PLT INSTRUCTOR OF A CESSNA 150 FAILED TO MAINTAIN THE RWY HDG AFTER TKOF CAUSING DEP CTL TO INTERVENE TO ASSURE LEGAL TFC SEPARATION.

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I HAVE BEEN A PART TIME CFI FOR 2 YRS. MOST TRAINING FLTS ARE TO THE PRACTICE AREA OR CROSS-COUNTRY TO UNCTLED ARPTS. I DECIDED I NEEDED TO GO OUT AND FLY IN THE ATC SYS TO BECOME MORE PROFICIENT. MY SOLO FLT WAS VFR USING VOR/LORAN FROM L12-ONT-CHO-RAL-HMT-L12. AFTER LNDG AT RAL; I REQUESTED THE VOR GND CHK LOCATED ON THE ARPT; THEN TAXI BACK TO RWY 27 FOR TKOF. I WAS INSTRUCTED TO REMAIN ON THE TWR FREQ. WITH THE VOR CHK COMPLETED; I WAS APPROVED TO TAXI TO RWY 27. WHILE TAXIING I ASKED THE TWR TO CONTACT SOCAL DEP FOR A CLRNC AND XPONDER CODE FOR A DEP TO THE SE INTO MARCH AFB CLASS C; WITH A LNDG AT HMT. WHEN I HAD COMPLETED MY PREFLT CHKLIST AND RECEIVED THE CURRENT ATIS INFO; I CONTACTED THE TWR TO TELL THEM I WAS READY TO DEPART RWY 27. RAL TWR GAVE ME MY XPONDER CODE AND MY CLRNC. I THOUGHT I HEARD SBOUND DEP APPROVED. WITH THE CLRNC TO DEPART I ENTERED RWY 27. OVER THE DEP END OF THE RWY THE TWR INSTRUCTED ME TO CONTACT SOCAL DEP. I MADE THE STANDARD L 15 DEG HDG CHANGE FOR RAL'S NOISE ABATEMENT PROGRAM WHILE CHKING IN WITH SOCAL. I THEN STARTED TO COMPLETE MY TURN TO THE S. IT WAS THEN THAT THE VERY ANGRY SOCAL CTLR ASKED ME WHAT I WAS DOING. WHY I HAD TURNED? THAT I WAS INSTRUCTED TO FLY RWY HDG. I IMMEDIATELY TURNED BACK TO COMPLY WITH HIS INSTRUCTIONS. I TOLD HIM I HAD SIDESTEPPED OVER (NOISE ABATEMENT) AND I WAS SORRY FOR THE CONFUSION. I WAS ABOUT 1/4 MI S OF THE ARPT. THE CTLR THEN CLRED ME TO MY SE HDG AND TO CONTACT THE NEXT SECTOR. I BELIEVE THAT THE CONFUSION CAME FROM THE TWR'S USE OF THE WORD 'APPROVED' IN MY CLRNC AND ME EXPECTING TO HEAR THAT I WAS GOING TO RECEIVE A CLRNC TO DEPART TO THE SE. I DID NOT HEAR THE TWR CLRNC AS MAINTAIN RWY HDG. I HEARD 'DEP APPROVED'. I THINK THAT WHEN A REQUEST IS MADE FROM AN ACFT; THAT THE WORD 'APPROVED' SHOULD NOT BE USED IN THE INSTRUCTIONS OR REPLY FROM THE CTLING FACILITY IF THERE IS A DIFFERENCE BTWN THE REQUEST AND WHAT ATC WANTS THAT ACFT TO DO.