Boeing 737 Center Pumps INOP: Fuel Really Unusable?

[LEM]

Everybody knows that if both center tank pumps are INOP the center tank fuel is not usable anymore.

Or is it?


CENTER TANK SCAVENGE JET PUMP.
When both Center Tank Fuel Pump Switches are turned OFF, the fuel scavenge shutoff valve opens. This allows...

You have already got my point!


After both center tank pumps failure, couldn't we use this feature to transfer fuel to N.1 tank?

After the 20 minutes cycle, couldn't we start more cycles by turning the center pumps ON momentarily then OFF?

LEM

[BOAC]

I think it will work, Lem, but the transfer rate is abysmally slow - you would NOT want to be waiting for.......................

[Dunflyin]

Why have your centre pumps failed ? Don't know ? Then leave well alone, nothing like a nice little spark in a Boeing centre tank to ruin your day.

[DDG]

LEM
1)The rate of fuel transfer will not be enough to support the fuel flow rates required by the engines,as it uses a small jet pump
2)The scavenge jetpump will only operate after a centre tank pump is switched off and will run for 20min to remove residual fuel in the centre tank
3)If you cycle the centre pump swictches every 20min you will have the scavenge jet pump running .
However you do run a risk of cavitating both the centre tank pumps as the inlets will be no longer sumerged(scavenge pump has done its job for 20min removing fuel),the reprime line for the centre tank boost pumps will be drained of fuel and be full of air.
By doing this the next time someone operates the centre tank pumps they will cavitate and destroy themselves as to reprime the centre tank reprime lines requires a minimium of 3000 to 4000kg of fuel in the centre tank.
Regards,DDG

[BOAC]

Unless our Pilots' Tech manual is wrong, these references to 'problems' with the centre tank pumps are not relevant, since my understanding is that it is No 1 Main tank pump that scavenges? The only time scavenge will not work is when this pump is u/s? No need to touch the ctre switches! I still think it will take all day!

[LEM]

This scenario calls for a center tank with loads of fuel, so the pumps will not be exposed to air.

I agree the transfer rate is very small, I was wondering if the logic of the 20 minutes could be restarted indefinitely, and if some engineer could provide the exact transfer rate figure...

[eng123]

Transfer rate approx 100-200 KGS per hour.Fuel scavenge jet pump motion is provided by #1 tank FWD boost pump.However.the scavenge shut-off valve [float operated] will not be open until #1 main tank fuel qty is below 1900 KGS.

[BOAC]

However.the scavenge shut-off valve [float operated] will not be open until #1 main tank fuel qty is below 1900 KGS

That does not fit with my experience on 732/3/4/5, eng. Is it NG type specific? It is understood that it is designed to EMPTY the center tank when the ctre tank pumps dry. At this stage there will probably be around 4000+ in each main.

[eng123]

I am talking NG here.Cant remember much about the classic system.