AN ACR B727 FLC DSNDED BELOW THEIR ASSIGNED ALT WHEN THE FLC BECAME DISTRACTED BY OTHER DUTIES AND FAILED TO LEVEL IN TIME. THIS ACFT DID NOT HAVE AN ALT CAPTURE MODE.

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ZDV CLRED OUR FLT 'TO DSND TO 24000 FT.' DEST WAS COS. COS ATIS SAID VISUAL APCHS TO RWY 35R WERE IN PROGRESS. WHILE DSNDING THROUGH FL250; ZDV GAVE A CONFUSING CLRNC; 'CLRED TO THE PETEY OM.' WE DID NOT KNOW WHERE PETEY WAS. UPON SEARCHING THE COS APCH CHARTS; I FOUND PETEY TO BE THE OM FOR RWY 35L; WHICH WAS CLOSED. RWY 35R WAS IN USE. VERY CONFUSING. OUR ACFT DID NOT HAVE AN ALT CAPTURE FUNCTION. SOME OF OUR FLEET DOES. DUE TO THE DISTRACTIONS; I MISSED LEVELING OFF AT FL240; AND DSNDED A LITTLE BELOW THE ALT. ZDV THEN CLRED US TO DSND TO 14000 FT; THEN QUERIED US ABOUT OUR ASSIGNED ALT AT FL240. NO CONFLICT OCCURRED. WHILE I WAS IN ERROR FOR NOT LEVELING OFF AT FL240; I FEEL ATC CONTRIBUTED TO THE ALTDEV BY GIVING US A CONFUSING CLRNC JUST WHEN WE NEEDED TO LEVEL OFF. COS IS STILL A FAIRLY NEW DEST FOR OUR AIRLINE. I HAVE BEEN TO COS ONLY ONCE; AND AM UNFAMILIAR WITH ARR PROCS. PUBLISHING A STAR TO COS WOULD HELP. SUPPLEMENTAL INFO FROM ACN 384349: FLT FROM MSP TO COS; RECEIVED DSCNT CLRNC TO 24000 FT. ACFT DSNDED THROUGH 24000 FT TO ABOUT 23650 FT BEFORE THE CAPT AND FO REALIZED WE WERE BELOW OUR CLRED ALT. THE CAPT WAS PF. FO HAD THE RADIOS. I WAS THE FE AND WAS 'OUT OF THE LOOP' AT THE TIME THE 24000 FT CLRNC WAS GIVEN AND AT THE TIME WE DSNDED THROUGH 24000 FT. (I WAS GETTING THE ATIS WHEN THE CLRNC WAS GIVEN AND GIVING A 'GOODBYE' PA AT THE TIME OF THE EXCURSION.)