WRONG ALTIMETER SETTING AS SELECTED BY FO; PF; LEADS FLT INTO AN ALTDEV ON DSCNT. ALT XING RESTR NOT MET.

Narrative

DSNDING FOR AN APCH INTO NEW YORK'S LGA ARPT; WE HAD BEEN GIVEN A DSCNT CLRNC TO CROSS SUMTO AT 13000 FT; ALTIMETER 30.68 (THIS IS APPROX). I HAD RECEIVED ATIS AND READ BACK THE ATIS TO THE FO WHO WAS FLYING. PASSING FL195 STARTED GETTING INTO SOME ROUGH TURBULENT AIR. I TURNED ON THE SEAT BELT LIGHT AND MADE A PA ANNOUNCEMENT ABOUT SEAT BELT LIGHT; ALSO GAVE PAX LATEST NEW YORK WX AND THANKED THEM FOR FLYING. DURING THE PA AS WE WERE PASSING FL180; THE FO RESET ALTIMETER TO 29.82. I REACHED DOWN AND RESET MY ALTIMETER TO WHAT THE FO HAD SAID; WHILE TALKING ON PA. WE LEVELED OFF AND CROSSED SOMTO AT 13000 FT. WE WERE GIVEN A FURTHER DSCNT CLRNC AND THE DSCNT AND APCH CHKLISTS WERE COMPLETED. AT THIS TIME WE REALIZED THE WRONG ALTIMETER SETTING. WE WERE 900 FT OFF AT SOMTO. (THERE WERE NO TFC CONFLICTS.) THE RECENT MIDAIR IN INDIA REINFORCES THE IMPORTANCE OF CORRECT ALTIMETER SETTINGS AND IT CAN BE GREATLY AGGRAVATED DURING THE HIGH OR LOW PRESSURE AREAS. CONTRIBUTING FACTORS: THIS WAS THE FIRST LEG OF A 3-DAY ROTATION REQUIRING AN EARLY MORNING SIGN- IN OF AC30; REQUIRING MY GETTING UP AT AA00 AM. EARLY DURING THE FLT; DISCUSSED HOW TIRED WE BOTH FELT AND THAT WE SHOULD BE EXTRA VIGILANT. IN THE PROCESS WE MISSED A VERY BIG BASIC (CORRECT ALTIMETER SETTING). TURB AT TIME NORMAL DSCNT CHKLIST; CAUSED A DISTR. WAITING TO RUN CHKLIST UNTIL AFTER 13000 FT LEVELOFF. 2-MAN ACFT AT TIME CAN BE VERY BUSY DOING THE WORK OF A 3-MAN ACFT.