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

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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.