B727 PREPARING FOR DEP FROM FOREIGN ARPT WHEN GATE AGENT REQUESTED FLC LET PAX PLT RIDE COCKPIT JUMP SEAT. PAX HAD TICKET AND FLC ASSUMED THE PLT PREFERRED TO RIDE IN COCKPIT; BUT IN FACT THE ACFT WAS FULL AND THE PAX PLT WASN'T INCLUDED IN THE WT AND BAL; THUS PUTTING THE ACFT OVER LEGAL ZERO FUEL WT.

Narrative

BEFORE DEP FROM SJO; THE GATE AGENT CAME TO THE ACFT AND ASKED IF WE MINDED IF A PLT COULD SIT IN THE JUMP SEAT. THEY ALSO SAID HE HAD A FULL FARE TICKET IN COACH; SO OUR ASSUMPTION WAS THAT HE HAD AN OPEN SEAT IN COACH AND JUST WANTED TO GET AWAY FROM A CROWDED CABIN. HE SHOWED US A BOARDING PASS WITH HIS SEAT ASSIGNMENT ON IT AND HIS IDENT BADGE. WHAT THE AGENT OR THE PLT FAILED TO TELL US WAS THAT HE SHOWED UP AT THE ARPT TOO LATE AND THEY GAVE HIS SEAT AWAY. HIS BOARDING PASS WAS FOR THE NEXT FLT AND WE FAILED TO CATCH ON TO THAT. AFTER TKOF; HE THANKED US FOR LETTING HIM JUMP SEAT OR HE WOULD HAVE TO WAIT FOR THE NEXT FLT. THAT CLUED US IN THAT THIS COULD BE A LEGALITY PROB. SO I ASKED THE FLT ATTENDANTS TO DO A HEAD COUNT AND IN FACT WE HAD 150 PAX IN BACK PLUS THE W. WE WERE STILL LEGAL ON EVERYTHING BUT THE ZERO FUEL WT. THIS PUT US OVER BY APPROX 55 LBS. WE RADIOED BACK TO SJO TO INFORM THEM AND TO HAVE THEM NOTIFY COMPANY. THIS MADE AN OVERWT LNDG INSPECTION NECESSARY AND WAS SUCCESSFULLY COMPLETED WITHOUT ANY SCHEDULE DISRUPTION OF THE ACFT. WE COULD HAVE AVOIDED THIS PROB IF WE WERE NOT LED TO BELIEVE THERE WAS AN OPEN SEAT FOR HIM IN BACK AND OUR CLOSEOUT DID NOT SHOW HIM IN THE W SINCE IT WAS A LAST MIN CHANGE AND WE BELIEVED THERE TO BE AN OPEN SEAT IN BACK FOR HIM. WE FULLY BELIEVED HIM TO BE IN THE 150 PAX COUNT DUE TO THE VERBAL INFO GIVEN TO US PLUS THE FACT THAT THESE SJO FLTS ARE ALWAYS SO WT RESTR AS THIS ONE WAS DUE TO ZERO FUEL WT; THAT WE COULDN'T IMAGINE THEM JUST PUTTING HIM ON AND THEN NOT EVEN PUTTING HIM ON THE CLOSEOUT OF A WT RESTR FLT.