FLC OF LTT CPR JET ACFT FAILED TO RESET THE PF'S ALTIMETER DURING DSCNT THROUGH FL180 RESULTING OVERSHOOTING LEVEL-OFF AT A XING ALT.

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DSNDING OUT OF FL240; RECEIVED CLRNC TO CROSS IMELL INTXN AT 12000 FT MSL AT 250 KTS. ALTIMETERS BELOW FL180 WERE APPROX 29.46 FOR ST LOUIS AREA. I WAS PIC IN L SEAT WITH SIC FLYING OUR LTT WITH AUTOPLT ENGAGED (IN TRANSFER MODE). I HANDLED CHKLIST ON DSCNT. OUT OF FL180; I CALLED FOR ALTIMETERS TO BE RESET TO 29.46 INCHES (SETTING AS CLOSE AS I REMEMBERED) AS PER CHKLIST. AFTER LEVEL-OFF AT 12000 FT; (AUTOPLT AUTOMATICALLY LEVELS ACFT TO SELECTED ALT SETTING) AND AFTER PASSING IMELL INTXN; WE RECEIVED HDG ASSIGNMENT TO APPROX 230 DEG WITH NO SPD RESTRICTION. A FEW MINS LATER; THE ST LOUIS APCH CTLR MENTIONED TO US THAT HE REALLY NEEDED US TO MAINTAIN 12000 FT FOR TFC. I THEN LOOKED DOWN AT MY PLT'S ALTIMETER AND SAW THAT WE WERE ACTUALLY AT 11500 FT MSL WITH CORRECT ALTIMETER SETTING. THEN LOOKED AT SIC'S ALTIMETER SETTING AND SAW IT WAS STILL SET AT 29.92 INCHES. SINCE AUTOPLT WAS TRANSFERRED; THE ACFT LEVELED OF 500 FT LOW. INITIATED IMMEDIATE CLB TO 12000 FT MSL AND CORRECTED SIC'S ALTIMETER TO CORRECT SETTING. INATTN ON BOTH OUR PARTS FOR NOT ASSURING ALL ALTIMETERS RESETS WAS REASON FOR ALT DEV. STRICT REVIEW OF CHKLIST PROCS WILL BE UNDERTAKEN.