Center fuel tank management / scavenge pumps

[alvin 0]

Hi,I have a question concerning necessary actions for managing center fuel tank pumps. It's rather a small detail, but it made me think about this.FCOM offers three alternative procedures with respect to center tank pumps management for climb/cruise:

Climb and Cruise Procedure [AD 2002-19-52 and AD 2002-24-51]Set both center tank fuel pump switches OFF when center tank fuel quantity reaches approximately 1000 pounds/500 kilograms.

Climb and Cruise Procedure [Alternate Method of Compliance (AMOC) to AD 2002-24-51]During climb or cruise, set one center tank fuel pump switch OFF when center tank fuel quantity reachesapproximately 2000 pounds/950 kilograms. Open the crossfeed valve to minimize fuel imbalance.Set the remaining center tank fuel pump switch OFF without delay and close the crossfeed valve when the Master Caution and FUEL system annunciator illuminate.

Climb and Cruise Procedure [Alternate Method of Compliance (AMOC) to AD 2001-08-24 and AD 2002-24-51 for Airplanes with Master Caution System Logic Change and Automatic Shutoff]Set both center tank fuel pump switches to OFF when a center tank fuel pump LOW PRESSURE light illuminates if the center tank is empty.

I understand NGX has automatic shutoff logic, so the only thing we should worry about is to turn center tank pumps off when low pressure gets illuminated?One additional thing that puzzled me was the scavenge pump. It seems to be not working, at least in my case. I switched central pumps off after 2hrs of flight, but the fuel in central tanks keeps switching between 0.47 and 0.48. FCOM2 says that scavenge pump is on once FWD fuel pump was turned on (it was), and is operational for the rest of the flight. Did I confuse something, or it is a bug?best,Jakub

[gear_monkey 0]

Hi,I have a question concerning necessary actions for managing center fuel tank pumps. It's rather a small detail, but it made me think about this.FCOM offers three alternative procedures with respect to center tank pumps management for climb/cruise:I understand NGX has automatic shutoff logic, so the only thing we should worry about is to turn center tank pumps off when low pressure gets illuminated?One additional thing that puzzled me was the scavenge pump. It seems to be not working, at least in my case. I switched central pumps off after 2hrs of flight, but the fuel in central tanks keeps switching between 0.47 and 0.48. FCOM2 says that scavenge pump is on once FWD fuel pump was turned on (it was), and is operational for the rest of the flight. Did I confuse something, or it is a bug?best,Jakub

The airline that I work with turns center fuel pumps off when quantity reaches 1650 lbs or less or when Fuel Low Pressure lights illuminate. They do this to avoid nuisance warnings at critical phases of flight...like during take-off as a result of fuel slosh.Other airlines my differ I'm sure.As for the scavange pump, it operates when the left fuel tank quantity decreases below 50% and the left fwd electric fuel pump is on.

[Houghton11 12]

The scavenge pump isn't a electric pumP or anything, it's a jet pump or flow ejector as its sometimes called. I'm pretty sure it will only begin to scavenge fuel out of the centre tank once the fuel in the wing tanks drops to a certain level.He whole scavenge system still make me scratch me head. Although I think the NGX's implementation of it is as per the latest AD above. Once you get low pressure, shut off the pumps.Daviarosoft can elaborate on this much more though, and probably help us understand.

[alvin 0]

Ok, thanks guys, now I see where I was wrong. In a later phase of my flight indeed the center tank started to be emptied, but only after left tank went below 50% of capacity, exactly as you wrote.

[davierosoft 13]

PMDG half fixed the scavenge system.The rate is now correct (if you want to see it working, go to the FUEL setup menu on the FMC, it will give you variations of 1kg or 1lb.The thing is not yet fixed is tat the scavenge is working either on ground and in air.As Houghton11 said the system is fully mechanic, with only one moving part, the scavenge valve that is in the main left tank.As you can see te valve is controlled by a float, so only the quantity in the left tank will decide if scavenge operates or not.Obviously, as the pump is a jet pump, the other necessary system is the flow from the the left fwd boost pump.Now, in the pmdg the scavenge stop its operation on ground, when it is useful to balance the APU fuel burned if the left tank is used to feed it.Not a big issue, with the rate adjusted there is no risk for an umbalance.