenEXT2: 000.0V     
XPC1   : 000.0V          APB1   : 000.0V          APB2   : 000.0V          XPC2   : 000.0V     
                                SSB < : 000.0V       SSB > : 000.0V     
             Bus AC Sync Left: 000.0V                          Bus AC Sync Right: 000.0V
BTB1 t : 000.0V          BTB2 t : 000.0V          BTB3 t : 000.0V          BTB4 t : 000.0V     
BTB1 b : 000.0V          BTB2 b : 000.0V          BTB3 b : 000.0V          BTB4 b : 000.0V     
Bus AC1: 114.9V/60.47A     Bus AC2: 115.2V/108.27A     Bus AC3: 114.7V/123.25A     Bus AC4: 114.8V/53.18A
GCB1   : 114.9V/60.47A     GCB2   : 115.2V/108.27A     GCB3   : 114.7V/123.25A     GCB4   : 114.8V/53.18A
GenEng1: 114.9V          GenEng2: 115.2V          GenEng3: 114.7V          GenEng4: 114.8V     

p6A2   : 114.9V/10.9A     p6A11  : 115.2V/13.44A     p6A20  : 114.7V/30.42A     p6A29  : 114.8V/9.15A
p6A5   : 114.9V/3.53A     p6A14  : 115.2V/3.02A     p6A23  : 114.7V/9.1A     p6A32  : 114.8V/1.83A
TRU1   : 28.03V/14.49A     TRU2   : 27.99V/12.4A     TRU3   : 28.01V/37.38A     TRU4   : 28.02V/7.52A
Bus DC1: 28.03V/14.49A     Bus DC2: 27.99V/12.4A     Bus DC3: 28.01V/37.38A     Bus DC4: 28.02V/7.52A
DCIR1 b: 000.0V          DCIR2 b: 000.0V          DCIR3 b: 000.0V          DCIR4 b: 000.0V     
DCIR1 t: 000.0V          DCIR2 t: 000.0V          DCIR3 t: 000.0V          DCIR4 t: 000.0V     
                                        Bus Tie DC: 000.0V
p6K6 : 28.03V/13.17A   p6A9 : 28.03V/7.88A   p6B9 : 28.03V/0.0A   p6B10: 28.03V/0.0A
p6B5 : 114.9V/0.0A   p6B23: 114.7V/5.92A   p6B7 : 114.9V/0.0A   p6B11: 115.2V/7.87A
p6K8 : 28.01V/11.9A   p6A26: 28.01V/0.0A
IBVS Cpt: 114.7V/5.92A  Bus Xfer Cpt: 114.7V/2.35A  IBVS F/O: 115.2V/7.87A  Bus Xfer F/O: 115.2V/0.48A
Bus GH: 115.1V/0.05A  Bus GS: 114.9V/24.05A  T150: 27.97V/0.53A  Bus GS28AC: 27.97V/0.53A
Bat APU: 27.95V/0.0A   Bat MN : 27.96V/0.0A   Bat ISFD: 27.93V ChrgMode: 0 p7B25: 28.01V/0.0A
Disab APU: 114.9V/0.06A  Chrg APU: 27.98V/0.28A  Bus Hot APU: 27.98V/0.28A
Disab MN : 114.9V/0.1A  Chrg MN : 28.04V/0.43A  Bat Pole MN: 28.04V/0.43A  Bus Hot MN : 28.04V/0.43A
p6K1: 28.04V/0.43A  p6G9: 28.04V/0.0A
p6J7: 28.04V/0.0A  p6J9: 27.98V/0.28A  Xfer APU: 27.98V/0.28A
p6K2: 28.04V/0.0A  p6K3: 28.04V/0.0A  Bus M4534/R7687B: 27.98V/0.28A
p6J8: 28.01V/10.45A  p6K4: 28.01V/13.5A  p6K5: 28.01V/0.0A
Main Bat Rel C: 000.0V/0.0A  Main Bat Rel B: 000.0V/0.0A  Main Bat Rel A: 000.0V/0.0A
Bat Xfer Rel C: 28.01V/10.45A  Bat Xfer Rel B: 28.01V/13.5A  Bat Xfer Rel A: 28.01V/0.0A
Bus APU Bat: 28.01V/10.45A  Bus MN Bat: 28.01V/13.5A  m855m866: 28.02V/1.1A  m853: 28.02V/1.1A
p6B24: 114.7V/7.76A  p6A25: 114.7V/0.0A  Inv MN in: 28.01V/0.0A  Inv MN out: 000.0V/0.0A
Xfer MN Stby: 114.7V/7.76A  Bus MN Stby: 114.7V/7.76A
p6J6: 27.98V/0.0A  p6J5: 27.98V/0.0A  Xfr APU Inv: 000.0V/0.0A  Inv APU: 000.0V/0.0A
p6B26: 000.0V/0.0A
Xfr Tow: 000.0V/0.0A  Inv Tow: 000.0V/0.0A  Tr Tow: 000.0V/0.0A  Bus Cgo Hdlg: 115.3V/0.0A
p6E22: 114.8V/0.54A  p6L1: 114.7V/0.0A  R7734b: 114.8V/0.4A  R7734a: 114.7V/0.27A  R7786d: 114.7V/0.27A
p6L2: 28.01V/0.37A  p6F24: 27.97V/0.0A 
BCU-1 in: 28.01V   BCU-2 in: 28.04V
ALTN EFIS:
Xfr2 APU Inv: 000.0V/0.0A
R8288>8289FMC: 000.0V/0.0A  R8288>8289: 000.0V/0.0A  R8288>8290: 000.0V/0.0A
R8289a: 000.0V/0.0A  R8289b: 114.7V/1.29A  R8289c: 114.7V/3.57A
R8290a: 000.0V/0.0A  R8290b: 115.2V/2.82A
p6J18: 28.01V/0.0A  p6B27: 114.7V/0.0A  p6B28: 115.2V/0.0A
vCount 9  aCount 34  dCount 7
I wonder what item draws 0.05A from the GH bus.

[Magoo]

The GH bus shouldn't be able to be powered in flight. It was designed this way to prevent the accidental opening of the cargo doors in flight so no electrical power possible to them so incidentally no power to aux 1 and 4 and refueling system.

[Hardy Heinlin]

The GH bus shouldn't be able to be powered in flight. It was designed this way to prevent the accidental opening of the cargo doors in flight ...

Do you mean the Ground Handling Bus or the Cargo Handling Bus? I think the cargo nose door is powered by the Cargo Handling Bus which is powered by APU-2 or EXT-2.

I guess in the above situation the Ground Handling Bus is powered by APU-1 because the APU is running (but the APU cannot power the AC bus system in flight). The 50 mA must be drawn by one of the two BCUs.

[Magoo]

I thought this was the case on the passenger airplanes, yes Cargo handling bus power by APU 2 or EXT 2 power available mode only.

[John H Watson]

Isn't the GHB logic controlled by 3 or more engine generators not above 6100rpms? Also, I see a 60 second time delay in the BCU.

What type of engines are you using?

6100rpms =
PW 73.3% N2
GE 74.6% N2
RR 67.4% N3

[Hardy Heinlin]

I powered the GH bus with the APU (inflight) ...

How did you do that? I just tried this in my PSX with the APU running during takeoff, but after liftoff the GHB got 0 v.

Did you pause the sim on the ground and then moved it up with the Instructor's altitude slider? The pause function will freeze the on-ground gear strut compression status and the 744 systems will think the aircaft is still on the ground.

[ASCTU744]

I loaded the "at final cruise alt" situ (Freighter with PW engines). I pulled the GRD SAFETY CB, started the APU and then pulled the LDG GEAR DSP&CONT ALTN CB (this allowed both AVAIL lights to be illuminated). Finally I positioned the throttles at idle to Close the APU GEN breakers. I re-checked this today and the GH bus still indicates 115.0V/0.05A.

[ASCTU744]

Isn't the GHB logic controlled by 3 or more engine generators not above 6100rpms? Also, I see a 60 second time delay in the BCU.

What type of engines are you using?

6100rpms =
PW 73.3% N2
GE 74.6% N2
RR 67.4% N3

When I increase thrust above 73.3% N1 the APU GEN breaker trips, the GHB still remains powered (AVAIL lights still illuminated).

[Hardy Heinlin]

I loaded the "at final cruise alt" situ (Freighter with PW engines). I pulled the GRD SAFETY CB, started the APU and then pulled the LDG GEAR DSP&CONT ALTN CB (this allowed both AVAIL lights to be illuminated). Finally I positioned the throttles at idle to Close the APU GEN breakers. I re-checked this today and the GH bus still indicates 115.0V/0.05A.

Well, that's obviously not a normal procedure :-) As you pulled those CBs, the system thinks it's on the ground with the engines shut down, thus the system commands the GHB to be powered which also powers either of the two BCUs in this case, which draws 50 mA.

[ASCTU744]

Well, that's obviously not a normal procedure :-) As you pulled those CBs, the system thinks it's on the ground with the engines shut down, thus the system commands the GHB to be powered which also powers either of the two BCUs in this case, which draws 50 mA.

XD, ah I see. But why is the GHB not operational for i.e. the AUX ACMP?

[Hardy Heinlin]

As long as the related DEM pump or EDP is pressurized, the AUX pump remains in standby mode.

You may try to depressurize both the DEM and EDP and see what the AUX pump will do then.

[ASCTU744]

As long as the related DEM pump or EDP is pressurized, the AUX pump remains in standby mode.

You may try to depressurize both the DEM and EDP and see what the AUX pump will do then.

The AUX still doesn't run, I depressurized the EDP and demand. The HYD synoptic indicates a yellow AUX pump symbol.

[Hardy Heinlin]

OK, that's normal. I can tell you why the AUX pump won't start:

(A) For HYD #4 on-ground mode you need to work with F22, not F23.

(B) F22 is an "in-air" mode CB. So when F22 is unpowered or pulled, it signals "in air", not "on ground".

(C) HYD #4 on-ground mode is not only linked with F22 but also with many mechanical factors in the gear system.


So you can't fool (B) because the "in air" mode remains when you pull that CB.
And you can't fool (C) because your gear struts are not compressed, the nose wheel is not down etc.

There are a lot of factors involved here to prevent the AUX pump start in flight :-) It's actually impossible.

[ASCTU744]

Thx 👍