A CORPORATE JET'S FLC FAILED TO RESET THEIR ALTIMETERS ON DSCNT; AND DSNDED BELOW THE CLRNC ALT. ATC QUERIED; AND THE FLC DETECTED THEIR ERROR WHEN TCASII ALERTED.

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ON APR/TUE/95 I WAS FLYING CAPT ON A FLT WHICH DEPARTED DECATUR; IL (DEC) ABOUT XA00 LCL; FOR CHICAGO (ORD). WE HAD ONLY BEEN LEVEL AT OUR CRUISING ALT OF FL210 FOR A SHORT TIME AND WERE CLRED DIRECT PLANO INTXN DIRECT (ORD) (PLANO WAS ON OUR FILED RTE). THE PLT IN THE R SEAT WAS OFF THE CTR FREQ GETTING THE ATIS WHEN ZAU CLRED US FOR A DSCNT AND ASKED US TO LOOK UP THE BRADFORD ARR TO (ORD) AND FIND THE BENKY INTXN. I READ BACK THE INSTRUCTIONS AND BEGAN THE DSCNT. A SHORT TIME LATER THE PLT IN THE R SEAT WAS BACK ON CTR AND SEARCHING FOR BENKY. CHICAGO THEN CLRED US TO CROSS BENKY AT 13000 FT AND GAVE US HOLDING INSTRUCTIONS AT BENKY. WE WERE RUSHING TO GET DOWN; SLOW DOWN AND SET THE RADIOS UP FOR THE HOLD. ALL APPEARED TO BE COMING TOGETHER JUST FINE AS WE ENTERED THE HOLD. AT THIS TIME CTR ASKED TO VERIFY WE WERE LEVEL AT 13000 FT - WE CONFIRMED. SHORTLY THEREAFTER WE GOT A TFC ALERT ON TCASII. I COULD SEE AN AIRLINER IN THE DISTANCE THAT ALSO APPEARED TO BE IN THE HOLD - ACCORDING TO TCASII AT 500 FT BELOW US. AT THIS POINT I NOTICED THE ALTIMETERS WERE STILL SET AT 29.92. I QUICKLY CORRECTED OUR ALT AND SET IN THE PROPER ALTIMETER SETTING OF 29.33. IN THE FUTURE I WILL BE MORE CAREFUL TO CHK THE ALTIMETER PASSING THROUGH FL180 AND ALSO TO CALL FOR THE DSCNT CHKLIST.