Narrative:

Level at 7000 ft MSL, en route to oak the controller issued instructions to descend to and maintain 6000 ft MSL. The clearance was acknowledged and as I set the new altitude into the alerter, the controller simultaneously came back and issued oncoming traffic to us. Given the range and distance of the issued traffic, it occurred to both myself and the captain, (who was flying the aircraft) that an error, either in the previously issued clearance or in the conflicting aircraft's position may have occurred. Due to the close proximity of the issued traffic, I immediately diverted my attention outside the aircraft, in an attempt to establish a visual on the target. The captain hesitated, consulted his TCASII, (which did not portray said traffic), and began a slow descent. I tried to re-establish communication with the controller to verify the previous message, but the controller was engaged with other aircraft. Simultaneously with the initiation of my visual search the TCASII started sounding off warnings regarding the potential for a hazardous situation. The captain arrested the min descent he had initiated and climbed slightly. The controller then admonished us for beginning our descent out of 7000 ft, saying she had told us to maintain 7000 ft when she issued the traffic. It was during this last verbal exchange that we gained a visual on the conflicting aircraft passing beneath us. This whole event happened in what seemed like a nanosecond. The WX was clear and visibility excellent. Radio traffic was extremely heavy and quite possibly the controller's retort of the original clearance to 6000 ft was blocked. I do know that no one on the flight deck consciously acknowledged it. The controller was relieved from the position prior to our leaving the sector. Upon landing, the captain called the ATC facility and was told something to the effect that the miscom was not our problem. He was told, many VFR targets transit the area and the potential for like incidents is unfortunately common. I do not have the big picture regarding command and control of this airspace, but if this is truly the case, some re-allocation of airspace around oak or additional frequencys to reduce radio congestion seem in order. Advances in technology such as TCASII, which idented the conflict potential before we had a visual, are not fool proof. Our rate of closure, the angle at which we closed, coupled with the rapid fire delivery of ATC instructions to digest and the inability to get an immediate verification of ATC's last communication, could have made this situation more significant. The human factors and workload side of the equation must be examined, so pilots and controllers are not faced with similar compromises.

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Original NASA ASRS Text

Title: LOSS OF SEPARATION ENDING IN TCASII RA TRAILS POOR CTLR TECHNIQUE AND ATTEMPT TO EVADE RESPONSIBILITY AS ACR MLG APCHS OAK.

Narrative: LEVEL AT 7000 FT MSL, ENRTE TO OAK THE CTLR ISSUED INSTRUCTIONS TO DSND TO AND MAINTAIN 6000 FT MSL. THE CLRNC WAS ACKNOWLEDGED AND AS I SET THE NEW ALT INTO THE ALERTER, THE CTLR SIMULTANEOUSLY CAME BACK AND ISSUED ONCOMING TFC TO US. GIVEN THE RANGE AND DISTANCE OF THE ISSUED TFC, IT OCCURRED TO BOTH MYSELF AND THE CAPT, (WHO WAS FLYING THE ACFT) THAT AN ERROR, EITHER IN THE PREVIOUSLY ISSUED CLRNC OR IN THE CONFLICTING ACFT'S POS MAY HAVE OCCURRED. DUE TO THE CLOSE PROX OF THE ISSUED TFC, I IMMEDIATELY DIVERTED MY ATTN OUTSIDE THE ACFT, IN AN ATTEMPT TO ESTABLISH A VISUAL ON THE TARGET. THE CAPT HESITATED, CONSULTED HIS TCASII, (WHICH DID NOT PORTRAY SAID TFC), AND BEGAN A SLOW DSCNT. I TRIED TO RE-ESTABLISH COM WITH THE CTLR TO VERIFY THE PREVIOUS MESSAGE, BUT THE CTLR WAS ENGAGED WITH OTHER ACFT. SIMULTANEOUSLY WITH THE INITIATION OF MY VISUAL SEARCH THE TCASII STARTED SOUNDING OFF WARNINGS REGARDING THE POTENTIAL FOR A HAZARDOUS SIT. THE CAPT ARRESTED THE MIN DSCNT HE HAD INITIATED AND CLBED SLIGHTLY. THE CTLR THEN ADMONISHED US FOR BEGINNING OUR DSCNT OUT OF 7000 FT, SAYING SHE HAD TOLD US TO MAINTAIN 7000 FT WHEN SHE ISSUED THE TFC. IT WAS DURING THIS LAST VERBAL EXCHANGE THAT WE GAINED A VISUAL ON THE CONFLICTING ACFT PASSING BENEATH US. THIS WHOLE EVENT HAPPENED IN WHAT SEEMED LIKE A NANOSECOND. THE WX WAS CLR AND VISIBILITY EXCELLENT. RADIO TFC WAS EXTREMELY HVY AND QUITE POSSIBLY THE CTLR'S RETORT OF THE ORIGINAL CLRNC TO 6000 FT WAS BLOCKED. I DO KNOW THAT NO ONE ON THE FLT DECK CONSCIOUSLY ACKNOWLEDGED IT. THE CTLR WAS RELIEVED FROM THE POS PRIOR TO OUR LEAVING THE SECTOR. UPON LNDG, THE CAPT CALLED THE ATC FACILITY AND WAS TOLD SOMETHING TO THE EFFECT THAT THE MISCOM WAS NOT OUR PROB. HE WAS TOLD, MANY VFR TARGETS TRANSIT THE AREA AND THE POTENTIAL FOR LIKE INCIDENTS IS UNFORTUNATELY COMMON. I DO NOT HAVE THE BIG PICTURE REGARDING COMMAND AND CTL OF THIS AIRSPACE, BUT IF THIS IS TRULY THE CASE, SOME RE-ALLOCATION OF AIRSPACE AROUND OAK OR ADDITIONAL FREQS TO REDUCE RADIO CONGESTION SEEM IN ORDER. ADVANCES IN TECHNOLOGY SUCH AS TCASII, WHICH IDENTED THE CONFLICT POTENTIAL BEFORE WE HAD A VISUAL, ARE NOT FOOL PROOF. OUR RATE OF CLOSURE, THE ANGLE AT WHICH WE CLOSED, COUPLED WITH THE RAPID FIRE DELIVERY OF ATC INSTRUCTIONS TO DIGEST AND THE INABILITY TO GET AN IMMEDIATE VERIFICATION OF ATC'S LAST COM, COULD HAVE MADE THIS SIT MORE SIGNIFICANT. THE HUMAN FACTORS AND WORKLOAD SIDE OF THE EQUATION MUST BE EXAMINED, SO PLTS AND CTLRS ARE NOT FACED WITH SIMILAR COMPROMISES.

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.