C172 PLT INITIATES EVASIVE ACTION FROM ASSIGNED RTE WHEN PERCEIVING POTENTIAL CONFLICT.

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I WAS GIVEN AN IFR RELEASE TO DEPART DVT AND PROCEED TO TUS. AFTER CONTACTING THE DEP FREQ; I WAS GIVEN AN AMENDED CLRNC TO MAINTAIN 5000 FT MSL AND EXPECT RADAR VECTORS TO V105. I WAS GIVEN A TA FOR A LEARJET (ACFT Y) DSNDING FROM 11000 FT MSL TO 4000 FT MSL AT MY 12 O'CLOCK POS; HDG OPPOSITE DIRECTION (NBOUND). I RPTED LOOKING FOR THAT TFC AND SCANNED THE HORIZON FOR THE TFC BUT WAS UNABLE TO LOCATE IT. I RECALL LOOKING DOWN AT THE NAV STACK WHEN THE PVT PLT WHO WAS SITTING IN THE R SEAT SHOUTED 'TFC 1 O'CLOCK POS AND DSNDING!' I IMMEDIATELY LOOKED TO MY 1 O'CLOCK POS AND SAW THE WINGTIP NAV LIGHTS ON AN ACFT DSNDING AND IT WAS NOW AT MY 12 O'CLOCK POS. R SEAT PLT AGAIN SHOUTED 'TFC DEAD AHEAD AND DSNDING!' I OBSERVED THE WINGTIP NAV LIGHTS GROWING FARTHER APART; HOWEVER; THERE WAS NO RELATIVE MOVEMENT OF THE ACFT IN MY WINDSCREEN. I ATTEMPTED TO CONTACT DEP CTL TO GET CLARIFICATION ON OUR TFC. HOWEVER; DUE TO THE CONGESTION; I WAS UNABLE TO GET THROUGH TO DEP. THE ACFT WAS CONTINUING ITS DSCNT AND THE WINGSPAN OF THE ACFT WAS GROWING LARGER AT AN ALARMINGLY FAST RATE; WITH AGAIN; NO RELATIVE MOVEMENT IN THE WINDSCREEN. THE OTHER PLT SHOUTED; 'WE NEED TO DO SOMETHING; OUR TFC IS COMING RIGHT AT US!' I ASSURED HIM THAT I HAD THE TFC IN SIGHT. IN ORDER TO AVOID A CONFLICT; I EXECUTED A R TURN WBOUND AND HELD MY ALT. I WAS STILL UNABLE TO CONTACT DEP DUE TO CONGESTION OF THE FREQ. AS I MADE MY TURN TO THE W; THE ACFT TURNED TO THE W AS WELL; AND THE ACFT'S WINGTIP NAV LIGHTS CONTINUED TO RAPIDLY GROW FARTHER APART WITH NO RELATIVE MOVEMENT OF THE ACFT IN MY WINDSCREEN. AT THIS TIME I MADE A DECISION THAT WE WERE ON A COLLISION COURSE; AND IF I FAILED TO TAKE EVASIVE ACTION; A COLLISION WOULD BE IMMINENT. IN THE INSTANT I DECIDED TO TAKE EVASIVE ACTION; IT APPEARED THAT THE ACFT BEGAN TO LEVEL OFF. I RESPONDED TO THIS SIT BY EXECUTING AN EMER DSCNT. I ALSO HEADED WBOUND IN MY DSCNT SO THAT IF THE TFC THAT I WAS AVOIDING WAS NOT THE LEARJET; I WOULD BE W OF THE LEARJET'S LOCATION. BY THE TIME WE WERE STABILIZED; THE CTLR CONTACTED US INQUIRING WHY WE WERE NOT AT 5000 FT MSL AND ADVISED US OF THE LEARJET TFC AT 4000 FT MSL; AND INSTRUCTED US TO IMMEDIATELY CLB AND MAINTAIN 5000 FT MFL. I BEGAN TO CLB IMMEDIATELY AND ANSWERED THE CTLR STATING WE HAD TFC ABOVE US (ACFT Z) APPEARING TO BE DSNDING DIRECTLY ON TOP OF US; AND THAT I HAD TO TAKE EVASIVE ACTION TO PREVENT AN NMAC. HE STATED THAT OUR ONLY TFC WAS A LEARJET AT 4000 FT MSL. I CLBED BACK TO 5000 FT MSL AND WE GOT A FREQ CHANGE. THE CTLR ON THE NEXT FREQ STATED THAT I NEEDED TO CONTACT TRACON. I CONTACTED CTR AND REQUESTED THAT THEY CANCEL MY IFR FLT PLAN TO TUS AND REQUESTED VFR FLT FOLLOWING BACK TO DVT. I CONTACTED THE WATCH SUPVR AND GAVE HIM ALL OF THE PERTINENT INFO. I WENT TO PHOENIX TRACON AND MET WITH A SUPVR AND DISCUSSED THE INCIDENT. AT THIS TIME I FEEL THAT I HAVE A FAIRLY GOOD UNDERSTANDING OF WHAT WENT WRONG. WE WERE GIVEN A TA OF THE LEARJET DSNDING FROM 11000 FT MSL TO 4000 FT MSL 12 O'CLOCK POS FROM OUR POS. THE LEARJET LEVELED OFF AT 4000 FT MSL PRIOR TO REACHING OUR POS. HOWEVER; DUE TO HVY ACTIVITY; THE CTLR WAS UNABLE TO GIVE ME A FOLLOW-UP CALL ON OUR TFC (TYPICAL IN THAT AREA; WORKLOAD PERMITTING). THE ACFT THAT WE SAW DSNDING ABOVE AND AT OUR 12 O'CLOCK POS WAS A B737 DSNDING FROM 8000 FT MSL TO 6000 FT MSL AND IT PROCEEDED TO PASS OVER THE TOP OF US. I DID NOT GET A TA ON THE B737 DUE TO THE FACT THAT IT WAS ON A DIFFERENT CTLR FREQ. WHEN I SAW THE B737 DSNDING; I WAS UNABLE TO GET THROUGH TO DEP TO ASK FOR CLARIFICATION ON OUR LEARJET TFC AND ANY OTHER TFC WHICH WAS IN OUR IMMEDIATE VICINITY. APPARENTLY THE B737 DID LEVEL OFF AT 6000 FT MSL. HOWEVER; IT APPEARED TO ME THAT IT WAS HDG DIRECTLY AT US. AT THE TIME OF THE INCIDENT; I WAS UNSURE WHETHER THE ACFT I SAW DSNDING WAS THE LEARJET (LATE IN HIS DSCNT) OR PERHAPS ANOTHER ACFT THAT WAS DSNDING TO 4000 FT MSL. THIS UNCERTAINTY IS WHAT LED ME TO TAKE EVASIVE ACTION. MY LOW HRS OF NIGHTTIME EXPERIENCE; COUPLED WITH THE BRIGHT LIGHTS BELOW; PLAYED A ROLE AS TO MY INABILITY TO ACCURATELY PERCEIVE DISTANCE. IN ADDITION; I HAD NEVER BEEN WITHIN 1000 FT OF A LARGER AIRLINER AND; DUE TO THE SIZE OF THE ACFT; IT APPEARED TO ME TO BE MUCH CLOSER THAN IT REALLY WAS. THIS WAS MY FIRST IFR FLT PLAN FLOWN WITHOUT A CFI ON BOARD (I HAD JUST EARNED MY INST RATING 12 DAYS PRIOR TO THE INCIDENT) AND I HAD A CERTAIN LEVEL OF NERVOUSNESS. THE INTENSITY IN THE COCKPIT WAS FURTHER ELEVATED DUE TO THE PLT IN THE R SEAT'S REACTION TO THE SIT. ANOTHER CONTRIBUTING FACTOR WAS THE FACT THAT I WAS NOT FAMILIAR WITH THE INTRICACIES OF THE AIRSPACE AND HOW IT IS CTLED. WHEN I VISITED PHOENIX TRACON; I WAS SHOWN WHERE THE INCIDENT OCCURRED ON THE RADAR SCOPE. IT HAPPENED IN A VERY BUSY CORRIDOR WHERE THERE ARE MULTIPLE ALTS ASSIGNED AND MULTIPLE CTLRS OPERATING THE AIRSPACE. ACCORDING TO THE PHOENIX TRACON SUPVR; THIS HAS BEEN A PROB AREA AND THERE ARE PLANS ON CHANGING THE CORRIDOR OUTLAY. I BELIEVE THAT EVERY NEW INST PLT SHOULD ATTAIN MORE NIGHT FLYING TRAINING THAN WHAT IS REQUIRED IN FAR PART 61; PRIOR TO EMBARKING ON AN IFR FLT AT NIGHT WITHOUT A CFI. I ALSO BELIEVE THAT IF A LOW HR PLT IS GOING TO FLY INTO CLASS B AIRSPACE THAT HE/SHE SHOULD VISIT THE CTL CTR AND GET A THOROUGH UNDERSTANDING OF THE PARTICULARS OF THAT INDIVIDUAL AIRSPACE; INCLUDING HEAVILY CONGESTED AREAS AND CORRIDORS. IN ADDITION; I BELIEVE IT WOULD BE USEFUL FOR PLTS TO KNOW ABOUT TFC WHICH WILL BE LEVELING OFF 1000 FT ABOVE THEM; ESPECIALLY IF IT IS A LARGER ACFT. I UNDERSTAND THAT THIS PRESENTS A PROB AT THE PHX CORRIDOR DUE TO THE MANNER IN WHICH THE VERT AIRSPACE LIMITS ARE SEGREGATED. HOWEVER; HAD I GOTTEN A TA ON THE B737 THIS INCIDENT MAY HAVE BEEN AVOIDED.