FLC IN AN MD80 DSNDS BELOW THEIR ASSIGNED ALT DUE TO A COCKPIT MGMNT PROB AND CLRNC INTERP.

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ON DSCNT INTO ORD; ZAU 135.15 GAVE US AN INITIAL DSCNT CLRNC TO CROSS BENKY AT 11000 FT. SHORTLY THEREAFTER; CTR INFORMED US ORD WAS CAT II/III ONLY; LNDG RWY 14R; RVR 3500 FT WITH WINDS 080 DEGS AT 17 KTS GUSTING TO 35 KTS. WE HAD PLANNED RWY 9R; AS WAS PUBLISHED BY ATIS. SINCE XWINDS WERE GREATER THAN 15 KTS AND RVR BELOW 4000 FT; WE TOLD CTR WE NEEDED RWY 9R. CTR SAID RWY 9R WAS BEING SNOW PLOWED AND GAVE US THE FOLLOWING HOLDING INSTRUCTIONS: 'HOLD AT BENKY SW AS PUBLISHED 20 MI LEGS; EFC AT XB15; HOLD AT FL180.' TO MAKE A LONG STORY SHORT; WE DSNDED TO 17300 FT BEFORE CORRECTING BACK TO FL180. CTR SAID THERE WAS NO TFC CONFLICT; BUT DID GIVE VECTORS TO ANOTHER ACR ACFT. I FEEL CONTRIBUTING FACTORS FOR OUR ALTDEV WAS THE HR OF THE DAY AFTER HAVING FLOWN DFW TO LAX; THEN LAX TO ORD. WHEN GIVEN THE ADVERSE WX CONDITIONS WE LOOKED INTO GETTING THE EFFECTIVE XWIND FROM THE CHARTS AT THE SAME TIME WE WERE GIVEN HOLDING INSTRUCTIONS. WE WROTE DOWN THE HOLDING INSTRUCTIONS; BUT DID NOT RESET THE ALT ALERT. SUPPLEMENTAL INFO FROM ACN 424732: FO FLYING DSCNT TO ORD; BOTH PLTS CHKING WX AND XWIND LIMITS CAT II/III APCHS. CTR GIVES WINDS OUT OF LIMIT PASSING FL200 FOR 11000 FT. WE GOT QUICK CLRNC TO 'HOLD NOW AT BENKY AS PUBLISHED FL180.' PASSING FL190 CTR CALLS TO CROSS FIX AT FL180. I MISTOOK THAT TO CONTINUE DSCNT TO 11000 FT NOT FL180. CONFIRMED WITH CAPT IF CORRECT TO KEEP DSCNT. WITH ALL THE BUSINESS GOING ON; HE DIDN'T HEAR ME. AT 17600 FT; CTR DIRECTED US TO CLB TO FL180. NO CONFLICT.