Potential conflict less than standard separation between an air carrier medium large transport and crossing traffic light transport jet VFR. TCAS RA set off twice.

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Attributes
ACN	179832
Time
Date	199105
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : dtw
State Reference	MI
Altitude	msl bound lower : 11800 msl bound upper : 12000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dtw
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other cruise other
Route In Use	enroute airway : v26
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	enroute : direct
Flight Plan	VFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 210 flight time total : 15000 flight time type : 12000
ASRS Report	179832
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer pilot : cfi
Experience	flight time last 90 days : 200 flight time total : 7500 flight time type : 400
ASRS Report	180062
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : far non adherence : required legal separation
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	none taken : detected after the fact
Consequence	Other
Miss Distance	horizontal : 6000 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other
Situations
ATC Facility	procedure or policy : unspecified

Narrative:

ZOB gave us a crossing restriction of cetus intersection at 12000'. During the descent, cle asked us to expedite, which we did, and leveled at 12000' before reaching cetus. At cetus we got a TCAS RA to descend at 1500 FPM. A descent was initiated and ZOB was advised. ZOB told us to maintain 12000' because there was no traffic. We had descended to 11800' and climbed back to 12000'. The TCAS RA then told us to climb at 1000 FPM. At that time we saw a jet at our 3 O'clock position, approximately 1 mi, going through our altitude in a steep descent. We advised ZOB and they said they had not traffic. They switched us to detroit approach control and approach informed us the jet was VFR. In my opinion, this was a valid TCAS alert and the jet should have been on an IFR flight plan due to the TCA in the area.

Original NASA ASRS Text

Title: POTENTIAL CONFLICT LESS THAN STANDARD SEPARATION BETWEEN AN ACR MLG AND CROSSING TRAFFIC LTT JET VFR. TCAS RA SET OFF TWICE.

Narrative: ZOB GAVE US A XING RESTRICTION OF CETUS INTXN AT 12000'. DURING THE DSNT, CLE ASKED US TO EXPEDITE, WHICH WE DID, AND LEVELED AT 12000' BEFORE REACHING CETUS. AT CETUS WE GOT A TCAS RA TO DSND AT 1500 FPM. A DSNT WAS INITIATED AND ZOB WAS ADVISED. ZOB TOLD US TO MAINTAIN 12000' BECAUSE THERE WAS NO TFC. WE HAD DSNDED TO 11800' AND CLBED BACK TO 12000'. THE TCAS RA THEN TOLD US TO CLB AT 1000 FPM. AT THAT TIME WE SAW A JET AT OUR 3 O'CLOCK POS, APPROX 1 MI, GOING THROUGH OUR ALT IN A STEEP DSNT. WE ADVISED ZOB AND THEY SAID THEY HAD NOT TFC. THEY SWITCHED US TO DETROIT APCH CTL AND APCH INFORMED US THE JET WAS VFR. IN MY OPINION, THIS WAS A VALID TCAS ALERT AND THE JET SHOULD HAVE BEEN ON AN IFR FLT PLAN DUE TO THE TCA IN THE AREA.

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.