ALTDEV DURING DSCNT THROUGH WX AT ORD.

Narrative

ON THE PMM 3 ARR INTO ORD; WE RECEIVED A DSCNT CLRNC FROM FL200 TO 15000 FT. ABOUT THE TIME WE WERE APCHING FL185; APCH CTL FREQ 133.2; TOLD US TO STAND BY FOR HOLDING INSTRUCTIONS AT PIVOT. WE SUBSEQUENTLY RECEIVED OUR HOLDING INSTRUCTION AND A FURTHER CLRNC TO DSND TO 13000 FT. DURING THIS TIME AS THE INTL OFFICER I WAS OFF FREQ OBTAINING THE CURRENT ATIS FROM ORD. WHILE I WAS OFF FREQ WE WERE SUBSEQUENTLY GIVEN A FURTHER CLRNC TO DSND TO 12000 FT. PASSING 13000 FT I QUESTIONED THE CREW AS TO OUR CLRED ALT AND WE CONFIRMED WITH APCH THAT OUR CLRNC WAS TO 12000 FT. SHORTLY AFTER WE LEVELED OFF WE ENTERED THE HOLD AT PIVOT; 12000 FT AT XX07Z. NOT EVEN 10 SECONDS LATER; APCH ASKED US TO CONFIRM OUR ALT. AT THIS TIME WE NOTED THAT OUR ALTIMETERS HAD NOT BEEN RESET FROM 29.92; LCL ALTIMETER WAS 29.10. THE ALTIMETERS WERE RESET AND AN IMMEDIATE CLB UP TO 12000 FT WAS UNDERTAKEN. MANY DISTRACTIONS TO INCLUDE HOLDING INSTRUCTIONS; REQUEST FOR AND EXPEDITED CLRNC PASSING 14000 FT; RAPIDLY CHANGING WX CONDITIONS; AND A CHANGE OF RWY AT CHICAGO CONTRIBUTED TO THE MISSING OF THE ALT RESET PASSING FL180. SUPPLEMENTAL INFO FROM ACN 325539: VERY HVY WORKLOAD ON ALL 3 CREW MEMBERS DUE TO WX IN THE CHICAGO AREA.