THE FLT TOOK OFF WITH THE 3RD REQUIRED FLT ATTENDANT DEAD HDG IN THE JUMP SEAT NOT REALIZING SHE WAS GOING TO SUBSEQUENTLY BE ONE OF THE REQUIRED FLT ATTENDANTS FOR THE FLT. THE ACFT CHANGE AND ASSOCIATED COMPLICATIONS LED TO THE ADDED FLT ATTENDANT ASSIGNMENT NOT HAPPENING PRIOR TO TKOF.

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ON THE SECOND DAY OF A 3 DAY TRIP; WE WERE PASSING THROUGH OUR HUB AND DID A QUICK ACFT CHANGE TO AN ACFT THAT WAS ON THE OPPOSITE SIDE OF THE ARPT. WE RUSHED ACROSS THE ARPT; GOT THE AIRPLANE READY AND NARROWLY DEPARTED ON TIME. THE PROB AROSE 20 MINS AFTER DEP WHEN A CREW COORDINATOR SENT US AN ACARS MESSAGE AND ASKED WHO OUR 'ADD;' OR THIRD; FLT ATTENDANT WAS. AT THAT MOMENT; THE CAPT SUMMONED THE LEAD FLT ATTENDANT UP TO THE COCKPIT AND SAID WE DID GET AN ADD FLT ATTENDANT DIDN'T WE? SHE HAD A BLANK LOOK; AND SAID 'OH MY GOSH; I FORGOT.' WE HAD BEEN FLYING A DC9-30 ON THE TRIP; BUT SWAPPED INTO A DC9-40. 12 MORE SEATS THAT REQUIRED 1 MORE FLT ATTENDANT. IT'S VERY DIFFICULT TO KEEP UP WITH BECAUSE WE MAY HAVE 4 DIFFERENT TYPES OF DC9'S IN ONE DAY; AND TWO OF THEM REQUIRE 2 FLT ATTENDANTS; AND 2 REQUIRE 3 FLT ATTENDANTS. AND; MANY OF THE TRIPS ON THE AIRPLANES THAT REQUIRE ONLY 2 FLT ATTENDANTS; WILL HAVE 3. I DON'T REALLY HAVE AN ANSWER TO PREVENT THIS FROM HAPPENING AGAIN. FORTUNATELY; WE HAD A FLT ATTENDANT ON A JUMP SEAT THAT WE COULD USE FOR THE DURATION OF THE FLT. BUT IT WAS JUST A STRING OF HUMAN ERRORS THAT LED TO THIS SIT. THE CREW COORDINATOR IS RESPONSIBLE FOR HAVING AN 'ADD FLT ATTENDANT' SHOW UP FOR THE FLT; THE GATE AGENT IS RESPONSIBLE TO MAKE SURE THAT THE REQUIRED CREW IS PRESENT BEFORE BOARDING THE PLANE; AND PRIMARILY THE LEAD FLT ATTENDANT IS RESPONSIBLE FOR THE OTHER FLT ATTENDANTS. AND THEN THERE'S ALWAYS THE CAPT WHO BARELY HAS TIME TO ACCOMPLISH HIS PRIMARY DUTIES; WHO USUALLY TAKES THE RAP.