An air carrier medium large transport descended in response to a TCASII command.

Attributes
ACN	270017
Time
Date	199405
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : dxo airport : dtw
State Reference	MI
Altitude	msl bound lower : 11700 msl bound upper : 12000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : zzz
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise other descent other
Route In Use	arrival other arrival star : star enroute airway : dtw
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Flight Phase	cruise other
Route In Use	arrival other arrival star : star
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 100 flight time total : 1200 flight time type : 7000
ASRS Report	270017
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : nmac other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were inbound to dtw on the mizar arrival at 12000 ft and overtaking visually another aircraft on the arrival at 11000 ft. We had just passed him when we had a TCASII RA to descend as an aircraft sbound at 12400 ft came on our TCASII on a collision course. We complied with RA and descended to approximately 11700 ft. The captain was flying and the first officer reported to dtw approach we had an RA. Captain called dtw approach on the ground. The supervisor informed us the controller was occupied with heavy traffic load including another air carrier flight. The dtw supervisor informed me the 12500 ft traffic was 'level,' VFR, and not working dtw ATC. The first officer saw the low wing, single engine aircraft pass over us. Controller had informed previous inbounds but not us due to workload and the other aircraft appeared VFR, level at 12500 ft. We had the current dtw altimeter setting set and we were level 12000 ft. At first, we thought the other aircraft was slowly climbing and would not be a factor, showing plus 300 ft, then plus 400 ft above use. When he appeared to stay at plus 400 ft, we descended keeping in mind the IFR traffic at 11000 and somewhat behind us. Only after calling dtw ATC did we know he was 'level.' we were committed, however, to descent due to TCASII command. Too often, VFR traffic try to fly over TCA's at major airports without knowledge of arrival and departure corridors and no contact with ATC. ATC should encourage and operators of VFR aircraft should solicit radar TA's and recommended rtes over or around terminal areas.

Original NASA ASRS Text

Title: AN ACR MLG DSNDED IN RESPONSE TO A TCASII COMMAND.

Narrative: WE WERE INBOUND TO DTW ON THE MIZAR ARR AT 12000 FT AND OVERTAKING VISUALLY ANOTHER ACFT ON THE ARR AT 11000 FT. WE HAD JUST PASSED HIM WHEN WE HAD A TCASII RA TO DSND AS AN ACFT SBOUND AT 12400 FT CAME ON OUR TCASII ON A COLLISION COURSE. WE COMPLIED WITH RA AND DSNDED TO APPROX 11700 FT. THE CAPT WAS FLYING AND THE FO RPTED TO DTW APCH WE HAD AN RA. CAPT CALLED DTW APCH ON THE GND. THE SUPVR INFORMED US THE CTLR WAS OCCUPIED WITH HVY TFC LOAD INCLUDING ANOTHER ACR FLT. THE DTW SUPVR INFORMED ME THE 12500 FT TFC WAS 'LEVEL,' VFR, AND NOT WORKING DTW ATC. THE FO SAW THE LOW WING, SINGLE ENG ACFT PASS OVER US. CTLR HAD INFORMED PREVIOUS INBOUNDS BUT NOT US DUE TO WORKLOAD AND THE OTHER ACFT APPEARED VFR, LEVEL AT 12500 FT. WE HAD THE CURRENT DTW ALTIMETER SETTING SET AND WE WERE LEVEL 12000 FT. AT FIRST, WE THOUGHT THE OTHER ACFT WAS SLOWLY CLBING AND WOULD NOT BE A FACTOR, SHOWING PLUS 300 FT, THEN PLUS 400 FT ABOVE USE. WHEN HE APPEARED TO STAY AT PLUS 400 FT, WE DSNDED KEEPING IN MIND THE IFR TFC AT 11000 AND SOMEWHAT BEHIND US. ONLY AFTER CALLING DTW ATC DID WE KNOW HE WAS 'LEVEL.' WE WERE COMMITTED, HOWEVER, TO DSCNT DUE TO TCASII COMMAND. TOO OFTEN, VFR TFC TRY TO FLY OVER TCA'S AT MAJOR ARPTS WITHOUT KNOWLEDGE OF ARR AND DEP CORRIDORS AND NO CONTACT WITH ATC. ATC SHOULD ENCOURAGE AND OPERATORS OF VFR ACFT SHOULD SOLICIT RADAR TA'S AND RECOMMENDED RTES OVER OR AROUND TERMINAL AREAS.

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.