Air carrier descended through an occupied altitude resulting in less than standard separation.

Attributes
ACN	140262
Time
Date	199003
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : fnt airport : dtw
State Reference	MI
Altitude	msl bound lower : 11600 msl bound upper : 12000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Route In Use	arrival other enroute airway : zob
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	140262
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : clearance non adherence : required legal separation non adherence : published procedure
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 18000 vertical : 400
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X inbound to dtw over the polar arrival fix was issued a crossing restriction to cross polar at 12000' and 250 KTS. The pilot read back 11000 and two fifty. Readback was missed. Air carrier X descended through 12000' and came into conflict with air carrier Y at 11000' southbound, also inbound to dtw. Air carrier X was issued a turn to 180 degree and climb to 12000' to avoid air carrier Y. He was also issued traffic. I discovered the problem when I noticed air carrier X mode C readout at 11600' and descending. There were numerous aircraft on frequency, and numerous landline xmissions, adding to the confusion. Additionally flint sector was being split from the lansing sector, causing further disruptions. Pilots should pay more exact attention to the clrncs they are issued, and I should have been listening more intently for his readback.

Original NASA ASRS Text

Title: ACR DESCENDED THROUGH AN OCCUPIED ALT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative: ACR X INBND TO DTW OVER THE POLAR ARR FIX WAS ISSUED A XING RESTRICTION TO CROSS POLAR AT 12000' AND 250 KTS. THE PLT READ BACK 11000 AND TWO FIFTY. READBACK WAS MISSED. ACR X DESCENDED THROUGH 12000' AND CAME INTO CONFLICT WITH ACR Y AT 11000' SBND, ALSO INBND TO DTW. ACR X WAS ISSUED A TURN TO 180 DEG AND CLIMB TO 12000' TO AVOID ACR Y. HE WAS ALSO ISSUED TFC. I DISCOVERED THE PROBLEM WHEN I NOTICED ACR X MODE C READOUT AT 11600' AND DESCENDING. THERE WERE NUMEROUS ACFT ON FREQ, AND NUMEROUS LANDLINE XMISSIONS, ADDING TO THE CONFUSION. ADDITIONALLY FLINT SECTOR WAS BEING SPLIT FROM THE LANSING SECTOR, CAUSING FURTHER DISRUPTIONS. PLTS SHOULD PAY MORE EXACT ATTN TO THE CLRNCS THEY ARE ISSUED, AND I SHOULD HAVE BEEN LISTENING MORE INTENTLY FOR HIS READBACK.

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.