Inflight Fuel Management

[MD82]

Hello ini,

as I progress through my long-range flights, I'm having fuel balancing problems between tanks. The rule is that as the fuel quantities between Tank 1, Tank 3 and the SUM of Tanks 2L and 2R become equal, fuel configuration changes to the "tank-to-engine" config. In this configuration, Tank 1 feeds ENG 1, Tank 3 feeds ENG 3 and Tanks 2L+2R feed ENG 2 (Hence the need for the sum of the 2 tanks). With all cross-feeds now closed and all engines consuming fuel at the same rate, the ratio of equal amounts between tanks 1, 2L+2R and 3 should persist. 

Here is what I am observing:

As I manage to re-match the fuel quantities exactly, you can see I am about to return to the tank-to-engine configuration.

Re-configuring is now complete, with all pumps ON and all cross-feed valves closed. Fuel quantity of 30.3k Lbs equal in tanks 1, 2L+2R and 3.

To make sure, all three engines are consuming fuel at the same rate.

After 20 minutes in the tank-to-engine configuration tanks 1 and 3 are at 28.7 while tanks 2L+2R are at 31.5. A difference of 2800 pounds of fuel. How can that be, if all cross-feed valves are closed and all three engines are consuming the same amount of fuel?

Cheers,

777Drvr