A VERY SHORT TKOF ROLL DURING A NIGHT OP WAS ABORTED WHEN THE PLT OF A C172 REALIZED THAT THE CLRNC FOR TKOF HAD NOT BEEN ISSUED BY THE TWR AT ONT; CA.

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I ALLOWED CIRCUMSTANCES TO OVERRIDE MY RULE OF 'THINK BEFORE YOU SPEAK.' WE WERE AT THE THRESHOLD END OF A VERY LONG RWY OF MORE THAN 12000 FT AT A MAJOR ARPT IN A LITTLE 4 PLACE CESSNA WHERE AT XA00 THE TFC WAS ALMOST NIL. AFTER A LONG WAIT BTWN 'TAXI INTO POS' AND 'TKOF' INSTRUCTIONS I FOUND MYSELF SAYING I WAS STARTING A TKOF ROLL; WHEN MY RULE OF 'THINK BEFORE YOU SPEAK' WOULD HAVE DICTATED TO CONFIRM FIRST AND SAY 'IN POS AND HOLDING' FOR A SECOND TIME OR 'CONFIRM; CLRED FOR TKOF.' FROM OUR ORIGINAL POS WELL BACK OF THE HOLD LINE; WHEN WE WERE READY WE HAD WATCHED A MEDIUM SIZED BUSINESS ACFT LAND. THE WIND WAS 3 KTS OR LESS AND THERE HAD BEEN NO TFC FOR THE PREVIOUS 10 MINS WHEN TWR CLRED US 'INTO POS AND HOLD.' I REPEATED THAT CLRNC AND SAID WE WOULD MOVE OUT SLOWLY IN THE 4 PLACE CESSNA TO LET ANY WAKE TURB FROM THE LNDG ACFT DISSIPATE. I TAXIED VERY SLOWLY OUT TO RWY CTR JUST IN FRONT OF THE THRESHOLD LIGHTS. BY THEN; PROBABLY NEARLY 2 MINS HAD PASSED SINCE THE ORIGINAL CLRNC HAD BEEN ISSUED INTO POS. AS WE SAT LOOKING DOWN THE RWY IT APPEARED THAT THE ARRIVING ACFT HAD GONE TO THE VERY FAR END OF THIS 12190 FT RWY BEFORE TURNING L TOWARD THE GA PARKING AREA BECAUSE OF CONSTRUCTION ON THE L RWY. CLRLY THAT AIRPLANE WAS SLOWLY TURNING OFF ABOUT 2 MI DOWN THERE. FOR SOME REASON; A QUIET NIGHT; AND PERHAPS SOME DISTR LOADING THE 2 PAX; I FOUND MYSELF SAYING 'ROLLING FOR TKOF.' ALMOST AT THE SAME INSTANT MY COPLT POINTED OUT THAT WE HAD NOT YET BEEN CLRED FOR TKOF. WE HAD ROLLED A FEW FT BUT I PROMPTLY APPLIED BRAKES; STOPPED AND TOLD THE TWR WE WERE HOLDING IN POS ON RWY 26R. THE POISED AND VERY CAPABLE TWR OPERATOR'S WORDS MET MINE AT THE SAME MOMENT SAYING 'YOU WERE NOT CLRED FOR TKOF.' I REPEATED BACK A SECOND TIME; 'HOLDING IN POS.' A VERY FEW SECONDS LATER; THE TWR CLRED US FOR TKOF; WHICH I REPEATED BACK AND THE REST OF THE FLT WENT ALONG UNEVENTFULLY. OUR PLANE HAD MOVED ONLY A FEW FT. THERE WAS NO SAFETY HAZARD. THE ARRIVING ACFT WAS OFF THE RWY AND FAR AWAY. BUT; I HAD MOMENTARILY ALLOWED OTHER THINGS TO SUBORDINATE MY CONSCIOUSNESS OF THE COMS AND SKIP MY RULE OF 'THINK BEFORE YOU SPEAK.' NIGHT/EARLY MORNING CONDITIONS OF POSSIBLE FOG OR LOWERING OVCST; AND THE RECENT ATTN TO A PAX NEED; WHILE THE ARPT WAS UNUSUALLY QUIET WITH MINIMAL TFC AND THE LONG TIME LAPSE BTWN COMS; I LET MY SUBCONSCIOUS GET INTO MY CONSCIOUS OPS AND I MOMENTARILY SKIPPED MY RULES ABOUT THINK FIRST; AND IF IN DOUBT; RECONFIRM BEFORE XMITTING. THIS EXPERIENCE WILL CERTAINLY ENHANCE MY GUARD AGAINST LETTING UNEXPECTED SITS AND LONG QUIET TIMES OR NIGHT CONDITIONS LOWER THE FOCUS ON CONSCIOUS PROCS.