Question on CTR tank feeding

[mizra108]

Hi!

So, on last flight I had an interesting observation for which I have a doubt and perhaps someone can shed some insight.

i was on the ground with wing tanks all full and CTR tank had only about 750lb. I turned the ctr tank fuel pumps for taxi but forgot to turn them off for takeoff since they should be off for takeoff and climb when it has less than a 1,000lb (i may be wrong though). Anyways, during the initial climbout, the fuel warning came on but it was going on and off (every 10s or so) the master caution I glimpsed at the fuel gauges, noticed the ctr tank has about 170lbs, i then looked up and noticed that both low pressure lights for the ctr tank where cycling on and off, as I had already evidenced by the master caution. By the time I flicked the pumps off, the ctr tank fuel amount was about 120lbs. Once off (both pumps), i noticed how, over a period of maybe 15mins or perhaps a little more, the fuel amount dwindled down to 0lbs in the tank. 
My doubt then is, if the pumps are off, why would the fuel from the ctr tank continue to be consumed (at a very low low rate). Is this a bug or a feature?

[Litjan]

Hi mirza,

your observations are correct - the master caution for the center fuel tanks will only come on if BOTH pumps show low pressure. Usually one of the pumps runs dry first, depending on the aircraft acceleration and attitude (fuel going forward or aft).

When you turn the pumps off, the low pressure lights extinguish (unlike the regular wing tank fuel pumps).

Also when you turn off the center pumps (I forgot which one of the two) you start the "scavenge pump". It will run for 15 minutes or so and evacuates the remaining fuel out of the center tank into one of the wing tanks.

Cheers, Jan