AN ACR CREW APCHING STL MISSED AN ASSIGNED ALT CROSSING CONSTRAINT WHILE EXAMINING THE APCH CHART BECAUSE VNAV DISENGAGED PRIOR TO DSCNT.

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ON QBALL 6 ARR INTO STL; WE WERE GIVEN CLRNC TO CROSS CGI AT FL260. FO WAS IN THE VNAV MODE AND THE ACFT CROSSED CGI 400 FT HIGH WHILE DSNDING. ATC DID NOT MENTION MISSING THE XING AND THERE DIDN'T APPEAR TO BE CONFLICTING ACFT. SUBSEQUENTLY; WE WERE GIVEN INSTRUCTIONS TO CROSS QBALL AT 15000 FT. THE ACFT HAD SWITCHED TO 'VNAV ALT' AND WE FAILED TO START THE DSCNT IN TIME TO MAKE THE XING. I INFORMED ATC THAT WE WEREN'T GOING TO MAKE THE XING AND THEY INSTRUCTED US TO EXPEDITE THE DSCNT. CONTRIBUTING FACTORS INCLUDED: I HAD NOT BEEN INTO STL IN ABOUT 15 YRS AND DURING THE DSCNT WE RECEIVED A NEW ATIS WHICH CHANGED OUR PLANNED RWY. THIS REQUIRED ADDITIONAL HEADS DOWN TIME ON MY PART. THE FO WAS FLYING; HAD 10 YRS EXPERIENCE IN THAT SEAT AND I FELT HE WAS A VERY CAPABLE PLT WHICH LED ME TO MISTAKENLY LET MY GUARD DOWN. SUPPLEMENTAL INFO FROM ACN 761953: FROM A PREVIOUS DSCNT CLRNC; THE VNAV MODE HAD KICKED OFF AND WAS NOW ON 'ALT HOLD.' I WAS REVIEWING MY STL APCH CHART AND WONDERING IF WE WERE GOING TO BE ASSIGNED RWY 12L OR RWY 12R FOR APCH. DURING THIS TIME OF REVIEW; I NEGLECTED TO SELECT THE VNAV MODE AND; THEREFORE; IT DID NOT DSND TO BE AT QBALL BY THE ASSIGNED ALT. WE WERE QUERIED BY ATC AT AROUND 2-3 MI PRIOR TO QBALL WHETHER WE HAD RECEIVED A DSCNT CLRNC FROM THE PREVIOUS CTLR. WE IMMEDIATELY RECOGNIZED OUR ERROR AND PROCEEDED TO DSND. AT THIS POINT; WE REALIZED THAT WE WOULD BE UNABLE TO MEET THE RESTR AND ADVISED ATC. HE RECLRED US TO 15000 FT WITH NO XING RESTR. THERE WAS NO CONFLICT WITH OTHER TFC. I ATTRIBUTE THIS ERROR TO MY UNFAMILIARITY WITH THE STL APCH AND AREA. ALSO; I FAULT MY INATTENTIVENESS TO THE VNAV MODE. IT IS RARE THAT I GO ANYWHERE OTHER THAN MY USUAL 4 ARPT DESTS. THIS IS A WAKEUP CALL FOR ME. I'VE BEEN FLYING THIS AIRPLANE FOR MANY YRS AND THIS INCIDENT REALLY HUMBLED ME.