A B747-200 CREW ACCEPTED AN 18 HR DUTY DAY AFTER AN ATLANTIC CROSSING WITH AN INTERMEDIATE STOP. THE CREW BECAME FATIGUED ON THE SECOND LEG WITH DETERIORATING WX AND AN IOE FO.

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HAD WX AND MAINT PROBS AT ZZZ1 WHICH DELAYED US FOR 4 HRS. THIS WOULD PUT US OVER A 16 HR DAY WHEN WE REACHED ZZZ. POLLED CREW AND WE ALL AGREED THAT WE WERE WELL RESTED AND WOULD TAKE A 2 HR EXTENSION. IT WAS GRANTED. WE DEPARTED JUST IN TIME TO COMPLETE THE DAY IN 18 HRS. UNEVENTFUL FLT TO ZZZ. VECTORS TO APCH WERE GOING NORMALLY ON DOWNWIND AND BASE. ATC REQUESTED THAT WE KEEP 210 KTS. ON BASE; ASKED FOR AND RECEIVED CLRNC TO SLOW DOWN. THE FO (AN OE STUDENT) WAS FLYING AT THIS TIME WITH THE AUTOPLT ENGAGED. WE WERE PUT ON AN INTERCEPT HDG AND TOLD TO INTERCEPT THE LOC TO RWY 9R. WHEN COURSE WAS ALIVE; WE WERE STILL AT 190 KTS. AT THIS SPD; THE AUTOPLT WOULD OVERSHOOT THE LOC. APCHS WERE IN PROGRESS ON THE PARALLEL RWY. TO PREVENT THE OVERSHOOT; I TOOK CTL OF THE ACFT AND DISCONNECTED THE AUTOPLT. I AGGRESSIVELY INTERCEPTED THE LOC. INTERCEPTION WENT WELL. WE WERE TOLD TO DSND TO 2700 FT AND CLRED THE ILS. I REENGAGED THE AUTOPLT AND STARTED A DSCNT. THE GS WAS COMING INTO VIEW AND THEN DISAPPEARED WITH A FLAG. I RESET THE NAV MODE SELECTOR AND THE NAV RADIO. THE FLAG DISAPPEARED. WE WERE BELOW GS BUT THAT WAS WHAT I EXPECTED UNTIL WE REACHED 2700 FT AND THE GS INTERCEPTION POINT. HOWEVER; I HAD MISREAD MY ALTIMETER AND I WAS ACTUALLY HEADING FOR 1700 FT. THE CREW CALLED ALT AND THEN I REALIZED MY ERROR. I DISCONNECTED THE AUTOPLT AND CLBED BACK TO 2700 FT. THE LOWEST ALT WE REACHED WAS 2200 FT. APCH CALLED AND SAID THEY HAD GOTTEN A LOW ALT WARNING AND REMINDED US WE WERE SUPPOSED TO BE AT 2700 FT. I TOLD THEM WE HAD INDEED GOTTEN LOW BUT WERE NOW LEVEL AT 2700 FT. THE REST OF THE APCH WAS RUSHED BUT THE LNDG WAS UNEVENTFUL. SEVERAL FACTORS CAME INTO PLAY HERE. THE CREW DUTY DAY EXTENSION WAS BASED ON WX BEING NO LOWER THAN 600/3 AT THE TIME OF OUR ARR. SHORTLY PRIOR TO DEP FROM ZZZ1 WE RECEIVED WORD THAT THE WX MAY GO DOWN TO 1/2 MI. THE ACTUAL WX TURNED OUT TO BE 300 FT OVCST WITH 9 MI VISIBILITY UNDER THE CLOUDS. IT WAS CLR ABOVE 500 FT. WE DID NOT SEE THE RWY UNTIL 300 FT. THE CHANGE IN CONDITIONS WE RECEIVED AT ZZZ1 SHOULD HAVE PROMPTED US TO REEVALUATE OUR DECISION FOR THE EXTENSION. ANOTHER FACTOR WAS FLYING WITH AN INEXPERIENCED OPERATING EXPERIENCE STUDENT. TRAINING ALWAYS REQUIRES MORE VIGILANCE THAN NORMAL LINE FLYING. ANOTHER UNEXPECTED FACTOR WAS THE TFC AT ZZZ. IT WAS MUCH HEAVIER THAN I EXPECTED. COMBINE THE BAD WX; TRAINING; AND HVY TFC LOAD THE DECISION TO EXTEND THE CREW DUTY DAY WAS PROBABLY A POOR ONE. THE REAL INSIDIOUS NATURE OF THIS INCIDENT WAS THAT I FELT THAT I WAS VERY ALERT THROUGHOUT THE APCH; BUT LOOKING BACK I CAN SEE THAT MY REACTIONS WERE SLOWED AND MY JUDGEMENT DID SUFFER. IT WASN'T UNTIL WE WERE IN THE BLOCKS THAT I REALIZED JUST HOW TIRED I WAS.