Narrative:

On descent into ord the first officer picked up the ATIS which advised of approachs being conducted to the west runways. The FMS was programmed and I briefed the runway 28 ILS. Approximately 25 NM from the airport we were told to turn right to intercept the runway 4R localizer; maintain 8000 ft until degan; cleared ILS runway 4R. We were unprepared for an immediate approach; but did our best getting the approach set up. What we didn't know was the GS is below 8000 ft MSL over degan. So; as we are trained; I armed the approach mode on the fgp as soon as the approach clearance was given and the frequency tuned. Since we were given this runway change on such short notice I let the autopilot fly while the first officer and I reviewed the approach and ran the checklists. I looked up just as the autoplt captured the GS and started a descent from 8000 ft. I noticed that we were about 2 miles short of degan and turned the autoplt off and returned to 8000 ft. Thankfully; we only descended about 150 ft below the assigned altitude before returning. I believe that approach controllers should either warn crews about intercept altitudes verbally or on the approach plate in this type of scenario. I don't believe that we caused any conflict; but we might not be so lucky next time.

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Original NASA ASRS Text

Title: LATE RWY CHANGE RESULTS IN DESCENT BELOW CLRED ALT AT ORD FOR FA50 FLT CREW.

Narrative: ON DSCNT INTO ORD THE FO PICKED UP THE ATIS WHICH ADVISED OF APCHS BEING CONDUCTED TO THE WEST RWYS. THE FMS WAS PROGRAMMED AND I BRIEFED THE RWY 28 ILS. APPROX 25 NM FROM THE ARPT WE WERE TOLD TO TURN R TO INTERCEPT THE RWY 4R LOC; MAINTAIN 8000 FT UNTIL DEGAN; CLRED ILS RWY 4R. WE WERE UNPREPARED FOR AN IMMEDIATE APCH; BUT DID OUR BEST GETTING THE APCH SET UP. WHAT WE DIDN'T KNOW WAS THE GS IS BELOW 8000 FT MSL OVER DEGAN. SO; AS WE ARE TRAINED; I ARMED THE APCH MODE ON THE FGP AS SOON AS THE APCH CLRNC WAS GIVEN AND THE FREQUENCY TUNED. SINCE WE WERE GIVEN THIS RWY CHANGE ON SUCH SHORT NOTICE I LET THE AUTOPILOT FLY WHILE THE FO AND I REVIEWED THE APCH AND RAN THE CHKLISTS. I LOOKED UP JUST AS THE AUTOPLT CAPTURED THE GS AND STARTED A DSCNT FROM 8000 FT. I NOTICED THAT WE WERE ABOUT 2 MILES SHORT OF DEGAN AND TURNED THE AUTOPLT OFF AND RETURNED TO 8000 FT. THANKFULLY; WE ONLY DESCENDED ABOUT 150 FT BELOW THE ASSIGNED ALTITUDE BEFORE RETURNING. I BELIEVE THAT APCH CTLRS SHOULD EITHER WARN CREWS ABOUT INTERCEPT ALTITUDES VERBALLY OR ON THE APCH PLATE IN THIS TYPE OF SCENARIO. I DON'T BELIEVE THAT WE CAUSED ANY CONFLICT; BUT WE MIGHT NOT BE SO LUCKY NEXT TIME.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.