Narrative:

During descent from 12000 ft, ATC advised traffic at 12 O'clock with type/altitude unknown. Traffic was observed on TCASII, displayed on the FMC track line, and closing. A scattered cloud layer below our altitude was present, and it appeared that our descent would take us into or through the clouds. As traffic was not visual, nor was its altitude displayed on TCASII, I requested an avoidance vector. ATC directed a 20 degree right turn. By the time that the vector was received, it appeared that only a left turn would assure lateral separation. The captain initiated a left turn and I advised ATC. After a short time, we cleared the TCASII traffic and resumed our original heading. This is a difficult problem to remedy. A mix of VFR/IFR traffic in the denver area, busy ATC frequencys, and WX constraints to visually acquiring traffic will cause many reoccurrences. ATC apparently does not issue avoidance vectors for non mode C VFR traffic unless pilot requested, and there is a tendency to delay requesting avoidance vectors as one searches for the traffic. When we decided to ask for a vector, we were unable to break in on a busy frequency. When we finally received clearance, it became apparent that the ATC assigned heading would not assure lateral clearance.

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Original NASA ASRS Text

Title: AN ACR TURBOJET TURNED OPPOSITE TO ITS VECTOR DIRECTION FOR TFC AVOIDANCE. TRACK HDG ERROR, ERROR ADMITTED. EVASIVE ACTION WRONG WAY TURN.

Narrative: DURING DSCNT FROM 12000 FT, ATC ADVISED TFC AT 12 O'CLOCK WITH TYPE/ALT UNKNOWN. TFC WAS OBSERVED ON TCASII, DISPLAYED ON THE FMC TRACK LINE, AND CLOSING. A SCATTERED CLOUD LAYER BELOW OUR ALT WAS PRESENT, AND IT APPEARED THAT OUR DSCNT WOULD TAKE US INTO OR THROUGH THE CLOUDS. AS TFC WAS NOT VISUAL, NOR WAS ITS ALT DISPLAYED ON TCASII, I REQUESTED AN AVOIDANCE VECTOR. ATC DIRECTED A 20 DEG R TURN. BY THE TIME THAT THE VECTOR WAS RECEIVED, IT APPEARED THAT ONLY A L TURN WOULD ASSURE LATERAL SEPARATION. THE CAPT INITIATED A L TURN AND I ADVISED ATC. AFTER A SHORT TIME, WE CLRED THE TCASII TFC AND RESUMED OUR ORIGINAL HEADING. THIS IS A DIFFICULT PROB TO REMEDY. A MIX OF VFR/IFR TFC IN THE DENVER AREA, BUSY ATC FREQS, AND WX CONSTRAINTS TO VISUALLY ACQUIRING TFC WILL CAUSE MANY REOCCURRENCES. ATC APPARENTLY DOES NOT ISSUE AVOIDANCE VECTORS FOR NON MODE C VFR TFC UNLESS PLT REQUESTED, AND THERE IS A TENDENCY TO DELAY REQUESTING AVOIDANCE VECTORS AS ONE SEARCHES FOR THE TFC. WHEN WE DECIDED TO ASK FOR A VECTOR, WE WERE UNABLE TO BREAK IN ON A BUSY FREQ. WHEN WE FINALLY RECEIVED CLRNC, IT BECAME APPARENT THAT THE ATC ASSIGNED HEADING WOULD NOT ASSURE LATERAL CLRNC.

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.