Air carrier Y descent through occupied altitude probably had less than standard separation from air carrier X. Possible system error.

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Attributes
ACN	241561
Time
Date	199305
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : dxo
State Reference	MI
Altitude	msl bound lower : 7000 msl bound upper : 7000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : dtw
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller non radar : 4 controller radar : 11
ASRS Report	241561
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : developmental
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Miss Distance	horizontal : 18000 vertical : 100
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X was on base leg for an arrival to runway 21L at dtw. Air carrier Y was on downwind (same side) for runway 21L at dtw. This was a training session with a specialist developing at slow to moderate rate. Air carrier X was left at 7000 but turned toward the airport. A situation closer to the airport distracted my attention for a couple of seconds. During this time, the developmental (apparently) descended air carrier Y to 5000 and issued a turn to widen him/it out. Air carrier Y did not respond to the turn but did descend, basically head-on into air carrier X (normally the downwind is kept at 8000 until sure of the situation). When the aircraft were 5-6 mi apart, I realized something was wrong (I had not heard the instruction to descend #2). Air carrier X was turned to 180 degrees and air carrier Y was turned to 220 degrees. Immediately after this was accomplished, air carrier X was issued air carrier Y as traffic. The pilot reported air carrier Y in sight and was instructed to maintain visual separation. This was accomplished 1 second before separation was lost.

Original NASA ASRS Text

Title: ACR Y DSCNT THROUGH OCCUPIED ALT PROBABLY HAD LTSS FROM ACR X. POSSIBLE SYS ERROR.

Narrative: ACR X WAS ON BASE LEG FOR AN ARR TO RWY 21L AT DTW. ACR Y WAS ON DOWNWIND (SAME SIDE) FOR RWY 21L AT DTW. THIS WAS A TRAINING SESSION WITH A SPECIALIST DEVELOPING AT SLOW TO MODERATE RATE. ACR X WAS L AT 7000 BUT TURNED TOWARD THE ARPT. A SIT CLOSER TO THE ARPT DISTRACTED MY ATTN FOR A COUPLE OF SECONDS. DURING THIS TIME, THE DEVELOPMENTAL (APPARENTLY) DSNDED ACR Y TO 5000 AND ISSUED A TURN TO WIDEN HIM/IT OUT. ACR Y DID NOT RESPOND TO THE TURN BUT DID DSND, BASICALLY HEAD-ON INTO ACR X (NORMALLY THE DOWNWIND IS KEPT AT 8000 UNTIL SURE OF THE SIT). WHEN THE ACFT WERE 5-6 MI APART, I REALIZED SOMETHING WAS WRONG (I HAD NOT HEARD THE INSTRUCTION TO DSND #2). ACR X WAS TURNED TO 180 DEGS AND ACR Y WAS TURNED TO 220 DEGS. IMMEDIATELY AFTER THIS WAS ACCOMPLISHED, ACR X WAS ISSUED ACR Y AS TFC. THE PLT RPTED ACR Y IN SIGHT AND WAS INSTRUCTED TO MAINTAIN VISUAL SEPARATION. THIS WAS ACCOMPLISHED 1 SECOND BEFORE SEPARATION WAS LOST.

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.