WDB ACFT ALMOST CONTACTS TAIL OF 747 STOPPED BTWN PARALLEL RWYS.

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WE WERE FLYING ACR A FLT FROM LAX TO IND. IT WAS DAY VMC. WE WERE HOLDING SHORT OF RWY 25L AT LAX. AN ACR B 747 (I DON'T REMEMBER THE FLT NUMBER) LANDED ON RWY 25L AND WAS TOLD TO TURN R OFF THE RWY HOLD SHORT OF RWY 25R AND REMAIN ON TWR FREQ. THE ACR B TURNED R 2/3 TO 3/4 OF THE WAY DOWN THE RWY. A COMMUTER METRO LANDED SHORTLY THEREAFTER AND TURNED OFF MIDFIELD OR SOONER. WE WERE CLRED FOR TKOF. THE TWR CTLR WAS VERY BUSY; TFC WAS HVY! WE WERE CLRED FOR TKOF. WE WERE LIGHT AND ACCELERATED RAPIDLY; SO RAPIDLY IN FACT WE MISSED OUR 80 KT AIRSPD CALL AND XCHKED OUR SPD AT 100 KTS. WE GET THE PWR SET; OUR AIRSPD XCHKED AND THE ENGINEER AND I BOTH REALIZED THAT THE TAIL OF THE 747 WAS TOO CLOSE TO OUR RWY. HOW MUCH TOO CLOSE? NOT SURE; BUT I BELIEVE OVER THE HOLD SHORT LINE BY A PRETTY GOOD BIT. BY NOW IT'S V1; WE ROTATE AND TKOF. NOTHING IS SAID; OTHER THAN A QUICK COMMENT PRIOR TO V1. WHEN WE TOOK THE RWY; THE 747 DIDN'T LOOK TOO CLOSE TO THE RWY. WHEN WE REALIZED HOW CLOSE WE WERE; IT WAS TOO LATE TO ABORT. HAD WE BEEN HEAVIER AND HAD A LONGER TKOF ROLL; OR HAD WE ABORTED WE WOULD HAVE EITHER HIT THE 747 TAIL OR AT LEAST BEEN VERY CLOSE TO IT. I HAVE HAD SIMILAR INCIDENTS AT MEM AND ORD; NONE THIS CLOSE BUT CASES WHERE THE TAIL POS OF AN ACFT WAS IN QUESTION. PLTS KNOW WHERE THEIR NOSE IS BUT NOT THEIR TAIL. TWR NEEDS TO CONFIRM COMPLETE ACFT CLRNC EITHER VISUALLY OR WITH THE PLTS BEFORE GIVING TKOF OR LNDG CLRNCS. LAX BETTER BE CAREFUL. I WILL BE TOO!