Narrative:

Flight was supposed to be from ZZZ to ZZZ2. About an hour into the flight it became apparent that we had a fuel problem -- either a fuel leak or a fuel quantity indication problem. The left tank fuel quantity kept dropping at an excessive rate. After complying with the QRH and doing a visual inspection; we were fairly certain that we had a quantity indication problem. Since we did not know how much fuel was in the left tank; the QRH directed us to land at the nearest suitable airport. We diverted to ZZZ3. We landed with an indication of 1850 pounds in the left tank and 5800 pounds in the right; although we assumed it was just an indication error and both tanks were probably pretty close to one another in quantity. After sticking the tanks we learned that both quantity gauges were reading accurately; at least at this point. We had landed with a 4000 pound imbalance. It turns out we were underfueled by 4000 pounds in ZZZ; but the left gauge showed full. It wasn't until an hour into the flight that it gave us more accurate readings. We spent the next two and a half hours working with contract maintenance; operations; dispatch; the fueler; etc; trying to make sure there wasn't a fuel leak; and then getting the left fuel quantity indicator; total fuel quantity indicator; and gross weight indicator properly placarded and the aircraft properly fueled. With this going on as well as passenger issues; we completely forgot about any possible inspection that might be required for landing with a fuel imbalance. It wasn't until we were climbing out on departure that it hit me. Upon landing in ZZZ2; we advised maintenance of our landing in ZZZ3 with an imbalance and they informed us that there was an inspection that was required. Although I thought of the 1500 pound imbalance limitation when the fuel imbalance started approaching the limit; I was convinced in my mind that we were actually in balance and the left quantity gauge was wrong. So I set myself up to believe that I was going to land with a balanced fuel load. Subsequent inspection proved we were out of balance. But by that point; we were jumping through so many hoops I completely forgot about this operations limit. I recommend that a note be added to the fuel leak or uncommanded fuel transfer emergency procedure stating that a maintenance inspection is required if fuel imbalance exceeds 1500 pounds. I believe this would have jogged my memory on the ground in ZZZ3.callback conversation with reporter revealed the following information: the reporter stated the aircraft fuel quantity system appeared normal during and after fueling. At cruise altitude flight crew experienced left wing fuel quantity dropping at a rate that was suspected as a leak or indication problem and the quick reference handbook required a diversion. During cruise maximum aileron trim was .5 unit left wing down. On landing; aircraft left wing was found 4000 pounds less fuel than right wing tank also creating a wing imbalance fuel load. Required inspection for imbalanced fuel load was not accomplished.

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Original NASA ASRS Text

Title: AN MD80 IN CRUISE DIVERTED DUE TO A FUEL QUANTITY INDICATION PROB. ON GND FOUND R WING TANK AT 5800 LBS AND L WING TANK AT 1800 LBS. ACFT HAD BEEN INCORRECTLY FUELED AT DEP POINT AND THE DISCREPANCY WAS NOT DISCOVERED DUE TO FUEL GUAGE ERROR.

Narrative: FLT WAS SUPPOSED TO BE FROM ZZZ TO ZZZ2. ABOUT AN HOUR INTO THE FLT IT BECAME APPARENT THAT WE HAD A FUEL PROB -- EITHER A FUEL LEAK OR A FUEL QUANTITY INDICATION PROB. THE L TANK FUEL QUANTITY KEPT DROPPING AT AN EXCESSIVE RATE. AFTER COMPLYING WITH THE QRH AND DOING A VISUAL INSPECTION; WE WERE FAIRLY CERTAIN THAT WE HAD A QUANTITY INDICATION PROB. SINCE WE DID NOT KNOW HOW MUCH FUEL WAS IN THE L TANK; THE QRH DIRECTED US TO LAND AT THE NEAREST SUITABLE ARPT. WE DIVERTED TO ZZZ3. WE LANDED WITH AN INDICATION OF 1850 LBS IN THE L TANK AND 5800 LBS IN THE R; ALTHOUGH WE ASSUMED IT WAS JUST AN INDICATION ERROR AND BOTH TANKS WERE PROBABLY PRETTY CLOSE TO ONE ANOTHER IN QUANTITY. AFTER STICKING THE TANKS WE LEARNED THAT BOTH QUANTITY GAUGES WERE READING ACCURATELY; AT LEAST AT THIS POINT. WE HAD LANDED WITH A 4000 LB IMBALANCE. IT TURNS OUT WE WERE UNDERFUELED BY 4000 LBS IN ZZZ; BUT THE L GAUGE SHOWED FULL. IT WASN'T UNTIL AN HOUR INTO THE FLT THAT IT GAVE US MORE ACCURATE READINGS. WE SPENT THE NEXT TWO AND A HALF HRS WORKING WITH CONTRACT MAINT; OPS; DISPATCH; THE FUELER; ETC; TRYING TO MAKE SURE THERE WASN'T A FUEL LEAK; AND THEN GETTING THE LEFT FUEL QUANTITY INDICATOR; TOTAL FUEL QUANTITY INDICATOR; AND GROSS WT INDICATOR PROPERLY PLACARDED AND THE ACFT PROPERLY FUELED. WITH THIS GOING ON AS WELL AS PAX ISSUES; WE COMPLETELY FORGOT ABOUT ANY POSSIBLE INSPECTION THAT MIGHT BE REQUIRED FOR LNDG WITH A FUEL IMBALANCE. IT WASN'T UNTIL WE WERE CLBING OUT ON DEP THAT IT HIT ME. UPON LNDG IN ZZZ2; WE ADVISED MAINT OF OUR LNDG IN ZZZ3 WITH AN IMBALANCE AND THEY INFORMED US THAT THERE WAS AN INSPECTION THAT WAS REQUIRED. ALTHOUGH I THOUGHT OF THE 1500 LB IMBALANCE LIMITATION WHEN THE FUEL IMBALANCE STARTED APCHING THE LIMIT; I WAS CONVINCED IN MY MIND THAT WE WERE ACTUALLY IN BAL AND THE L QUANTITY GAUGE WAS WRONG. SO I SET MYSELF UP TO BELIEVE THAT I WAS GOING TO LAND WITH A BALANCED FUEL LOAD. SUBSEQUENT INSPECTION PROVED WE WERE OUT OF BALANCE. BUT BY THAT POINT; WE WERE JUMPING THROUGH SO MANY HOOPS I COMPLETELY FORGOT ABOUT THIS OPS LIMIT. I RECOMMEND THAT A NOTE BE ADDED TO THE FUEL LEAK OR UNCOMMANDED FUEL TRANSFER EMER PROC STATING THAT A MAINT INSPECTION IS REQUIRED IF FUEL IMBALANCE EXCEEDS 1500 LBS. I BELIEVE THIS WOULD HAVE JOGGED MY MEMORY ON THE GND IN ZZZ3.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT FUEL QUANTITY SYSTEM APPEARED NORMAL DURING AND AFTER FUELING. AT CRUISE ALT FLT CREW EXPERIENCED L WING FUEL QUANTITY DROPPING AT A RATE THAT WAS SUSPECTED AS A LEAK OR INDICATION PROB AND THE QUICK REFERENCE HANDBOOK REQUIRED A DIVERSION. DURING CRUISE MAXIMUM AILERON TRIM WAS .5 UNIT L WING DOWN. ON LNDG; ACFT L WING WAS FOUND 4000 LBS LESS FUEL THAN R WING TANK ALSO CREATING A WING IMBALANCE FUEL LOAD. REQUIRED INSPECTION FOR IMBALANCED FUEL LOAD WAS NOT ACCOMPLISHED.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.