Narrative:

This report is being filed to serve as a learning measure for others who may fall into similar circumstances. On the morning of our departure from lax to cun; my first officer; brought to my attention; the possibility of being close to the maximum taxi; and thus possible takeoff weight limitation; in spite of the common practice of taking only 'minimum fuel' for our trip. The flight was simply full of people and bags; while remaining one of the most popular trips in the system. At 'push' time; we ensured that the aircraft fell below the limitation prior to pushing back from the gate. We met the limitation by only a mere 100 pounds; by the time 5 additional 'claim-at-gate' bags were added to the total ZFW computation. The flight departed the gate on time; and in an effort to meet the additional takeoff weight restr; we decided to taxi slow; and when parked along the taxiway; kept the engine power slightly higher; to try to expedite our fuel burn-off. It was at this time that we (the crew) noticed that our gross weight display on the 'takeoff reference' page of our FMC display began to decrease in value; but the associated takeoff weight remained unchanged at a value slightly higher than permitted for takeoff. This confused both of us; as we expected both values would decrease. After consulting with the system handbook; dispatch; and finally operations; a unanimous agreement was reached via radio conversation; whereby it was determined that the FMS was not outputting erroneous information; rather; the takeoff weight figure was indicating that the associated 'large font' takeoff speeds were being generated from a numerical factor; that was calculated from a parameter-set of the FMC; representing a sort of 'ceiling' to the figures; that would be conservative in nature; providing a buffer to the resultant speeds. This calculation can be seen; when consulting the performance handbook; and references to weights much higher than those actually used; will be viewed from a table at the top of the 160000 pound column. The airplane takeoff weight limitation is of course; lower: 154500 pounds. We wanted to insure that the actual takeoff weight was not exceeded; and experienced a takeoff delay; while determining this curious disparity. Thanks to dispatch; the flight finally departed safely. The flight handbook does not clearly explain why the takeoff weight does not change as would be expected by the flight crew; so I hope this report will serve to clear up that point.

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Original NASA ASRS Text

Title: B737 CREW LEARNS A NEW IDIOSYNCRASY OF THE FMC ON TAXI OUT AT LAX.

Narrative: THIS RPT IS BEING FILED TO SERVE AS A LEARNING MEASURE FOR OTHERS WHO MAY FALL INTO SIMILAR CIRCUMSTANCES. ON THE MORNING OF OUR DEP FROM LAX TO CUN; MY FO; BROUGHT TO MY ATTN; THE POSSIBILITY OF BEING CLOSE TO THE MAX TAXI; AND THUS POSSIBLE TKOF WT LIMITATION; IN SPITE OF THE COMMON PRACTICE OF TAKING ONLY 'MINIMUM FUEL' FOR OUR TRIP. THE FLT WAS SIMPLY FULL OF PEOPLE AND BAGS; WHILE REMAINING ONE OF THE MOST POPULAR TRIPS IN THE SYS. AT 'PUSH' TIME; WE ENSURED THAT THE ACFT FELL BELOW THE LIMITATION PRIOR TO PUSHING BACK FROM THE GATE. WE MET THE LIMITATION BY ONLY A MERE 100 LBS; BY THE TIME 5 ADDITIONAL 'CLAIM-AT-GATE' BAGS WERE ADDED TO THE TOTAL ZFW COMPUTATION. THE FLT DEPARTED THE GATE ON TIME; AND IN AN EFFORT TO MEET THE ADDITIONAL TKOF WT RESTR; WE DECIDED TO TAXI SLOW; AND WHEN PARKED ALONG THE TXWY; KEPT THE ENG PWR SLIGHTLY HIGHER; TO TRY TO EXPEDITE OUR FUEL BURN-OFF. IT WAS AT THIS TIME THAT WE (THE CREW) NOTICED THAT OUR GROSS WT DISPLAY ON THE 'TKOF REF' PAGE OF OUR FMC DISPLAY BEGAN TO DECREASE IN VALUE; BUT THE ASSOCIATED TKOF WT REMAINED UNCHANGED AT A VALUE SLIGHTLY HIGHER THAN PERMITTED FOR TKOF. THIS CONFUSED BOTH OF US; AS WE EXPECTED BOTH VALUES WOULD DECREASE. AFTER CONSULTING WITH THE SYS HANDBOOK; DISPATCH; AND FINALLY OPS; A UNANIMOUS AGREEMENT WAS REACHED VIA RADIO CONVERSATION; WHEREBY IT WAS DETERMINED THAT THE FMS WAS NOT OUTPUTTING ERRONEOUS INFO; RATHER; THE TKOF WT FIGURE WAS INDICATING THAT THE ASSOCIATED 'LARGE FONT' TKOF SPDS WERE BEING GENERATED FROM A NUMERICAL FACTOR; THAT WAS CALCULATED FROM A PARAMETER-SET OF THE FMC; REPRESENTING A SORT OF 'CEILING' TO THE FIGURES; THAT WOULD BE CONSERVATIVE IN NATURE; PROVIDING A BUFFER TO THE RESULTANT SPDS. THIS CALCULATION CAN BE SEEN; WHEN CONSULTING THE PERFORMANCE HANDBOOK; AND REFS TO WTS MUCH HIGHER THAN THOSE ACTUALLY USED; WILL BE VIEWED FROM A TABLE AT THE TOP OF THE 160000 LB COLUMN. THE AIRPLANE TKOF WT LIMITATION IS OF COURSE; LOWER: 154500 LBS. WE WANTED TO INSURE THAT THE ACTUAL TKOF WT WAS NOT EXCEEDED; AND EXPERIENCED A TKOF DELAY; WHILE DETERMINING THIS CURIOUS DISPARITY. THANKS TO DISPATCH; THE FLT FINALLY DEPARTED SAFELY. THE FLT HANDBOOK DOES NOT CLRLY EXPLAIN WHY THE TKOF WT DOES NOT CHANGE AS WOULD BE EXPECTED BY THE FLT CREW; SO I HOPE THIS RPT WILL SERVE TO CLR UP THAT POINT.

Data retrieved from NASA's ASRS site as of January 2009 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.