WRONG RWY TKOF PLANNED ON MISREPRESENTED ACFT GROSS WT.

Narrative

DURING THE LAST YR MY CARRIER HAS INITIATED A CONTRACT WITH XYZ TO PROVIDE A COMPREHENSIVE PACKAGE OF SVCS; INCLUDING COMPUTERIZED PERFORMANCE AND WT AND BAL COMPUTATIONS. ON A NUMBER OF OCCASIONS; I HAVE NOTICED THAT VARIOUS OF THE ACFT WHICH WERE INCLUDED IN THIS CONTRACT DID NOT PERFORM AS THEY MIGHT HAVE BEEN EXPECTED (IE; DIFFICULTY LIFTING OFF DUE TO MISTRIM; NOSE COMING OFF THE GND EARLY; ACFT UNABLE TO CLB TO PLANNED ALTS). THE FINAL WT AND BAL AND PERFORMANCE DATA IS PROVIDED TO US BY ACARS PRINTOUT. WE RECEIVED A 'FINAL' ACARS CLOSEOUT WHICH WAS PREDICATED ON A TKOF WT OF 404;000 LBS. THIS VARIED FROM OUR PLANNED' TKOF WT BY ABOUT 20000 LBS; HOWEVER; SUCH VARIATIONS ARE NOT UNUSUAL. ALL TKOF BUGS AND TRIM WAS SET FOR THE 404;000 LB WT; AND WE PLANNED ON A RWY AND FLAP SETTING WHICH WOULD NOT HAVE BEEN POSSIBLE AT THE HIGHER (424000) WT. DUE TO AN ATC DELAY; THE FE MADE A ROUTINE CALL TO ADVISE LCL OPS PERSONNEL ABOUT THE DELAY. AT THAT POINT WE DETERMINED THAT THE ACARS INFO WAS WRONG. WHERE WE HAD BEEN TRAINED THAT ACARS WAS ALWAYS THE LATEST AND BEST INFO AVAILABLE; OUR OWN INVESTIGATION SHOWED THAT THE DATA CAN EASILY BE WRONG. IN FACT; I NOW BELIEVE THAT IT IS OFTEN WRONG; AND THERE ARE FEW CLUES WHICH MIGHT LEAD A CREW TO QUESTION THE DATA.