Fuel In Centre

[blocks_off]

Hi 

Before start, if there is less than 1000kgs in the centre tanks, I leave off the centre pumps until after take off (assuming of course there is fuel in centre!)

I noticed for the first time, when I loaded the fuel I had 830kgs in the center and so the centre pumps were off. Taxiing out I did see a very small reduction - down to 800kgs just after take off. Assume some fuel still gets drawn from the centre in that case ?

Thanks

- Carl

Edited August 6, 2018 by blocks_off

[mgeiss]

Fuel still get's drawn from the tanks with the pumps off. IIRC you can even start the engines (and fly) with all fuel pumps off, as fuel is feeded to them by gravity.

The fuel pumps are needed to ensure positive pressure under all flight conditions.

[Litjan]

You have observed the scavenger fuel pump working. Whenever you turn a center fuel pump off, you trigger this scavenge pump. Its a small electrical pump that will run for a short while and attempts to drain all remaining fuel out of the center tank into the left tank (iirc). It is possible to trigger this pump again and again (not recommended) - the last bit of fuel will slosh around the center tank and depending on the attitude the scavenge pump may not get it all...

Jan

 

[blocks_off]

Thanks - so this is of course why when in the air and switching off the centres the remaining few kilos are used ?

[mmerelles]

the aircraft has a scavenge jet pump that drains the little remaining fuel on the center tanks to main tank 1 so all fuel in the center tank is used.

 

On the classic series 737-300, the scavenge jet pump activates manually by the pilot when you turn off the center tank fuel pump, and it works for 20 minutes, transfer rate is very low.

On the 737-NG the process is automatic, when the center tank is low and the main tanks reaches half their capacity, the scavenge pump activates and drains remaining fuel to main tank 1, process runs for the rest of the flight.

[Litjan]

11 hours ago, mgeiss said:

Fuel still get's drawn from the tanks with the pumps off. IIRC you can even start the engines (and fly) with all fuel pumps off, as fuel is feeded to them by gravity.

The fuel pumps are needed to ensure positive pressure under all flight conditions.

In fact the fuel is getting "sucked" into the engines by the engines fuel pumps through a dedicated set of bypass lines - however, this fuel is getting sucked out of the wingtanks, only. So if you have fuel in the center tank, and BOTH of the center tanks pumps fail, you could be in a difficult situation: The center fuel becomes unusable, and you may be getting into trouble with your trim - using the fuel from the wings only will drive your CG forward, possibly out of limit. So even if your fuel remaining in the wingtanks is high enough to reach a comfortable alternate or even your destination, CG considerations may require you to land much sooner than that.

Jan

 

[mgeiss]

Hallo Jan,

danke für die Klarstellung!

[blocks_off]

Thanks

- Carl