C172 PLT DSNDED BELOW ASSIGNED ALT DURING A VFR DSCNT IN ORDER TO AVOID FLYING INTO CLOUDS RESULTING IN ANOTHER ACFT AN EVASIVE ACTION DIVE TO AVOID RPTR OF WHICH HE NEVER OBSERVED.

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I WAS IN COM WITH DTW APCH AND RECEIVING VFR FLT ADVISORIES JUST W OF THE CLASS E AIRSPACE AND DSNDING FROM 7500 FT; HAVING BEEN CLRED BY ATC FOR A VFR DSCNT AND DIRECT TO MY DEST. AT THIS POINT I WAS ABOVE A SCATTERED TO BROKEN LAYER OF CLOUDS. ALTHOUGH I HAD BEGUN A DSCNT AT A MODERATE APPROX 500 FPM; IT BECAME CLR THAT AS I APCHED THE LAYER OF CLOUDS IT WOULD BE NECESSARY TO MANEUVER TO MAINTAIN VFR CLRNCS FROM CLOUDS. SHORTLY AFTER FINDING A GOOD SIZE CLRING; AND STARTING A 1000-1500 FPM DSCNT; ATC INFORMED ME (AT EXACTLY WHAT ALT I AM UNSURE) TO LEVEL AT 4500 FT DUE TO TFC. I DID NOT NOTE THE POS OR INTENTIONS OF THE TFC; BUT DID CONFIRM THE ALT. I DID NOT KNOW EXACTLY WHAT THE CEILING WAS; BUT KNEW IT TO BE AROUND THIS LEVEL. SO; 4500 FT MIGHT BE LOW ENOUGH; AND IF NOT; I REASONED I WOULD HAVE TIME TO CONTACT ATC; IF NECESSARY. PASSING THROUGH 5000 FT IT BECAME CLR THAT WHILE I HAD DROPPED BELOW THE LCL CLOUD TOPS; THIS WAS INSUFFICIENTLY LOW TO DROP BELOW THE CLOUDS. TRAVELING IN A CLRING TOWARD THE OPPOSITE CLOUD WALL; I LEVELED MY DSCNT AT 4500 FT AND SIMULTANEOUSLY CONTACTED ATC TO TERSELY INFORM THEM '(CALL SIGN) UNABLE TO MAINTAIN VFR; REQUEST 3500 FT.' THIS MESSAGE WAS APPARENTLY IN CONFLICT WITH ANOTHER CALL; SO I WAITED FOR CLR FREQ (APPROX 10 SECONDS); CALLED AGAIN; AND WAS AGAIN STEPPED ON. RUNNING OUT OF CLRING AND OPTIONS; AND UNSURE OF THE NEARBY TFC (WAS IT CLBING AWAY FROM ME?); I DECIDED TO BEGIN A RAPID DIVE TO MAINTAIN VFR CLRNCS FROM THE CLOUDS; AGAIN TRYING AND FAILING FOR A THIRD TIME TO GET THROUGH TO ATC. ALTHOUGH I REALIZED I HAD BEEN CLRED TO 4500 FT; FLYING INTO THE CLOUD WAS CLRLY NOT AN OPTION; AND AT THE TIME TRYING TO CLB BACK ABOVE THE CLOUD LAYER DID NOT SEEM FEASIBLE. IT IS THEN THAT THE TFC I PRESUME I WAS NEAR (AS I NEVER MADE VISUAL CONTACT WITH THE OTHER ACFT); RPTED HAVING TO DIVE TO AVOID TFC -- HAVING TO AVOID ME! ATC CALLED AND REQUESTED MY ALT; WHICH I RPTED AS 3500 FT. ATC ASKED; RHETORICALLY; IF I HAD SEEN THE TFC (NEGATIVE); AND BLUNTLY STATED; 'THAT IS WHY I TOLD YOU TO HOLD AT 4500 FT; SIR.' I REPLIED THAT I WAS UNABLE TO MAINTAIN VFR AND THAT I ATTEMPTED 3 TIMES TO RPT EXACTLY THAT. SHORTLY THEREAFTER; ATC ASKED ME WHAT ALT I WAS GOING TO; AS I HAD NOW DROPPED TO 2300 FT. I BEGAN CLBING AND RPTED 2500 FT. AFTER WHICH ATC DROPPED ME; TELLING ME TO SQUAWK VFR AND TO CONTACT ANN ARBOR; WHOSE AIRSPACE I WAS ENTERING. MY ERRORS WERE FAILING TO POSITIVELY IDENT THE LOCATION AND INTENTIONS OF THE CONFLICTING TFC; NOT ANTICIPATING THE CLOUD LAYER THICKNESS; AND CONSEQUENTLY DECIDING TO CONTINUE THE DSCNT THROUGH THE CLOUDS WITHOUT A POSITIVE IMPRESSION OF THE HT OF THE CEILING. IT WOULD HAVE BEEN BETTER TO HAVE INFORMED ATC THAT 4500 FT MIGHT NOT WORK BECAUSE OF THE CLOUDS AND REMAIN HIGHER FOR THE TIME BEING; RATHER THAN 'TRYING IT.' IN MY DEFENSE; MAINTAINING VFR CLRNC; RATHER THAN FLYING INTO CLOUDS AT MY ASSIGNED ALT; DID PROVIDE ENOUGH SEPARATION AND TIME FOR THE OTHER ACFT TO MANEUVER TO AVOID A COLLISION. IT WOULD HAVE BEEN BETTER IF THE CTLR HAD TOLD ME 'REMAIN AT OR ABOVE 4500 FT FOR CLBING TFC AT YOUR XX O'CLOCK POS.' ONLY SAYING 'LEVEL OFF AT 4500 FT' DIDN'T EXPLICITLY LEAVE ME ANY OPTIONS -- HE HAD GIVEN ME A CLRNC THAT DID NOT PERMIT ME TO MAINTAIN VFR. WHILE SCOLDING ME BEFORE UNDERSTANDING THE SIT WAS UNFORTUNATE; I FEEL IT WAS UNPROFESSIONAL OF THE CTLR TO SIMPLY DUMP ME TO ANN ARBOR (SORT OF 'GET OUT OF MY AIRSPACE') BEFORE ENSURING THE SIT WAS UNDER CTL. IT IS CLR THAT THE CTLR HAD BEEN IN A RELAXED MOOD (PRIOR TO THIS SIT). PERHAPS IF THIS CTLR WAS BETTER PREPARED TO HANDLE THE ISSUES OF VFR FLYING; NOT JUST IFR; HAD BEEN A LITTLE LESS CAUSAL ABOUT THE SIT; AND/OR I HAD CONVEYED UNCERTAINTY ABOUT THE CLOUD LEVEL I WAS MOVING THROUGH; THIS SIT WOULD HAVE BEEN ENTIRELY AVOIDED.