A B-737 FLC DSNDED THROUGH CLRNC ALT CITING DISTR; A NONSTANDARD AUTOPLT; AND PNF'S LOSS OF SITUATIONAL AWARENESS AS CONTRIBUTING FACTORS.

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ON DEP CLB FROM STL TO CLE WE EXPERIENCED L WING BODY OVERHEAT LIGHT. RAN CHKLIST; ISOLATED L SIDE OF PNEUMATIC SYS AND CLOSED #1 ENG BLEED. STOPPED CLB 25000 FT (1 PACK). IN CRUISE; ACFT WAS NOT MAINTAINING PRESSURIZATION. CABIN AT 9000 FT AND CLBING SLOWLY. WE ASKED FOR LOWER ALT AND BEGAN A SERIES OF STEP-DOWN DSCNTS. WE WERE GIVEN L 15 DEGS AND DSCNT TO 16000 FT FROM 17000 FT. THE HDG TOOK US INTO AREA OF PRECIPITATION. CAPT WAS ON THE PA. THEN I TURNED; STARTED DSCNT AND REACHED OVER TO ADJUST RADAR PICTURE. I WAS CONCERNED ABOUT ICING AND OUR INABILITY TO SHED ICE OFF L WING DUE TO OUR SET-UP ON PNEUMATICS. PREOCCUPIED WITH RADAR; FAILED TO LEVEL AT 16000 FT AND CAUGHT ALT AT 15300 FT. IMMEDIATELY CLBED BACK TO 16000 FT. THIS ACFT DOES NOT HAVE ALT CAPTURE FEATURE ON AUTOPLT. SOME ACFT DO; SOME DO NOT. NONSTANDARD FLEET. CONTRIBUTING FACTORS FOR THE ALT BUST: 1) CAPT NOT IN THE LOOP. 2) FAILURE OF PF TO MONITOR DSCNT. 3) NONSTANDARD AUTOPLT. SUPPLEMENTAL INFO FROM ACN 302390: CTLR GAVE A TURN TO EXPEDITE DSCNT AND I WENT OFF TO MAKE PA TO PAX AT 18000 FT PRIOR TO INITIATION OF STERILE COCKPIT. COPLT WAS LOOKING AT RADAR AND PASSED THROUGH 16000 FT. COMPANY HAS B-737-100/200 IN FLEET. SOME HAVE ALT SELECT FUNCTION ON AUTOPLT; SOME DO NOT. THIS ACFT DID NOT.