A C172 PLT TURNED OFF HIS VHF COM RADIO THEN LATER ATTEMPTED TO MAKE THE STANDARD CTAF CALLS ON THE INOP RADIO. NOT RECEIVING ANY REPLY; HE TAXIED ONTO THE RWY IN FRONT OF AN APCHING ACFT.

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ON JUL/XA/97 I TOOK MY FIRST PAX FOR A FLT FROM PAO TO CASTLE ARPT. AS I HAVE BEEN INSTRUCTED; I BROADCAST MY XWIND; DOWNWIND; BASE LEG; AND FINAL APCH ON CTAF 123.0. THERE WAS NO RESPONSE TO A REQUEST FOR LNDG ADVISORY NOR ANY EVIDENCE OF TFC AT CASTLE. WHILE TAXIING FROM THE RWY TO THE RUNUP AREA; I WAS EXPLAINING THE PROCS TO MY PAX. BECAUSE HE WAS HAVING DIFFICULTY HEARING ME OVER THE COMS FROM OTHER ARPTS ALSO ON 123.0; I TURNED OFF THE COMS CHANNEL. BEFORE TAXIING ONTO THE RWY FOR TKOF; I TURNED AT THE HOLD LINE AT ABOUT 75 DEGS TOWARD THE FINAL APCH. I OBSERVED NO ONE. WHEN I BROADCAST MY INTENTION TO TKOF; I HEARD NOTHING. SHORTLY AFTER I BEGAN MY TKOF ROLL; ANOTHER SMALL PLANE FLEW PAST ME TO THE R; AND I REALIZED THAT THE OTHER PLANE HAD BEEN ON FINAL APCH WHEN I HAD TAXIED INTO PLACE FOR TKOF. I WONDERED WHY THE PLT HAD NOT BROADCAST HIS APCH. CHKING THE COMS CHANNEL; I NOTED THAT MY COMS SWITCH WAS STILL IN THE OFF POS AND KNEW THAT IT WAS I WHO HAD MADE A SERIOUS ERROR IN SAFETY. I HAD VIOLATED THE OTHER PLT'S RIGHT-OF-WAY AND ENDANGERED BOTH OF US. MY INSTRUCTOR HAD EMPHASIZED MANY TIMES THAT I MAKE BROADCASTS OF ALL ACTIONS BEING TAKEN IN NON TWRED ARPTS AND THAT I ALWAYS MAKE A 90 DEG TURN TOWARD FINAL APCH BEFORE TAXIING PAST THE HOLD LINE; JUST IN CASE ANOTHER PLT WHO IS NOT USING A RADIO IS ON FINAL APCH. THE FAA EXAMINER WHO GAVE ME MY CHK RIDE COMPLIMENTED ME ON THE SAFETY MEASURES I TOOK AT NON TWRED ARPTS. HOWEVER; I MADE 2 SERIOUS ERRORS ON THE SAME TKOF: 1) I SHOULD HAVE MADE A FULL 90 DEG TURN TOWARD THE FINAL APCH. 2) I SHOULD ALWAYS MAKE CERTAIN THAT THE COMS CHANNEL OVER WHICH I AM BROADCASTING IS ON AND WORKING. I PLAN TO DO BOTH OF THESE THINGS IN MY FUTURE FLTS; AND I FEEL FORTUNATE THAT THERE WAS NOT AN ACCIDENT AS A RESULT OF MY CARELESSNESS.