Narrative:

This aircraft had a history reported to me by maintenance control of approximately three months with 'center tank fuel quantity with erroneous indications'. The breakdown of this event and why I refused this aircraft due to safety concerns is in several parts that will make up the whole. I hope this helps in setting the stage; showing what was happening; and why this aircraft ultimately was taken out of service.part 1During preflight; the fuel quantity before fueling indicated 8;500 pounds of fuel. After initial fueling was complete; a fuel revision was added for a total of 17;200; and sometime during the fueling process the fueler saw that there was approximately 27;000 pounds indicated in the center tank which gave us a total fuel of 44;000 pounds. Due to the MEL procedure for the center fuel tank quantity erroneous/inoperative; there was no fuel to be allowed in the center tank. Unsure as to whether the fueler may have inadvertently added fuel to the center tank; and out of an abundance of caution; I asked maintenance to come out and verify that the center tank was in fact empty. They did a 'stick' check of the center tank and confirmed that it was indeed empty.part 2After maintenance had checked the center tank empty; we got into a discussion about the ramifications of the faulty reading and how it affected the FMC; it's calculations for altitude; and airspeed limits; and center of gravity (cg) as well as trim calculations that we would experience in flight as the fuel burned down; but the total fuel reading calculated 27;000 pounds more fuel than we actually had on board. I spoke with load planning; dispatch; maintenance control; and nobody could give me a good answer as to what would happen as the artificially high gross weight skewed the actual aircraft performance. I reviewed a write up of the aircraft and it confirmed what I suspected could happen did happen. This write up indicated erratic function of the airspeed high/low buffet indications that were inaccurate due to the higher projected gross weight based on the erroneous center tank fuel reading.three days later; there was another write up regarding erratic function of the center tank fuel indication.already on a placard; but apparently allowed to have center tank fuel; the constantly changing fuel quantity caused problems with FMC calculations and during their descent VNAV became erratic and unusable. According to maintenance control; this particular aircraft was apparently responsible for and resulted in and emergency revision to the MEL that states that 'if the fuel quantity (fqis) is providing erroneous fuel quantity data; the FMC fuel quantity cannot be manually entered. In this case; VNAV should not be used and flight crew need to be aware of the fuel quantity on board to calculate reference speeds.' so the crew must manually calculate v-speeds for landing.I called and talked to this captain and asked him what his experience was with this aircraft. He told me that after the center tank fuel had burned down to '0' the center quantity became erratic. It fluctuated up to 27;000 pounds and varied from that constantly throughout the remainder of the flight. As it did so; he got an amber 'configuration' warning on the display; and the airspeed warning system fluctuated from the alpha floor(a) to the max overspeed limit (8) and the max and minimum buffet 'zipper' also moved around erratically.part 3There were many write-ups that I saw; and not all mirrored each other. Also; there were 3 different mels that I saw used to defer this problem; there were also several attempts I was told about trying to solve what was going on. There was some confusion on the part of maintenance control and what we could or could not do as it related to MEL; and inputting the 'actual' fuel on board; attempting to override the fuel quantity that was erroneously displayed in the center tank. Our particular situation would not allow us to change the actual fuel onboard. Trying tochange anything else; like zero fuel weight (ZFW) was not only not an option; but it would be equally erratic as the center tank fuel varied; thus giving you equally erroneous readings for the FMC to use for calculations. My concern is that there needs to be more clear and concise information provided for this particular problem. There are many variables that when trying to get the 'first flight' out 'on time' that pilots need to consider. My biggest concerns were validated with the information I got concerning the FMC calculations and its effect on the speed window; alpha floor and overspeed warnings that I confirmed over the phone. My other concern is that of FMC calculations of cg and trim. Nowhere did I find any reference to this and how the erroneous fuel reading might or might not affect this performance issue.with all this in mind; I elected to refuse the aircraft for this revenue flight. I was also asked by dispatch if I would ferry it. I declined. I was then notified by tracking that I was going to ferry this aircraft. I told her that I had already declined to do that with dispatch. She called me back soon after and told me I was deadheading home.improve overall understanding of each scenario involved with [this] MEL to include better information on what to expect; and what systems are affected and how; including a warning for erroneous airspeed indications possible inflight including alpha floor to overspeed warnings and fluctuations in the 'zipper' indication as well as possible 'configuration' warning for the fuel system if center tank fuel becomes erroneous after burned off; and pumps are subsequently turned off as well; or are not on in the first place with no fuel in the center tank; but an erroneous reading triggers that indication.[don't] take three months to try and resolve an issue that has a very narrow function; but large ramifications on aircraft performance and flight reliability. The hardest decision to make that had an impact on the passengers; crew and others downline was to refuse this jet today. I still believe it was the right decision. I would humbly hope next time; that it could be resolved before it had to be turned down.

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Original NASA ASRS Text

Title: B737-800 Captain reported refusing an aircraft with an MEL for center tank fuel quantity due to the erroneous indications produced affecting the FMC and PFD airspeed limits. An inoperative center tank fuel quantity would have been acceptable.

Narrative: This aircraft had a history reported to me by Maintenance Control of approximately three months with 'center tank fuel quantity with erroneous indications'. The breakdown of this event and why I refused this aircraft due to safety concerns is in several parts that will make up the whole. I hope this helps in setting the stage; showing what was happening; and why this aircraft ultimately was taken out of service.Part 1During preflight; the fuel quantity before fueling indicated 8;500 LBS of fuel. After initial fueling was complete; a fuel revision was added for a total of 17;200; and sometime during the fueling process the fueler saw that there was approximately 27;000 LBS indicated in the center tank which gave us a total fuel of 44;000 LBS. Due to the MEL procedure for the center fuel tank Quantity erroneous/inoperative; there was no fuel to be allowed in the Center Tank. Unsure as to whether the fueler may have inadvertently added fuel to the center tank; and out of an abundance of caution; I asked Maintenance to come out and verify that the center tank was in fact empty. They did a 'stick' check of the center tank and confirmed that it was indeed empty.Part 2After Maintenance had checked the center tank empty; we got into a discussion about the ramifications of the faulty reading and how it affected the FMC; it's calculations for altitude; and airspeed limits; and Center of Gravity (CG) as well as trim calculations that we would experience in flight as the fuel burned down; but the total fuel reading calculated 27;000 LBS more fuel than we actually had on board. I spoke with Load Planning; Dispatch; Maintenance Control; and nobody could give me a good answer as to what would happen as the artificially high gross weight skewed the actual aircraft performance. I reviewed a write up of the aircraft and it confirmed what I suspected could happen did happen. This write up indicated erratic function of the airspeed high/low buffet indications that were inaccurate due to the higher projected gross weight based on the erroneous center tank fuel reading.Three days later; there was another write up regarding erratic function of the center tank fuel indication.Already on a placard; but apparently allowed to have center tank fuel; the constantly changing fuel quantity caused problems with FMC calculations and during their descent VNAV became erratic and unusable. According to Maintenance Control; this particular aircraft was apparently responsible for and resulted in and Emergency Revision to the MEL that states that 'If the Fuel Quantity (FQIS) is providing erroneous fuel quantity data; the FMC fuel quantity cannot be manually entered. In this case; VNAV should not be used and flight crew need to be aware of the fuel quantity on board to calculate reference speeds.' so the crew must manually calculate V-speeds for landing.I called and talked to this Captain and asked him what his experience was with this aircraft. He told me that after the center tank fuel had burned down to '0' the Center quantity became erratic. It fluctuated up to 27;000 LBS and varied from that constantly throughout the remainder of the flight. As it did so; he got an amber 'CONFIG' warning on the display; and the airspeed warning system fluctuated from the Alpha Floor(A) to the Max Overspeed limit (8) and the max and minimum buffet 'zipper' also moved around erratically.Part 3There were many write-ups that I saw; and not all mirrored each other. Also; there were 3 different MELs that I saw used to defer this problem; there were also several attempts I was told about trying to solve what was going on. There was some confusion on the part of Maintenance Control and what we could or could not do as it related to MEL; and inputting the 'actual' fuel on board; attempting to override the fuel quantity that was erroneously displayed in the center tank. Our particular situation would not allow us to change the actual fuel onboard. Trying tochange anything else; like Zero Fuel Weight (ZFW) was not only not an option; but it would be equally erratic as the center tank fuel varied; thus giving you equally erroneous readings for the FMC to use for calculations. My concern is that there needs to be more clear and concise information provided for this particular problem. There are many variables that when trying to get the 'first flight' out 'on time' that pilots need to consider. My biggest concerns were validated with the information I got concerning the FMC calculations and its effect on the speed window; alpha floor and overspeed warnings that I confirmed over the phone. My other concern is that of FMC calculations of CG and Trim. Nowhere did I find any reference to this and how the erroneous fuel reading might or might not affect this performance issue.With all this in mind; I elected to refuse the aircraft for this revenue flight. I was also asked by Dispatch if I would ferry it. I declined. I was then notified by tracking that I was going to ferry this aircraft. I told her that I had already declined to do that with Dispatch. She called me back soon after and told me I was deadheading home.Improve overall understanding of each scenario involved with [this] MEL to include better information on what to expect; and what systems are affected and how; including a warning for erroneous airspeed indications possible inflight including alpha floor to overspeed warnings and fluctuations in the 'Zipper' indication as well as possible 'CONFIG' warning for the fuel system if center tank fuel becomes erroneous after burned off; and pumps are subsequently turned off as well; or are not on in the first place with no fuel in the center tank; but an erroneous reading triggers that indication.[Don't] take three months to try and resolve an issue that has a very narrow function; but large ramifications on aircraft performance and flight reliability. The hardest decision to make that had an impact on the passengers; crew and others downline was to refuse this jet today. I still believe it was the right decision. I would humbly hope next time; that it could be resolved before it had to be turned down.

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.