B737-800 FLT CREW DISPUTES MIA ATCT LCL CTLR'S CLRNC FOR TKOF ON RWY 8R WHEN PRECEDING LNDG B747 WAS STOPPED WITH AFT HALF OF ACFT STILL ON RWY SIDE OF HOLD LINE AT EXIT M10.

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AS WE WERE CLRED ONTO THE RWY 8R AT MIA AND THEN CLRED FOR TKOF; THE PREVIOUS FLT TO TKOF; ACFT Z; TOLD TWR THAT THE ACFT Y; A 747; WHICH HAD JUST LANDED BEFORE HIS TKOF WAS 'NOT CLR OF THE RWY.' WE COULD SEE THE 747 AT A STOP ON M10; PERPENDICULAR TO THE RWY AND THE TAIL RELATIVELY NEAR THE EDGE. TWR THEN CAME ON THE RADIO AND FOR ALMOST ONE MINUTE ANNOUNCED OVER AND OVER THAT 'HE WAS CLR;' 'WE HAVE SENSORS THAT TELL US HE IS CLR' AND SUCH. WE THEN CLARIFIED WITH TWR THAT HE HAD CLRED US FOR TKOF. TWR CONFIRMED THAT WE WERE CLRED. THE 747 HAD NOT MOVED. WHAT WE SAW WAS THE 747 WITH THE REAR GEAR JUST OVER THE HOLD LINE (FACING AWAY FROM THE RWY) AND AT LEAST 50 FT OR MORE OF THE ACFT ON THE RWY SIDE OF THE HOLD LINE. WE DON'T THINK THE TAIL WAS OVER THE EDGE LINE FOR THE RWY. AT CRUISE; WE CONTACTED ACFT Z AND DISCUSSED WHAT WE SAW. OUR CHART MANUALS CALL THE HOLD SHORT LINE THE EDGE OF THE 'RWY SAFETY ZONE.' AFTER LNDG; THE CAPT CALLED MIA TWR WHO EXPLAINED THAT THEY BELIEVE THAT GOING AWAY FROM THE RWY AN ACFT CAN BE 'OVER THE LINE;' BUT GOING TOWARDS THE RWY; NOTHING CAN BE OVER THE LINE. THIS WAS NORMAL OPS FOR THEM. IN THE LAST WEEK; I'VE ASKED MANY PLTS WHO ALL AGREE WE ARE TRAINED TO BELIEVE THE HOLD LINE IS SACRED. FROM OUR TRAINING WE ALL BELIEVE THAT THE 747 WAS 'NOT CLR OF THE RWY.' THERE IS NEVER ANY DIFFERENCE PUBLISHED IN ANY FAA MANUAL ABOUT DIRECTION OF TRAVEL. WITH RWY INCURSION A 'HOT ISSUE' IS MIA TWR RIGHT? CAN THE WHOLE BACK END OF A 747 OVERHANG THE HOLD LINE ON THE RWY SIDE AND STILL BE LEGAL?