NON COMPLIANCE WITH ALT CLRNC; ALT UNDERSHOT BY THE CREW OF A LEAR 45 ON APCH TO ORD; IL.

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WHILE OPERATING UNDER PART 91; WE WERE BEING VECTORED FOR THE ILS RWY 27R AT ORD. SIMULTANEOUS ILS APCHS WERE BEING CONDUCTED TO RWY 27L AND RWY 27R. CHICAGO APCH ISSUED US A CLRNC TO DSND TO 4000 FT MSL. MY FO WAS THE PF AND I WAS PERFORMING PNF DUTIES. THE FO WAS NOT UTILIZING THE AUTOPLT AT THE TIME OF THIS EVENT. DURING THE DSCNT; I ASKED THE FO TO BRIEFLY ARREST THE DSCNT SO THAT THE INDICATED AIRSPD WOULD DECAY ENOUGH TO FACILITATE FLAP DEPLOYMENT OF 20 DEGS. AT THE TIME OF MY REQUEST; OUR ALTIMETER INDICATED 4200 FT MSL. VERY SHORTLY THEREAFTER; WE WERE GIVEN AN INTERCEPT HDG FOR THE LOC. I REPLIED TO THE CLRNC AND RESET THE HDG BUG TO THE INTERCEPT HDG. A FEW MOMENTS LATER; WE RECEIVED ANOTHER RADIO CALL FROM THE CTLR QUESTIONING OUR ALT. AT THAT POINT; I LOOKED UP AND OUR ALTIMETER INDICATED 5000 FT MSL. I THEN LOOKED AT THE ALT ALERTER; AND IT INDICATED 4000 FT. I REALIZED THAT WE WERE 1000 FT HIGH; AND I REPLIED TO THE CTLR THAT 'WE ARE LEVEL AT 5000 FT. I'M NOT SURE WHY WE STOPPED THE DSCNT.' THE CTLR REPLIED WITH SOME URGENCY; 'IT APPEARS THAT YOU HAVE BEEN CLBING. I HAVE TFC AT 5000 FT. I NEED YOU AT 4000 FT NOW.' I IMMEDIATELY RETARDED THE PWR LEVERS AND APPLIED FORWARD PRESSURE TO THE CTL YOKE WHILE SIMULTANEOUSLY VERBALLY DIRECTING THE FO TO DSND. I THEN REMEMBERED THAT WE HAD BEEN AT 4200 FT ONLY A FEW SECONDS EARLIER. I REALIZED AT THAT POINT THAT THE PF; INSTEAD OF RESUMING THE DSCNT TO 4000 FT; HAD ACTUALLY CLBED BACK TO 5000 FT MSL. MOMENTS AFTER BEGINNING THE DSCNT; THE TCASII PROVIDED A BASIC TA ALERT AT AN ALT OF APPROX 4500 FT. AN AURAL ALERT OF 'TFC' WAS HEARD. THERE WAS NO RA ISSUED BY OUR TCASII; NOR WAS THERE AN AUDIO WARNING TO DSND. THE INST APCH WAS CONTINUED. MY LACK OF SUPERVISION OVER THE PF'S ALT CTL WAS A CONTRIBUTING FACTOR. MY DESIRE TO REDUCE OUR AIRSPD AND SUBSEQUENTLY DEPLOY 20 DEGS OF FLAPS WAS PREMATURE; AND COULD HAVE WAITED UNTIL THE PF HAD STABILIZED THE DSCNT. MY PNF DUTIES OF REPLYING TO THE INTERCEPT CLRNC AND SETTING THE HDG BUG CONTRIBUTED TO A REDUCED PERFORMANCE OF MY SUPERVISORY DUTIES. A LACK OF EXPERIENCE IN ORD'S EXTREMELY HIGH DENSITY TFC ENVIRONMENT WAS A CONTRIBUTING FACTOR TO THE LACK OF SITUATIONAL AWARENESS FOR BOTH PLTS. THE PF HAD NEVER FLOWN INTO ORD; AND ALTHOUGH I HAVE LIMITED EXPERIENCE AT HIGH DENSITY ARPTS; PRIOR TO THIS DAY I HAD VERY LITTLE IF NO EXPERIENCE AT ORD WHILE SIMULTANEOUS APCHS WERE BEING CONDUCTED TO PARALLEL RWYS. THE USE OF THE AUTOPLT DURING THIS EVENT COULD HAVE PLAYED A SIGNIFICANT ROLE IN DECREASING OUR WORKLOAD AND COULD HAVE PREVENTED OUR REDUCTION IN SITUATIONAL AWARENESS.