C172 TFC ACFT MONITORING I-50 AT 1400 FT BELOW THE CLASS B AIRSPACE. MONITORING CTAF FREQ FOR UNCTLED ARPT WHERE HE NORMALLY TURNS TO RETURN OVER RTE; ENCOUNTERS SMA ACFT AT SAME ALT CLOSING VERY RAPIDLY. TAKES EVASIVE ACTION BY SHARP L TURN AND INADVERTENTLY CLBS TO 1700 FT ENTERING THE CLASS B. DSNDS IMMEDIATELY.

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WHILE FLYING OUR NORMAL TFC RTE; WE HAD TO TAKE EVASIVE ACTION FROM ANOTHER AIRPLANE. AT THIS TIME WE WERE FOLLOWING RTE SEBOUND AT OUR NORMAL 1400 FT. WE FLY AT THIS SAME ALT EVERY DAY; DUE TO THE FACT THAT WE FLY JUST OUTSIDE AND UNDER THE NEARBY TCA LIMITS; WHICH AFTER THE 7 MI SURFACE RING BEGINS AT 1500 FT. THIS LEAVES VERY LITTLE ROOM FOR ERROR. WE FLEW RTE SE TO FREEWAY ARPT -- A SMALL UNCTLED FIELD AND THEN REVERSED COURSE. NORMAL PROC IS TO MONITOR THE LCL UNICOM/TFC FREQ. BUT BEING AN UNCTLED FIELD; PLTS ARE ENCOURAGED BUT NOT REQUIRED TO HAVE RADIOS TO RPT THEIR POS. WE REVERSED COURSE TO HEAD W AFTER JUST PASSING THE FIELD. UPON REVERSAL I SAW AN ACFT OFF OUR R WINGTIP AT APPROX THE SAME ALT HDG TOWARDS OUR R WINGTIP -- VERY CLOSE AND CLOSING FAST. SO I SHARPLY TURNED L; AND HE DID THE SAME. DURING THIS SURPRISE I WAS SO WRAPPED UP IN WHAT WAS GOING ON THAT I HAD INADVERTENTLY CLBED TO 1700 FT (200 FT ABOVE THE 1500 FT TCA LIMIT). IT ONLY TOOK A MIN AND AS SOON AS I REALIZED THIS MISTAKE; I IMMEDIATELY DSNDED TO OUR NORMAL ALT; 1400 FT BELOW THE TCA. LOOKING BACK I DON'T THINK MONITORING THE UNICOM FREQ AND CAREFUL VIGILANCE ARE ENOUGH. OUR NORMAL PROC OF REVERSING COURSE SO CLOSE TO AN ARPT IS AN UNNECESSARY RISK. IT TAKES VERY LITTLE TIME TO GO A MI PAST IT. THE AIRPLANE WAS PROBABLY ON A DOWNWIND LEG AND I PROBABLY WOULD HAVE SEEN HIM FURTHER OUT. ALSO; AT THE ALT WE FLY AT SO CLOSE TO TCA AIRSPACE IT'S GOOD TO BE PREPARED FOR SITS THAT UNEXPECTEDLY ARISE; AND TO REMEMBER ALWAYS THAT ALT RESTR.