Hello captains,last flight was very strange about what happened. According normal procedure I turned off center fuel pumps when center tank fuel under 500 Kgs / 1000 lbs. They remained OFF for the entire flight.After landing I was very impressed to see that center tank had 0 kgs of fuel. Crossfeed valve obviously closed.How it is possible? Many thanks bye.

Gravity Kade Bellaney

FCOM 2 12.20.2: Center Tank Fuel Scavenge Jet PumpWith the main tank fuel pump No. 1 FWD Switch ON, the center tank fuel scavenge jet pump operates automatically to transfer any remaining center tank fuel to main tank No. 1. Fuel transfer begins when main tank No. 1 quantity is about one-half. Once the fuel scavenge process begins, it continues for the remainder of the flight. T

FCOM 2 12.20.2: Center Tank Fuel Scavenge Jet PumpWith the main tank fuel pump No. 1 FWD Switch ON, the center tank fuel scavenge jet pump operates automatically to transfer any remaining center tank fuel to main tank No. 1. Fuel transfer begins when main tank No. 1 quantity is about one-half. Once the fuel scavenge process begins, it continues for the remainder of the flight. T

Ok, thank you. Does not this procedure lead to a fuel imbalance between main tank 1 and 2? Last night I had the main tank 1 with more fuel (about 200 kgs) then main tank 2. I guess there is another logical reply to my question...Bye.

Ok, thank you. Does not this procedure lead to a fuel imbalance between main tank 1 and 2? Last night I had the main tank 1 with more fuel (about 200 kgs) then main tank 2. I guess there is another logical reply to my question...Bye.

If the imbalance is more then 1000lbs ? dont know the correct number right out of my head use the crossfeed ... if its less dont bother, thats what the manual said :) edit:if tank1 have more than 1000lbs? more activate crossfeed and disable fuelpumps for eng2 until its balanced again, then start fuelpump for eng2 again while deactivating the crossfeedbut do not use crossfeed during takeof or landing

I couldn't for the life of my figure out why I was suddenly getting an imbalance just before T/D on every flight. Now I know! I thought Boeing had recommended leaving 1,000lbs in the center tank to avoid fire/explosion ala TWA 800. Doesn't this center tank scavenge pump logic go against the recommended procedures? Do these procedures not apply to the 737?

someone from maintenance or so would have to explain in depth but the center tank pumps are cooled using the fuel, when the fuel level is 1000lbs or below, there may be instances or aircraft attitudes that may prevent the center fuel pumps from being submerged in fuel enough to cool them, so they would be running and pumping little or no fuel, this causes metal to metal friction and heat and sparks which can ignite the vapor. The scavenge pump i assume is located in a different part of the center tank, possibly the lowest part of the tank so it would not suffer from this problem, unless of course it is cooled some other way

Exactly how much fuel was in the centre tank when you turned the pumps off?

The scavenge pump i assume is located in a different part of the center tank, possibly the lowest part of the tank so it would not suffer from this problem, unless of course it is cooled some other way

The scavenge pump is a "motive fuel" or "jet" pump, no moving parts. You're actually using fuel from #1 Main Tank to suck the fuel from the Center Tank. That's also why the fuel goes only to #1 tank when this is in operation.

I had 1000lbs in the center tank with pumps off the entire flight. Wing tanks had around 7000 each with pumps on. At some point prior tod to but well into the flight, all of the center tank fuel was moved to tank 1 without me doing anything...leaving the center empty. Make sense?My question was not around friction or spark caused by the scavenge pump but about the tank being automatically run dry...which I thought was a no-no

You turned the center fuel pumps off?

I had 1000lbs in the center tank with pumps off the entire flight. Wing tanks had around 7000 each with pumps on. At some point prior tod to but well into the flight, all of the center tank fuel was moved to tank 1 without me doing anything...leaving the center empty. Make sense?

The part that doesnt make sense is this. Off the top of my head i think the pump transfers fuel from centre to left main at a rate of a couple hundred pounds an hour. So doing the math it makes sense that it would take a few hours of flying for the centre to transfer all 1000 pounds to the left main. BUT the transfer doesnt take place until the maons are about half full (which is approx 4300 plus change). 4300 pounds in the left main would take about an hour and 40 minutes (approximately). So if it takes a few hours to transfer the centre tank fuel to left main BUT only just over an hour and a half to empty the mains therefore you would be just about out of fuel before the centre tank even came close to transferring. Whenever we have turned the centre tank pumps off at 1000 pounds in the real 737 we have always landed with fuel still in the centre tank. It has never completely transfered to the left main.

The part that doesnt make sense is this. Off the top of my head i think the pump transfers fuel from centre to left main at a rate of a couple hundred pounds an hour. So doing the math it makes sense that it would take a few hours of flying for the centre to transfer all 1000 pounds to the left main. BUT the transfer doesnt take place until the maons are about half full (which is approx 4300 plus change). 4300 pounds in the left main would take about an hour and 40 minutes (approximately). So if it takes a few hours to transfer the centre tank fuel to left main BUT only just over an hour and a half to empty the mains therefore you would be just about out of fuel before the centre tank even came close to transferring. Whenever we have turned the centre tank pumps off at 1000 pounds in the real 737 we have always landed with fuel still in the centre tank. It has never completely transfered to the left main.

Thanks Jack, very informative. This has happened on the last 5 flights that I've flown. I'm trying to remember the exact behavior, but on my last flight I had a little left in the center tank (pumps off) when I picked up the imbalance. I opened cross-feed and turned off tank 2 pumps (center pumps were already off). Watching the fuel gauge, the center tank appears to be used directly...meaning there was no decrease in tank 1 fuel until the center tank was dry. Is this expected behavior? You guys may want to check the effectiveness (or over-effectiveness) of the scavenge pump. I will fly another flight now, and see if I can catch some screenshots.

Ok, flight complete and screenshots below My completely anecdotal analysis appears to indicate that once the threshold is reached (~4.5k lbs) the fuel system is coded such that the center tank directly feeds engine 1. I say this because, regardless of fuel flow (went from cruise to idle descent), wing tank 1 fuel remained constant. If the scavenge pump were modeled, then I would expect to see wing tank 1 fuel be used based on fuel flow net the scavenge pump rate. Instead, wing tank 1 remained perfectly constant regardless of engine 1 fuel flow. Interestingly, once I landed, fuel seemed to be draw from wing tank 1. Hopefully that makes sense... --On the ground showing fuel and configuration --In flight, right after fuel started being consumed from the center tank (notice the 4.48 and 2.4 fuel flow) --About to land (notice 4.48)  --Shutting down at the gate (notice that taxiing in used fuel from wing tank 1 again)

I thought Boeing had recommended leaving 1,000lbs in the center tank to avoid fire/explosion ala TWA 800. Doesn't this center tank scavenge pump logic go against the recommended procedures? Do these procedures not apply to the 737?

The fuel pumps will run dry and begin to heat up. Keeping 1000lbs in the tank would prevent that from happening (the fuel cools the pumps)Beoing has, however, added and retrofitted a system that will prevent that. If a pump runs dry the decreased amperage and increased pump rpm will trigger a shutdown of that pump. That's when the warnings come on.