Narrative:

In cruise flight just past the point of equal time enroute to ZZZ; an insufficient fuel alert message flashed. It then went away. It returned and this became the new norm - this message coming and going about every 30 seconds. Also; the time to our next waypoint became erratic and changing. Also; our FMS predicted landing fuel began to vary from quite low to insufficient to make destination. We began to trouble shoot with manual time speed calculations and also manual fuel flow over time calculations. The fuel totalizer (gauges) looked fine/normal. It was only predicted/projected values in the FMS that were erratic and incorrect. We notified FSS radio of a possible time error for next waypoint of more than two minutes; and that we were having computer problems with unreliable predictions of time at next fix. We looked over multiple procedures in the QRH and flight manual but nothing seemed to fit our scenario. We were certain that we did not have a fuel leak based on fuel aboard for our given time in flight. It suddenly dawned on me that since the FMS problems were all due to predictive and/or calculation errors; and that since we were in managed speed using cost index; perhaps I could alleviate some of the computer's issues by hard coding speeds in the VNAV pages. This had the almost immediate effect of solving/removing all of the problems we were experiencing. The remainder of the flight proceeded normally and without issue. To my recollection; our times crossing fixes were always within two minutes of reported for next. We wrote the problem up and reported to maintenance technicians upon landing.

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Original NASA ASRS Text

Title: B737 Flight Crew reported issues with the FMC fuel burn computations.

Narrative: In cruise flight just past the point of equal time enroute to ZZZ; an Insufficient Fuel alert message flashed. It then went away. It returned and this became the new norm - this message coming and going about every 30 seconds. Also; the time to our next waypoint became erratic and changing. Also; our FMS predicted landing fuel began to vary from quite low to insufficient to make destination. We began to trouble shoot with manual time speed calculations and also manual fuel flow over time calculations. The fuel totalizer (gauges) looked fine/normal. It was only predicted/projected values in the FMS that were erratic and incorrect. We notified FSS Radio of a possible time error for next waypoint of more than two minutes; and that we were having computer problems with unreliable predictions of time at next fix. We looked over multiple procedures in the QRH and Flight Manual but nothing seemed to fit our scenario. We were certain that we did not have a fuel leak based on fuel aboard for our given time in flight. It suddenly dawned on me that since the FMS problems were all due to predictive and/or calculation errors; and that since we were in managed speed using Cost Index; perhaps I could alleviate some of the computer's issues by hard coding speeds in the VNAV pages. This had the almost immediate effect of solving/removing all of the problems we were experiencing. The remainder of the flight proceeded normally and without issue. To my recollection; our times crossing fixes were always within two minutes of reported for next. We wrote the problem up and reported to maintenance technicians upon landing.

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.