Narrative:

I was working the lansing sector at ZOB. It was a fairly normal day. I had a SW3 (air carrier X) inbound to dtw on the polar 1 arrival, a DA20 (ccay) inbound to mbs and a GA aircraft inbound to ptk, direct sprtn for the sprtn 3 arrival. He was about 10 or 15 mi west of mbs. Normally the DA20 would go into mbs at 11000 ft, the SW3 into dtw at 11000 ft and the GA was at 11000 ft. Dtw approach had requested earlier that morning that we bring the propellers in at 9000 ft. So I descended the SW3 from 11000 ft to 9000 ft, the DA20 from 15000 ft to 10000 ft, and left the GA at 11000 ft. As the DA20 and the SW3 got closer, I turned the DA20 15 degree right, because the SW3 was descending slowly. When the SW3 was level at 9000 ft and the DA20 at 10000 ft. I turned the DA20 back direct mbs. When I switched the DA20 to mbs approach, the conflict alert went off. Without unkeying I asked the DA20 to say his altitude but he had already switched. He passed about 3 1/2 mi north of the SW3 at about 200 or 300 ft above him. The pilot had descended on his own, without clearance, to 9000 ft. As this was occurring, I was giving a position relief briefing to another controller. Upon reviewing the tape, when the pilot acknowledged the direct mbs, he also said 'down to 9000 ft.' I didn't hear that part of his transmission, probably because I was giving the briefing. The sector was starting to get busy, and I probably wanted to get the briefing done so the next controller could get in there and situated.

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Original NASA ASRS Text

Title: ARTCC RADAR CTLR RECEIVING TRAINING CLRS A CGA JET TO AN OCCUPIED ALT BY MISTAKE. TRAINEE INTENDED TO DSND THE ACFT TO 10000 FT, BUT ISSUED A CLRNC TO 9000 FT.

Narrative: I WAS WORKING THE LANSING SECTOR AT ZOB. IT WAS A FAIRLY NORMAL DAY. I HAD A SW3 (ACR X) INBOUND TO DTW ON THE POLAR 1 ARR, A DA20 (CCAY) INBOUND TO MBS AND A GA ACFT INBOUND TO PTK, DIRECT SPRTN FOR THE SPRTN 3 ARR. HE WAS ABOUT 10 OR 15 MI W OF MBS. NORMALLY THE DA20 WOULD GO INTO MBS AT 11000 FT, THE SW3 INTO DTW AT 11000 FT AND THE GA WAS AT 11000 FT. DTW APCH HAD REQUESTED EARLIER THAT MORNING THAT WE BRING THE PROPS IN AT 9000 FT. SO I DSNDED THE SW3 FROM 11000 FT TO 9000 FT, THE DA20 FROM 15000 FT TO 10000 FT, AND LEFT THE GA AT 11000 FT. AS THE DA20 AND THE SW3 GOT CLOSER, I TURNED THE DA20 15 DEG R, BECAUSE THE SW3 WAS DSNDING SLOWLY. WHEN THE SW3 WAS LEVEL AT 9000 FT AND THE DA20 AT 10000 FT. I TURNED THE DA20 BACK DIRECT MBS. WHEN I SWITCHED THE DA20 TO MBS APCH, THE CONFLICT ALERT WENT OFF. WITHOUT UNKEYING I ASKED THE DA20 TO SAY HIS ALT BUT HE HAD ALREADY SWITCHED. HE PASSED ABOUT 3 1/2 MI N OF THE SW3 AT ABOUT 200 OR 300 FT ABOVE HIM. THE PLT HAD DSNDED ON HIS OWN, WITHOUT CLRNC, TO 9000 FT. AS THIS WAS OCCURRING, I WAS GIVING A POS RELIEF BRIEFING TO ANOTHER CTLR. UPON REVIEWING THE TAPE, WHEN THE PLT ACKNOWLEDGED THE DIRECT MBS, HE ALSO SAID 'DOWN TO 9000 FT.' I DIDN'T HEAR THAT PART OF HIS XMISSION, PROBABLY BECAUSE I WAS GIVING THE BRIEFING. THE SECTOR WAS STARTING TO GET BUSY, AND I PROBABLY WANTED TO GET THE BRIEFING DONE SO THE NEXT CTLR COULD GET IN THERE AND SITUATED.

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.