B737-800
Flight Crew Air Transport Pilot (ATP)
Workload; Troubleshooting; Human-Machine Interface; Distraction; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem
Aircraft; Manuals; Procedure
We were dispatched under MEL 28 [for] inoperative center tank fuel gauge. The MEL states that the crew may have to manually input the fuel into the FMC to get accurate data. On the ground the center tank gauge read zero even though it had been fueled with approximately 10;000 lbs of fuel (fuel was metered into the A/C and we had a fuel slip verifying this; plus maintenance stuck the tank as a back-up). With the tank reading zero we were able to manually enter the correct fuel into the FMC. Passing through 18;000 ft we verified we were capable of our cruise altitude of 37;000 ft. FMC stated max cruise altitude of 38;400 ft. After approximately 90 mins at cruise with the center tank nearly empty the center tank suddenly indicated 27;800 lbs. This immediately translated to an increase in the FMC indicated gross weight of 28;000 above actual gross weight. Concurrently the high speed and low speed buffet margin yellow caution tapes closed together and we received an FMC scratchpad message of 'buffet margin'.  In no way was the A/C in near a buffet margin. This was simply an FMC anomaly caused by a default center tank reading. We actually descended to 35;000 ft at ATC's request for traffic and this opened up the buffet margin indicators some. However; with the center tank now empty; and still indicating 27;800 lbs we were unable to manually enter the actual fuel load into the FMC. Due to weather we needed to climb back to 37;000 ft so we verified our actual capability in the performance manual and also verified it by reducing the ZFW (Zero Fuel Weight) in the FMC by 28;000 lbs. This fooled the FMC and gave us our actual gross weight with both these parameters verified; we began the climb to 37;000 ft. Around 36;000 ft the tapes came together again; we got the 'buffet alert' scratchpad message and this time the autopilot disconnected. Once level at 37;000 ft the FO and I discussed if there would be any issues if we once again fooled the FMC by reducing our ZFW in the FMC by the amount indicated in the center tank (27;800 lbs). We deduced we would be okay and did so. This brought the buffet margin tapes back to where they belonged and we continued uneventfully to [our destination]. This MEL doesn't address the possibility of this situation and doesn't address the possibility of not being able to manually enter the correct fuel onboard into the FMC.
A B737-800 Captain reported that the center fuel tank indication was inoperative with a metered 10;000 lbs in it. A manual fuel entry gave the FMC correct weights; but at FL370 the tank suddenly indicated 27;800 which created high and low speed buffer alerts. The event occurred twice before the crew reduced the FMC ZFW for the proper aircraft weight.