Narrative:

Assigned descent to 5000 ft MSL, aircraft descended through 5000 ft to 4700 ft, corrective action taken immediately. Time below 5000 ft MSL approximately 10 seconds maximum. Crew diverted by concern for TCA (class B) altitude assignments and associated airspeed restrs (200 KIAS). Supplemental information from acn 418142: my attention (PNF) was diverted by concern for TCA (class B) floor, and descending below it. I saw on the chart that the floor was 5000 ft and thought -- no problem to worry about until they descend us lower. I looked up and saw us passing 4900 ft, and warned the captain.

Google
 

Original NASA ASRS Text

Title: B727 CARGO CREW HAD ALT EXCURSION ON DSCNT.

Narrative: ASSIGNED DSCNT TO 5000 FT MSL, ACFT DSNDED THROUGH 5000 FT TO 4700 FT, CORRECTIVE ACTION TAKEN IMMEDIATELY. TIME BELOW 5000 FT MSL APPROX 10 SECONDS MAX. CREW DIVERTED BY CONCERN FOR TCA (CLASS B) ALT ASSIGNMENTS AND ASSOCIATED AIRSPD RESTRS (200 KIAS). SUPPLEMENTAL INFO FROM ACN 418142: MY ATTN (PNF) WAS DIVERTED BY CONCERN FOR TCA (CLASS B) FLOOR, AND DSNDING BELOW IT. I SAW ON THE CHART THAT THE FLOOR WAS 5000 FT AND THOUGHT -- NO PROB TO WORRY ABOUT UNTIL THEY DSND US LOWER. I LOOKED UP AND SAW US PASSING 4900 FT, AND WARNED THE CAPT.

Data retrieved from NASA's ASRS site as of July 2007 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.