Narrative:

We departed narita, japan, at XX30 nov/xa/98 en route to anc. It's an all night flight -- 5 hours 50 mins. We were asked to keep our speed up on descent to accommodate a flight behind us. The first officer was flying. Below 10000 ft we were given vectors away from the direct track to the airport to increase spacing with the aircraft in front of us. We were cleared to 2000 ft and had the aircraft ahead of us in sight, but there was low fog in the area below us. We were then vectored toward the localizer. At 4000 ft I said '2 to go' (2000 ft), and at 3000 ft I said '1 to go' (1000 ft). We were then cleared for the approach and I said to the first officer 'now we can go down to 1600 ft,' the initial approach altitude to runway 6R at anc. The so and I were then focused on the traffic ahead and when I looked at the altimeter we were at 1400 ft and descending. I said, watch your altitude we're too low. We were about 12 mi west of the airport. The first officer corrected back to 1600 ft but in the process we descended to about 1250 ft before the descent was arrested. Approach control called and said to check our altitude as the first officer was already correcting back. A normal approach and landing was subsequently accomplished. RVR was 2400 ft and the wind was light and variable. A human error due to inattn and being up all night contributed to this mistake.

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Original NASA ASRS Text

Title: FO OF AN ACR 4 ENG WDB OVERSHOT DSCNT ALT DURING APCH RESULTING IN FO CORRECTING BACK AT ONCE AND ATC CALLED TO HAVE CREW CHK THEIR ALT.

Narrative: WE DEPARTED NARITA, JAPAN, AT XX30 NOV/XA/98 ENRTE TO ANC. IT'S AN ALL NIGHT FLT -- 5 HRS 50 MINS. WE WERE ASKED TO KEEP OUR SPD UP ON DSCNT TO ACCOMMODATE A FLT BEHIND US. THE FO WAS FLYING. BELOW 10000 FT WE WERE GIVEN VECTORS AWAY FROM THE DIRECT TRACK TO THE ARPT TO INCREASE SPACING WITH THE ACFT IN FRONT OF US. WE WERE CLRED TO 2000 FT AND HAD THE ACFT AHEAD OF US IN SIGHT, BUT THERE WAS LOW FOG IN THE AREA BELOW US. WE WERE THEN VECTORED TOWARD THE LOC. AT 4000 FT I SAID '2 TO GO' (2000 FT), AND AT 3000 FT I SAID '1 TO GO' (1000 FT). WE WERE THEN CLRED FOR THE APCH AND I SAID TO THE FO 'NOW WE CAN GO DOWN TO 1600 FT,' THE INITIAL APCH ALT TO RWY 6R AT ANC. THE SO AND I WERE THEN FOCUSED ON THE TFC AHEAD AND WHEN I LOOKED AT THE ALTIMETER WE WERE AT 1400 FT AND DSNDING. I SAID, WATCH YOUR ALT WE'RE TOO LOW. WE WERE ABOUT 12 MI W OF THE ARPT. THE FO CORRECTED BACK TO 1600 FT BUT IN THE PROCESS WE DSNDED TO ABOUT 1250 FT BEFORE THE DSCNT WAS ARRESTED. APCH CTL CALLED AND SAID TO CHK OUR ALT AS THE FO WAS ALREADY CORRECTING BACK. A NORMAL APCH AND LNDG WAS SUBSEQUENTLY ACCOMPLISHED. RVR WAS 2400 FT AND THE WIND WAS LIGHT AND VARIABLE. A HUMAN ERROR DUE TO INATTN AND BEING UP ALL NIGHT CONTRIBUTED TO THIS MISTAKE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.