AN MD10 PLT CONFERRED WITH THE COMPANY REGARDING CLOSE 8 HR IN A 24 HR SCHEDULING PRIOR TO DEP ONLY TO HAVE A MECHANICAL SIT REQUIRING RETURN TO DEP STATION.

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THIS EVENT STARTED WITH MY QUESTIONING THE COMPANY DUTY OFFICER ABOUT POSSIBLE DUTY TIME AND BLOCK TIME EXCEEDENCES; POSSIBLY THE 8 IN 24 RULE. MY ONT-MEM LEG WAS 3 HRS 58 MINS AND I WAS SCHEDULED FOR 3 HRS 57 MINS OUTBOUND MEM-LAX. THE DUTY OFFICER ADVISED ME I WOULD BE LEGAL EVEN IF THERE WAS A TAXI OUT DELAY THAT WOULD PUT ME OVER 8 IN 24. AFTER A CONSIDERABLE TAXI OUT DELAY; WE DEPARTED MEM. IMMEDIATELY AFTER TKOF; A 'TAT PROBE HEAT' ALERT WAS DISPLAYED. THE CIRCUIT BREAKER WAS HEARD TO POP SOME TIME AFTER THE ALERT. WE DID OUR BEST TO STAY OUT OF VISIBLE MOISTURE AND ICING WHILE I TROUBLESHOT THE PROB WITH APPROPRIATE CHKLIST; WHICH HAD VERY LITTLE GUIDANCE ON CONSEQUENCES OF LOSS OF TAT HEAT. THE ONLY REF TO CONSEQUENCES OF LOSS OF TAT HEAT WAS FOUND IN THE MEL; WHICH OF COURSE IS FOR USE PRIOR TO TKOF. THE MEL LIMITED FLT TO CLR OF ICING CONDITIONS; AND SINCE IT WAS IMPOSSIBLE TO CONTINUE THE FLT WITHOUT ENTERING VISIBLE MOISTURE; I DECIDED TO DUMP FUEL AND RETURN FOR UNEVENTFUL LNDG AT MEM.