FO OF A B737-200 OVERSHOT ALT ON DSCNT DUE TO LOOKING FOR TFC AND CAUGHT HIS MISTAKE WHEN THE ALT ALERTER SOUNDED. HE RETURNED IMMEDIATELY TO ASSIGNED ALT.

Narrative

APCHING ANC WE WERE GIVEN CLRNC TO DSND TO 3000 FT. WE FOLLOWED COMPANY PROC BY BOTH OF US VERIFYING AND ACKNOWLEDGING THE 3000 FT SETTING IN THE ALT ALERTER. CONDITIONS WERE NIGHT VMC AND WE HAD THE TFC WE WERE APPARENTLY BEING VECTORED TO FOLLOW IN SIGHT ON THE OTHER SIDE OF THE ANC VOR. THE ARPT WAS ALSO IN SIGHT; BUT WE HAD NOT YET BEEN CLRED FOR A VISUAL APCH. I WAS PNF; AND AS WE APCHED 3000 FT; I STARTED MAKING SOME CHANGES TO THE FMS. A FEW SECONDS LATER; THE ALT ALERTER SOUNDED AT 2700 FT. THE FO (PF) MADE AN IMMEDIATE CORRECTION BACK TO 3000 FT. WE DSNDED TO 2600 FT AT THE LOWEST POINT. NO OTHER TFC WAS INVOLVED. I'M NOT SURE WHAT REALLY CAUSED THE DEV; BUT IT WAS PROBABLY A COMBINATION OF BEING ON DUTY MORE THAN 11 HRS; AND HAVING AN EXPECTATION THAT WE WERE ABOUT TO BE CLRED FOR THE VISUAL APCH TURN INTO THE THOUGHT THAT WE HAD BEEN CLRED FOR THE VISUAL. ALSO; IF I HAD NOT BEEN HEADS DOWN AT THE FMS; I WOULD HAVE BEEN ANOTHER BACKUP. SUPPLEMENTAL INFO FROM ACN 422690: WE HAD VISUAL CONTACT WITH TFC -- NO CONFLICT. I BELIEVE WE ADHERED TO CORRECT ALT AWARENESS PROC. I THINK THAT THE LONG DUTY DAY MAY HAVE BEEN A FACTOR. WE WERE AT THE END OF AN 11 1/2 HR DUTY DAY. I WAS MANUALLY FLYING THE ACFT AND BOTH PLTS WERE HEADS UP KEEPING AN EYE ON THE B757 LIGHTS AGAINST A BACKDROP OF CITY LIGHTS. WHILE SCANNING THE CITY SKYLINE AND KEEPING VISUAL CONTACT WITH THE B757; I MOMENTARILY DSNDED BELOW THE ASSIGNED ALT AND IMMEDIATELY CORRECTED. TO PREVENT THIS FROM HAPPENING AGAIN; I WOULD FLY THE ACFT ON AUTOPLT (WITH AUTOTHROTTLES COUPLED) -- ESPECIALLY GIVEN SUCH A LONG DUTY DAY OF 11 1/2 HRS. I WOULD ALSO SUGGEST THAT OR REINFORCE THAT THE PNF BE 'OUTSIDE' LOOKING FOR TFC AND PF PRIMARILY RELY ON FLYING INSIDE GAUGES. I NEED TO ADD THAT INITIALLY BOTH PLTS ASSUMED WE WERE CLRED FOR VISUAL; AND PERHAPS DID NOT RESET THE ALT ALERTER.