ACR EXCURSION FROM ASSIGNED ALT WHILE MAKING TURN ONTO FINAL APCH COURSE.

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ARRIVING AT SEA AT END OF 13 HR BACK OF CLOCK TRANS- PACIFIC FLT (I WAS FLYING) BEING VECTORED ON LONG DOWNWIND E OF LOC FOR ILS RWY 16R SEA. GIVEN L 90 DEG TURN TO 90 DEG INTERCEPT ANGLE TO ILS RWY 16R AND CLRED FOR ILS APCH TO RWY 16R TOLD TO MAINTAIN 5000 FT TILL ERYKA 18 DME FIX. IMMEDIATELY AFTER TURNING TO 90 DEG INTERCEPT HDG WE KNEW WE WOULD HAVE A LARGE OVERSHOOT OF THE FINAL APCH COURSE. THE CAPT WAS BUSY MAKING ONE OF MANY REQUIRED RADIO FREQ CHANGES AS I BEGAN TURNING. BECAUSE OF LARGE OVERSHOOT AND ABOUT 10 MI VISIBILITY; I ELECTED TO USE VISUAL REFS TO TURN FINAL AND CORRECT BACK TO COURSE. AS I WAS ROLLING OUT ON AN INTERCEPT HDG; I REALIZED I HAD DSNDED 400 FT BELOW ASSIGNED ALT BEFORE REACHING ERYKA. I IMMEDIATELY CORRECTED BACK TO 5000 FT. SO ALSO CALLED ALT AS I REALIZED I HAD DSNDED. PROBS: 90 DEG INTERCEPT ANGLE; OTHER TFC IN AREA TO BE AWARE OF. HIGH LEVEL OF FATIGUE OF ENTIRE CREW. RADIO FREQ CHANGE AT CRITICAL TURN TO INTERCEPT. MY CONCERN ABOUT LARGE OVERSHOOT AND MY TRANSITION TO VISUAL REFS FOR HELP. AT NEARLY 20 DME I HAD PLENTY OF TIME TO CORRECT BACK TO COURSE. I SHOULD NOT HAVE ALLOWED MYSELF TO BECOME DISTRACTED ALTHOUGH OTHER TFC IN THE AREA WAS A CONCERN.