B737-700
Flight Crew Last 90 Days 484; Flight Crew Type 16000
Situational Awareness; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Flight Crew Overcame Equipment Problem
Aircraft; MEL
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.
Dispatched on flight with an inop Left Main Wing Tank Quantity Gauge MEL (display blank) AND a Right Center Tank Pump inop MEL. During climb out as MEL procedures indicated; the fuel cross feed was opened. This should have allowed the center fuel tank with the SINGLE pump to fuel both engines due to the center tank pumps on the newer model aircraft producing a higher line pressure. We almost immediately noticed that although the center tank continued to drain so did the right main tank!With the right main draining at a fast rate of approximately 4;500 pounds per hour we quickly would have found ourselves in an imbalanced situation between main tanks. Compounding that problem is the unclear verbiage in the MEL as to whether an imbalance warning would even be triggered with a blank main tank display; and if it is triggered how would you police where the imbalance lies and by how much as there is no way to know quantity in left main tank and no way to know with center pump pressure suspect if any has been used from left main (as it clearly was being used from the right main).By quickly doing a fuel audit (fuel used from push versus sum of displayed center and right main quantity); we determined that the left main was untouched and suspected an approximate 600 to 700 pounds main tank imbalance. The right wing main tank pumps were turned off and the center (with cross feed still open) was drained almost to zero. Another fuel audit was completed with same end result of an estimated 600 to 700 pound imbalance. We balanced the main tanks by using indicated cruise burn rate versus clock and got down to within an estimated 100 pounds.Reason for the ASAP is that there is; in my opinion; a huge 'gotcha' in the combination of these two MEL's when combined and needs to be reviewed.Does the Imbalance Warning still function; and if so then how can you accurately police it with a blank gauge.MEL verbiage info to Crew change to reflect number 1 above.Policy chance to consider the center tank remain empty alleviating the above.Info addition to MEL if Center is fueled to state that the single center tank might not provide enough pressure to feed BOTH engines with cross feed open (as Maintenance Control subsequently explained to us was our situation and a known situation with the Newer Model Aircraft. This information will at least allow the Crews to be hyper aware of the potential problem of a quick imbalance. We were forced to deal with an unknown variable with a rapidly approaching imbalance while trying to climb on a SID out of [a busy terminal environment] with multiple prohibited airspace in a dynamic flight environment.
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.