LGT ACR ACFT FLC HAD LAST MIN ACFT CHANGE AND DIDN'T NOTICE THE FLT PLAN WAS FOR A DIFFERENT ACFT; NOT ONLY NUMBER BUT ALSO ACFT TYPE. THUS; THE FUEL LOAD WAS NOT CORRECT. RPTR CHK PLT CAUGHT THE PROB.

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I BOARDED A ROUTINE FLT FROM LAX-SEA; SEP/XX/97; AT THE END OF BOARDING PROCESS TO RETURN TO MY BASE. AS CHK AIRMAN I DECIDED TO DO SPOT CHK OBSERVATION OF FLC. CREW HAD SENIOR CAPTAIN AND VERY NEW FO. DURING THE PUSHBACK TAXI PHASE I WATCHED THE FO TO SEE HOW WELL HE HAD 'CLIMATIZED' TO THE PROCS/ACFT. DURING CLBOUT I FINALLY REALLY FOCUSED ON THE TOTAL FUEL ONBOARD AND THOUGHT IT WAS A LITTLE LOW FOR THAT PHASE. WHEN I MENTIONED THIS TO THE CAPT; WE IMMEDIATELY REVIEWED PAPERWORK. IT APPEARED TO BE IN ORDER AND THE MINIMUM REQUIRED SEEMED ADEQUATE FOR THE 1 HR 58 MIN FLT TO SEA. THERE HAD BEEN AN ACFT SWAP AT THE LAST MOMENT AND WE/I REALIZED IT HAD BEEN A B737-400 PREVIOUSLY. THE FLT PLAN COMPUTER CTL MATCHED THE DISPATCH RELEASE WITH A STATEMENT INDICATING FLT PLAN HAD BEEN CHKED WITH DISPATCH FOR ACCURACY FOR NEW PLANE; BUT; FLT PLAN WAS FOR A B737-400 WHICH BURNS LESS FUEL ENRTE; WHILE THE TOTAL ALLOTTED DID SEEM ADEQUATE FOR AN MD80 ACFT -- THE XCHKS WERE INACCURATE BECAUSE WE HAD A B737 FLT PLAN. THE PLANE ARRIVED IN SEA WITH 4.8 -- WE TARGET 5.0 MIN ON VFR ARR WHICH IT WAS -- BUT IT WAS NOT PLANNED AS SUCH ON THE FLT PLAN. RECOMMEND: 1) FOR INSTRUCTORS STEPPING ON FLT DECK CHK FUEL TOTAL BEFORE PUSHBACK ALWAYS. 2) CHK TO SEE IF SOME IDENT MARK ON BODY OF FLT PLAN OTHER THAN TAIL NUMBER CAN BE MADE TO IDENT TYPE ACFT.