B727 CREW HAD ALT OVERSHOOT IN C90 CLASS B AIRSPACE.

Narrative

UPON JOINING LOC JUST OUTSIDE THE MARKER AND CLRED FOR THE APCH; THE FO (PF) STARTED A DSCNT BEFORE JOINING GS; AND DSNDED 300-400 FT BEFORE I RECOGNIZED ERROR AND POINTED IT OUT TO HIM. HE THOUGHT HE HAD HEARD 2000 FT (ALT) IN LAST CLRNC INSTEAD OF 2600 FT (ALT); ALTHOUGH I DID NOT CHANGE THE ALT ALERT TO 2000 FT (ALT). WE STARTED TO CORRECT ALT AS THE GS WAS COMING ALIVE AND JOINED THE GS AND CONTINUED A NORMAL APCH AND LNDG. CONTRIBUTING FACTORS: ATC HAD JUST GIVEN US A CLRNC WHICH CONTAINED A HDG; AN ALT; CLRED FOR AN APCH CLRNC; AIRSPD RESTR; AND A TWR CONTACT POINT. WE WERE ALSO WATCHING FOR TFC IN FRONT OF US. WE HAD NOT YET JOINED THE LOC WHEN WE RECEIVED THE CLRNC; SO IT WAS VERY BUSY. I WAS BUSY READING BACK CLRNC; LOOKING FOR TFC AND GETTING A GOOD VISUAL ON RWY AND MISSED THE ALT. I THINK BEING IN THE TURN MADE IT A LITTLE BIT HARDER TO FEEL THE DSCNT. THE ALT ALERT WAS WORKING PROPERLY AND DID NOT SOUND. WE WERE ALSO CONFIGURING FOR AIRSPD REDUCTION WHICH MAKES IT MORE DIFFICULT TO FEEL ALT CHANGES. OUR PROC FOR ALT CHANGES IS FOR BOTH PEOPLE TO VERIFY NEW ALT VERBALLY AFTER IT IS SELECTED IN ALT ALERT AND BEFORE ANY DSCNT OR CLB IS INITIATED. PF (FO) DSNDED WITHOUT DOING THIS BECAUSE IT WAS STILL SET AT 2600 FT. WHAT I LEARNED AND WILL CHANGE: I WILL BE MORE DILIGENT IN LISTENING TO ANY VARIATIONS OF THE APCH BRIEFING. I WILL VERIFY WITH MY APCH PLATE AS THE FO IS BRIEFING THE ENGINEER. AT NIGHT UNDER LOW LIGHT THE PLATES ARE SOMETIMES HARD TO READ. ALSO; I WILL STEP UP MY AWARENESS DURING VERY BUSY TIMES DURING APCH IN THE TERMINAL ENVIRONMENT. SUPPLEMENTAL INFO FROM ACN 486260: ACFT WAS TRACKING ILS RWY 4R LOC AT MDW AND WAS LEVEL AT 2600 FT MSL. WE WERE 2-3 MI OUTSIDE ERMIN OM WHEN FO UNDERSTOOD ATC TO CLR ACFT TO 2000 FT MSL. FO BEGAN DSCNT. CAPT DID NOT HEAR ALT CLRNC; NOR DID HE CALL ATC FOR ALT VERIFICATION. FE DID NOT HEAR ALT CLRNC OR REQUEST VERIFICATION. ACFT PASSED OM AT APPROX 2200 FT VERSUS 2600 FT. FO HAD REVIEWED APCH PLATE AND SAW NONE AUTHORIZING 2600 FT MSL WITH ATC APPROVAL; BUT WITH DIM LIGHTING; THOUGHT THE NOTE AUTHORIZED 2000 FT MSL. ATC DID NOT QUESTION OUR LOWER THAN AUTH (ASSIGNED) ALT. ACFT LANDED WITHOUT FURTHER INCIDENT AND TAXIED TO GATE. NOTES: FO'S DAY BEGAN AT XA30 FOR ORIGINAL FERRY DEP. ORIGINAL DEP WAS SCHEDULED FOR XD00. DEP TIME WAS CHANGED SEVERAL TIMES. FO HAD LITTLE OPPORTUNITY FOR REST DUE TO MULTIPLE PHONE CALLS TO AND FROM CREW SCHEDULING. CAPT AND FE FAILED TO REQUEST CLARIFICATION/VERIFICATION FROM ATC. FO FAILED TO SUGGEST TO THE CAPT THAT HE SHOULD REQUEST ALT VERIFICATION FROM ATC WHEN FO REALIZED CAPT HAD NOT HEARD ALT ASSIGNMENT. CONTRIBUTING FACTORS: NIGHT; DIM LIGHTING; BORDERLINE FATIGUE (FO).