747X: Can I stop fuel transfer from center tanks?

[Guest Daniel42]

During my flight yesterday I ran into a small problem.I had fuel in the main and center tanks, but because the APU had been running rather long, there was a difference between main 2 and main 3 that I tried to fix by using only main 3 for a while.But as soon as I setup this configuration, fuel transfer from the center tanks to main 2 was initiated automatically (center tanks were far from empty).When the amounts in main 2 and 3 were equal again, I switched back to the normal configuration which used the center tanks again. Yet, the transfer from the center tanks to main 2 remained active. (Indicated by the thick green arrow)Is there a way to halt this transfer?Daniel van Os(FSXA, 747X with service pack)

[744pilot 0]

It's fully automatic and it's designed to dissipate the difference between mains 2 and 3 caused by the APU fuel burn.The only way to stop it would be to shutdown the scavange pump through the circuit-breakers...which are not modelled in the PMDG a/c.Something you learned for next time, I guess :(

[Qavion 19]

Strange... Sounds like something is broken or your aircraft fuel load is configured incorrectly.There are only four situations the scavenge pump will operate.1)If either CWT pump has been selected ON for 5 minutes and then developed low pressure (Not possible with "far from empty" centre tank)2)If Reserve Fuel Tank transfer valves are open (this will only happen if the fuel in the inboard main tanks is below 6000 gallons.. about 18,000kilos). With a standard fuel load, if you have fuel in your CWT, your inboard main tanks should be full (apart from minor APU fuel burn).3)If CMC Maintenance Ground Test is running (not modelled).4)If Scavenge Pump Maintenance Switch on overhead panel is manually selected ON(not modelled) If the #2 main tank is relatively full, even if the scavenge pump is running, it would have to force fuel through a mechanical check valve to put fuel into the #2 tank. The check valve is there to stop fuel leaking from the #2 tank back into the CWT. The weight of fuel in the #2 tank will probably slow down fuel transfer.Did you have the correct fuel configuration?Cheers.Q> Ян

[Guest Daniel42]

From what I remember:Main 1, 4: 17 tons eachMain 2, 3: 38 tons eachCenter : 28 tonsRes : 4 tons eachStab : empty(Fuel loaded using the PMDG utility)The manual does mention that the scavenge pump will also operate when the crew turns off the center wing tank pumps, but only as soon as the transfer from the reserve tanks has started, which was not the case.Daniel

[G-CIVA 62]

Why?Steve Bell

[Qavion 19]

"The manual does mention that the scavenge pump will also operate when the crew turns off the center wing tank pumps, but only as soon as the transfer from the reserve tanks has started,... "Ah.. true... I missed one of the logic routes in the aircraft wiring schematics... I guess that makes it 5 ways to activate the pump!So.... having confirmed that your fuel configuration was ok, then something is definitely amiss.I guess it's time to hand over the problem to the PMDG programmers :(I believe Robert Randazzo did a lot of the fuel system programming ;)Cheers.Q> イアン

[Qavion 19]

"Ah.. true... I missed one of the logic routes in the aircraft wiring schematics... I guess that makes it 5 ways to activate the pump!"Actually, after further reading... not really 5 ways, but...The "both CWT pumps off" signal is only to allow the scavenge pump to latch on (stay running) for 2 hours.So, any CWT pump low pressure (i.e. CWT almost empty) plus both CWT pumps off = scavenge for two hours.. orReserve tanks transfer command plus both CWT pumps off = scavenge for two hours.If you let any CWT pump run until the CWT is empty, scavenging should start immediately. Switching off both CWT pumps should latch the scavenging for 2 hours (or until the scavenge pump runs dry for 5 minutes)If you switch off the CWT pumps prematurely (still lots of fuel in the CWT), the scavenge transfer should not begin until your reserve valves get a signal to open.If you were still balancing the fuel (in your scenario) and the reserve valves were commanded open because the inboard tanks had dropped to a certain level, the two hour timer would have been set. However, as I understand it, switching on the CWT pumps again should have reset the timer and the scavenging should have stopped.Curiouser and curiouser :)Anyway, I'll keep reading to see if I've missing something..Cheers.Q>P.S. (Edit) In normal circumstances (in the air), do you get CWT scavenging immediately after the pumps go low pressure?