Narrative:

Small transport X is on 040 degree heading at 4000 ft. Small aircraft Y is descended to 4000 ft on 360 degree heading. Both aircraft are being vectored to VFR tower controled airport (chicago meigs). Small aircraft Y mode C is not working or intermittent at best. Small transport X is descended to 3000 ft. Small aircraft Y altitude is checked and he is leaving 5200 ft. Small transport X is told to expedite descent to 3000 ft. Small aircraft Y reads back descend to 3000 ft. (I, as the controller, do not copy this readback until hearing the tape later.) I later clear small transport X to 2500 ft. Small aircraft Y is descended from 4000 ft to 3000 ft when I notice his mode C showed 3300 ft for 1 sweep so I question his altitude. Pilot says he is leaving 3200 ft for 3000 ft. I climb him back to 4000 ft and descend small transport X to 2000 ft.

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Original NASA ASRS Text

Title: SMA Y UNAUTH DSCNT TO OCCUPIED ALT HAD LTSS FROM SMT X. PLTDEV. SYS ERROR.

Narrative: SMT X IS ON 040 DEG HDG AT 4000 FT. SMA Y IS DSNDED TO 4000 FT ON 360 DEG HDG. BOTH ACFT ARE BEING VECTORED TO VFR TWR CTLED ARPT (CHICAGO MEIGS). SMA Y MODE C IS NOT WORKING OR INTERMITTENT AT BEST. SMT X IS DSNDED TO 3000 FT. SMA Y ALT IS CHKED AND HE IS LEAVING 5200 FT. SMT X IS TOLD TO EXPEDITE DSCNT TO 3000 FT. SMA Y READS BACK DSND TO 3000 FT. (I, AS THE CTLR, DO NOT COPY THIS READBACK UNTIL HEARING THE TAPE LATER.) I LATER CLR SMT X TO 2500 FT. SMA Y IS DSNDED FROM 4000 FT TO 3000 FT WHEN I NOTICE HIS MODE C SHOWED 3300 FT FOR 1 SWEEP SO I QUESTION HIS ALT. PLT SAYS HE IS LEAVING 3200 FT FOR 3000 FT. I CLB HIM BACK TO 4000 FT AND DSND SMT X TO 2000 FT.

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.