FLC OF B727 ON NIGHT VISUAL APCH DSNDS BELOW MSA. CAPT NOTICES RADAR ALTIMETER AT 1000 FT AND CALLS FOR MISSED APCH.

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THE EVENT INVOLVES CLOSER THAN DESIRABLE PROX TO TERRAIN. FLT WAS A FERRY FLT EWR TO IFP (BULLHEAD CITY; AZ). NEAR THE DEST AT NIGHT; UNLIMITED VISIBILITY; CLRNC WAS GIVEN TO 11000 FT WITH CLRNC FOR VISUAL APCH WHEN VISUAL CONTACT WAS MADE WITH IFP. THE ORIGINAL CLRNC WAS TO EED VOR DIRECT IFP; EED IS S OF IFP 25 MI. THE PUBLISHED APCH FOR IFP BEGINS AT EED VOR AT 6000 FT AND PROCEEDS N TO IFP. IT IS COMMON; HOWEVER; WHEN VFR TO INTERCEPT THE APCH COURSE N OF EED TO EXPEDITE THE APCH. OUR ACFT WAS APCHING FROM THE E AT 11000 FT WHEN BULLHEAD CITY WAS IN SIGHT AT 25 MI. WE BEGAN DSCNT TO 6000 FT; FO IS FLYING. AT SOME POINT; CLRNC WAS GIVEN DIRECT TO BULLHEAD CITY. THE FO WAS INTENT ON DOING THIS. I; HOWEVER; EXPLAINED THERE WAS HIGH TERRAIN BTWN US AND THE ARPT AND THE BETTER APCH WAS FROM THE S IN THE VICINITY OF EED VOR AND PROCEED N. THIS REQUIRED 3 SEPARATE EXPLANATIONS TO GET HIM TO MODIFY OUR COURSE TO THE S AWAY FROM A DIRECT COURSE TO BULLHEAD. FINALLY; WE WERE HEADED SW TO A POINT APPROX 6 MI N OF EED WHICH IN MY EXPERIENCE IN A DAYTIME OP WOULD HAVE PROVIDED ADEQUATE TERRAIN CLRNC AT 6000 FT. THE QUESTION WAS RAISED CONCERNING OUR MSA. THE APCH CHART WAS REFED AND IN WHAT I PERCEIVED TO BE OUR QUADRANT THE CHART STATED A MSA OF 6100 FT; WHICH ADDED TO MY CONFIDENCE IN OUR ALT. WE PROCEEDED ALL THE TIME KEEPING BULLHEAD IN SIGHT. AT SOME POINT; WHILE PROCEEDING SW TO INTERCEPT THE APCH COURSE; I OBSERVED THAT THE RADAR ALT WAS READING 2500 FT BUT AT ONE POINT IT ABRUPTLY DECREASED TO 1000 FT WHEREUPON I COMMANDED A CLB; TO THE BEST OF MY KNOWLEDGE THIS WAS AS CLOSE TO THE GND AS WE GOT. FROM THIS POINT THE APCH AND LNDG PROCEEDED NORMALLY. LOOKING BACK; THERE WERE SEVERAL FACTORS THAT LED TO DSNDING TO AN ALT THAT DID NOT PROVIDE SUFFICIENT TERRAIN CLRNC; BUT ONE IMPORTANT ONE WAS A FAILURE ON MY PART TO UNDERSTAND THE MSA DIAGRAM AT THE TOP OF THE APCH CHART. AT TIME OF THE INCIDENT; I PLACED OUR POS IN THE LOWER R QUADRANT WITH AN MSA OF 6100 FT WHEN IN FACT WE WERE IN THE UPPER R QUADRANT WITH A HIGHER MSA. THIS PROB AROSE BECAUSE I NOW BELIEVE THE DIAGRAM IS BASED ON THE APCH FIX EED 25 MI S; NOT CTR OF THE ARPT. CONTRIBUTING FACTORS: TRYING TO USE LCL KNOWLEDGE AT NIGHT AND AFTER BEING ON DUTY FOR 14 HRS; CONCERN WITH FUEL REMAINING AFTER A 5 HR FLT WITH A DESIRE TO EXPEDITE THE APCH TO BULLHEAD. ATC SHOULD PROBABLY NOT OFFER DIRECT BULLHEAD UNTIL PASSING EED; AND THE DIFFICULTY JUDGING DISTANCE ON A CLR NIGHT.