Narrative:

Small transport X flight plan was den to dro. Small transport X was about 10 mi north of dro descending from FL240 to FL160. I was stepping small transport X down reference the terrain. Small transport X was cleared to FL140. When the next altitude was available I asked 'small transport X descend and maintain 12000 ft and are you going to request the ILS approach? Previous aircraft initially have had to start the ILS and have needed to get down to at least about 9000 ft before they can go VFR.' 'yea, small transport X requesting ILS we're down to 12000 ft now.' 'small transport X roger, cleared direct dro, direct hindy, execute procedure turn inbound, you can expect the ILS, maintain 12000 ft.' small transport X reply, '12000 ft, direct dro and hindy, for the procedure turn for the ILS.' at dro small aircraft Y was cleared dro..fmn..gup..sjn..fhu. Climbing to 11000 ft. I had planned for small transport X to fly over the dro VOR south to hindy (14 DME south) intersection on the ILS approach turn inbound. When my departure (small aircraft Y) was south en route to fmn and small transport X was clear of small aircraft Y then I could clear small transport X for the ILS/sme runway 2 approach to dro. After small aircraft Y departed I reclred small aircraft Y to 10000 ft so that I could eventually get small transport X down to 11000 ft, I did not want small transport X to be too high for the G/south. Small aircraft Y leveled at 10000 ft and small transport X was never cleared for lower. When small transport X was in his procedure turn I noticed he was descending. Conflict alert was going off. I tried twice to stop small transport X at 11000 ft. Sep was lost and I eventually did recieve a reply from small transport X that he was level at 11000 ft.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR SMT OVERSHOT ALT ON DESCENT CAUSED LESS THAN STANDARD SEPARATION WITH SMA ON CLIMBOUT OF DRO.

Narrative: SMT X FLT PLAN WAS DEN TO DRO. SMT X WAS ABOUT 10 MI N OF DRO DSNDING FROM FL240 TO FL160. I WAS STEPPING SMT X DOWN REFERENCE THE TERRAIN. SMT X WAS CLRED TO FL140. WHEN THE NEXT ALT WAS AVAILABLE I ASKED 'SMT X DSND AND MAINTAIN 12000 FT AND ARE YOU GOING TO REQUEST THE ILS APCH? PREVIOUS ACFT INITIALLY HAVE HAD TO START THE ILS AND HAVE NEEDED TO GET DOWN TO AT LEAST ABOUT 9000 FT BEFORE THEY CAN GO VFR.' 'YEA, SMT X REQUESTING ILS WE'RE DOWN TO 12000 FT NOW.' 'SMT X ROGER, CLRED DIRECT DRO, DIRECT HINDY, EXECUTE PROC TURN INBND, YOU CAN EXPECT THE ILS, MAINTAIN 12000 FT.' SMT X REPLY, '12000 FT, DIRECT DRO AND HINDY, FOR THE PROC TURN FOR THE ILS.' AT DRO SMA Y WAS CLRED DRO..FMN..GUP..SJN..FHU. CLBING TO 11000 FT. I HAD PLANNED FOR SMT X TO FLY OVER THE DRO VOR S TO HINDY (14 DME S) INTXN ON THE ILS APCH TURN INBND. WHEN MY DEP (SMA Y) WAS S ENRTE TO FMN AND SMT X WAS CLR OF SMA Y THEN I COULD CLR SMT X FOR THE ILS/SME RWY 2 APCH TO DRO. AFTER SMA Y DEPARTED I RECLRED SMA Y TO 10000 FT SO THAT I COULD EVENTUALLY GET SMT X DOWN TO 11000 FT, I DID NOT WANT SMT X TO BE TOO HIGH FOR THE G/S. SMA Y LEVELED AT 10000 FT AND SMT X WAS NEVER CLRED FOR LOWER. WHEN SMT X WAS IN HIS PROC TURN I NOTICED HE WAS DSNDING. CONFLICT ALERT WAS GOING OFF. I TRIED TWICE TO STOP SMT X AT 11000 FT. SEP WAS LOST AND I EVENTUALLY DID RECIEVE A REPLY FROM SMT X THAT HE WAS LEVEL AT 11000 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.