Narrative:

We flew the flight with the following conditions: right center tank fuel boost pump - inop/MEL; left main fuel quantity indicator - inop/MEL (blank); center tank fueled to 3300 pounds.following the relevant right center boost pump MEL procedure; we opened the cross feed valve after takeoff; leaving all main fuel pumps; and the single operating (left) center pump; on. We normally see the center tank decrease before any main tank fuel is used; given the center pump's higher operating pressure. However; we quickly noticed the center and operating right main tank fuel quantity both decreasing equally. We surmised this was because; while an operating center boost pump would 'overpower' both main pumps on that side; saving the (left) main tank; it would not keep the far side main tank fuel (right) from being used; even with the cross feed open (this was confirmed in a subsequent conversation with mx control).we used the fuel used switch to confirm the total fuel used for both engines matched the fuel used only from the center tank and the right main. In other words; the left main tank's fuel was not being burned and an imbalance was rapidly developing; though; in this case; we could not see the imbalance because the left main quantity display was inop.we solved the problem by continuously monitoring the fuel used switches as the center tank depleted; then using conventional cross feed techniques based on fuel flow rate; elapsed time; and repeated fuel audits; to prevent an imbalance; and rebalance the main tanks as best we could; by cross feeding for what we calculated was the correct minutes and seconds; and then confirming with total fuel used versus the quantities we could see.the problem was that while the center tank boost pump MEL contained a rather cryptic note to 'be aware of a potential fuel imbalance developing;' it did not contemplate that the pilot might not be able to 'see' the imbalance developing due to a second fuel quantity indicator inop MEL. It also did not note that single side main fuel might be used while the other main tank remained undetectably full; resulting in a 1;000 pound imbalance developing very rapidly; well before reaching cruise in a crowded terminal area or delayed climb (by my calculations; in only 10 to 12 minutes). Further; I was not certain whether the crew would get an imbal alert without both fuel quantity indicators; or; if they would; how difficult it would be for them to diagnose the problem correctly; especially if there was still center tank fuel in that tank complicating the math.I suggest that the fuel quantity indicator and center boost pump MEL's should not be allowed simultaneously; center tank fuel be disallowed with both MEL's; or; at minimum; that crews be made aware of this risk with a more strongly worded vigilance note in the MEL; explaining the risk of an undetected imbalance rapidly developing after takeoff and how to monitor for it.

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Original NASA ASRS Text

Title: B737 flight crew reported that they had a fuel imbalance after following MEL procedures for one fuel boost inoperative and a fuel quantity indicator inoperative.

Narrative: We flew the flight with the following conditions: Right Center Tank Fuel Boost Pump - Inop/MEL; Left Main Fuel Quantity Indicator - Inop/MEL (blank); Center Tank fueled to 3300 pounds.Following the relevant right Center Boost Pump MEL procedure; we opened the cross feed valve after takeoff; leaving all main fuel pumps; and the single operating (left) center pump; ON. We normally see the center tank decrease before any main tank fuel is used; given the center pump's higher operating pressure. However; we quickly noticed the center and operating right main tank fuel quantity BOTH decreasing EQUALLY. We surmised this was because; while an operating center boost pump would 'overpower' both main pumps on that side; saving the (left) main tank; it would NOT keep the far side main tank fuel (right) from being used; even with the cross feed open (this was confirmed in a subsequent conversation with MX Control).We used the Fuel Used switch to confirm the total fuel used for both engines matched the fuel used only from the center tank and the right main. In other words; the left main tank's fuel was NOT being burned and an imbalance was rapidly developing; though; in this case; we could not SEE the imbalance because the left main quantity display was inop.We solved the problem by continuously monitoring the Fuel Used switches as the center tank depleted; then using conventional cross feed techniques based on fuel flow rate; elapsed time; and repeated fuel audits; to prevent an imbalance; and rebalance the main tanks as best we could; by cross feeding for what we calculated was the correct minutes and seconds; and then confirming with total fuel used versus the quantities we could see.The problem was that while the Center Tank Boost Pump MEL contained a rather cryptic note to 'be aware of a potential fuel imbalance developing;' it did not contemplate that the Pilot might not be able to 'see' the imbalance developing due to a second Fuel Quantity Indicator Inop MEL. It also did not note that single side main fuel might be used while the other main tank remained undetectably full; resulting in a 1;000 pound imbalance developing very rapidly; well before reaching cruise in a crowded terminal area or delayed climb (by my calculations; in only 10 to 12 minutes). Further; I was not certain whether the Crew would get an IMBAL Alert without both fuel quantity indicators; or; if they would; how difficult it would be for them to diagnose the problem correctly; especially if there was still center tank fuel in that tank complicating the math.I suggest that the Fuel Quantity Indicator and Center Boost Pump MEL's should not be allowed simultaneously; center tank fuel be disallowed with both MEL's; or; at minimum; that Crews be made aware of this risk with a more strongly worded vigilance note in the MEL; explaining the risk of an undetected imbalance rapidly developing after takeoff and how to monitor for it.

Data retrieved from NASA's ASRS site 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.