CAPT OF A B737-200 FAILED TO MAKE XING ALT DURING DSCNT DUE TO TRAINING CONVERSATION BY THE NON FLYING FO; TO A TRAINEE IN THE JUMP SEAT; AND FATIGUE OF LONG DUTY HRS.

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THE JUMP SEAT WAS OCCUPIED BY A NEW HIRE PLT WHO WOULD BE WITH US FOR THE ENTIRE TRIP TO OBSERVE THE OP AS A PREPARATION TO HIS IOE. HE WAS GOING TO BE ASSIGNED TO ACR X OPS. I ENCOURAGED THE FO TO POINT OUT SIGNIFICANT POINTS IN THE FLT WHERE HE WOULD BE TASKED AND TO DESCRIBE VARIABLE AND SUGGEST TECHNIQUES. ZBW GAVE US A CLRNC TO CROSS 45 NM SW OF PVD VOR AT 11000 FT. WE WERE NOW AT ABOUT 80-90 NM FROM PVD AND OUR ALT WAS SOMEWHERE IN THE 20'S (FL240 OR FL230?). WE STARTED THE DSCNT RIGHT AWAY AT 1200 FPM WITH THE AUTOPLT. I KNEW THIS WASN'T SUFFICIENT TO MEET THE RESTR BUT WAS USED TO INITIATE THE DSCNT IN ORDER THAT THE TRANSITION TO A NOSE DOWN/PWR IDLE CONFIGN WOULD BE MORE SMOOTH. HERE IS WHERE I FEEL THE FATAL FLAW TOOK PLACE. I ALLOWED THE FO TO TALK WITH THE JUMP SEAT RIDER FAR LONGER THAN I SHOULD HAVE. I CALLED FOR THE DSCNT CHK PASSING THROUGH FL180 AND BEGAN TO BRIEF THE APCH. THE FO HAD SOME DIFFICULTY LOCATING HIS APCH PLATE AND CONTINUED TO DISCUSS HIS DUTIES WITH THE JUMP SEAT RIDER. I SHOULD HAVE PUT A STOP TO THAT AT THAT TIME BUT I KNEW I COULD DO THIS SOLO IF NECESSARY. I WAS STILL BRIEFING THE APCH WHEN ZBW CALLED US BACK AND CLRED US TO 9000 FT. I REALIZED THEN I HAD FORGOTTEN TO MONITOR MY ALT AND WAS NOW ABOUT 42 NM FROM PVD VOR AND STILL ABOVE 13000 FT. ZBW ASKED IF WE WERE AWARE THAT WE MISSED OUR ALT AND I TOLD HIM THAT WE NOW REALIZE THAT. WHEN WE SWITCHED OVER TO PVD APCH WE WERE TOO HIGH TO LAND STRAIGHT IN TO RWY 5 AND HAD TO BE VECTORED IN A BOX PATTERN TO GET DOWN. THERE WERE UNUSUALLY HIGH TAILWINDS AT LOW ALT WHICH EXACERBATED THE PROB. I DON'T THINK THERE WAS A CONFLICT WITH OTHER TFC.