MD80 Fuel System

[MAYDAYMAYDAY]

Can anyone with experience on the MD80 explain why the main tank fuel quantity indication increases after level off? After a 4 hour flight the quantity can be seen as high as 9600lbs (full tank is 9250).

Thanks in advance.

[bafanguy]

The only thing I can think of is "fuel migration" from center to mains. It's due to leaking one-way check valves as the CTR fuel pressure is higher than the mains with all pumps on, fuel moves to the mains from CTR. MTC manual says 500#/hr migration from CTR to mains is OK...mains to CTR...no migration is allowed...it will ground the airplane. Ask me another question.

[idg]

MM,
Two questions come to mind. First is it a real indication? ie does the fuel used plus the fuel in tanks equal the fuel on board at start. If this sum comes out positive then it is an indication problem and the temperature compensators are not working properly therefore giving an over-read of the quantity.

If the sum is equal then second question would be (not knowing the MD80) does the fuel cool the generator or oil system in the engine? If this is true and the fuel is recirculated to the tanks this could be slowly increasing the tank contents.

[bafanguy]

idg,

Good question. My experience with both DC9 and MD-80's is that, while the total of fuel-used counters and fuel remaining should equal fuel on board at start ( allowing for a bit of APU burn perhaps ), they frequently add up to LESS that the orginal fuel load. The question then becomes, where did the unaccounted-for fuel go ? A leak ? After asking the very knowledgeable MTC instructors at my former airline, I was told that the fuel-used counters are not necessarily maintained or calibrated to much accuracy. Therefore, you'll often see what appears to be discrepencies that appear to be a fuel leak. For all the times I added up fuel used + fuel remaining and got a confusing result, it NEVER was a leak...could be some time, but wasn't in my approx. 15,000 hours in the two airplane types.