AN ACR B727 FLC RPTS AN ENCOUNTER WITH A B747'S JET BLAST ON SHORT FINAL APCH TO RWY 12 AT MIA FROM WHICH THEY MADE A GAR AFTER A FIRM LNDG. THE B747 WAS GOING INTO POS ON RWY 9L AND ITS EXHAUST BLEW DIRECTLY ACROSS THE APCH END OF RWY 12.

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ON FINAL TO VISUAL APCH TO RWY 12 AT MIA. CONDITIONS WERE CLR; DAYLIGHT; WIND 080 DEGS AT 10 KTS. ACFT WAS ON STABLE VISUAL APCH; ON GS AND AIRSPD OF 143 KIAS. APPROX 1 1/2 NM FROM RWY THRESHOLD WE OVERHEARD A B747 CLRED INTO POS FROM TXWY Q TO RWY 9L. ACFT WAS IN TKOF POS ON RWY 9L WHEN WE CROSSED THE THRESHOLD OF RWY 12 STILL ON SPD AND GS. AT APPROX 80 FT AGL WE FELT A SLIGHT BURBLE AND THE ACFT BEGAN TO FLOAT AND AIRSPD INCREASED 10 KTS. PWR WAS REDUCED SLIGHTLY AND THEN ACFT BEGAN TO SINK SUDDENLY WITH IMMEDIATE LOSS OF 15-20 KIAS. ACFT TOUCHED DOWN IN LNDG ATTITUDE; BUT AT HIGH ENOUGH SINK RATE TO BOUNCE ACFT BACK INTO THE AIR. THE THROTTLES HAD BEEN INCREASED JUST PRIOR TO TOUCHDOWN TO ARREST SINK RATE. AFTER APPROX 1000 FT OF FLOAT; CAPT CALLED 'VREF' (130 KIAS) AND ACFT ABRUPTLY SANK BACK TO RWY WITH A FIRMER THAN NORMAL TOUCHDOWN AND BOUNCED BACK INTO THE AIR. DUE TO THE UNSTABLENESS OF THE ACFT AND INCREASING OSCILLATIONS; A GAR WAS EXECUTED. A NORMAL GAR WAS ACCOMPLISHED TO A SUBSEQUENT NORMAL LNDG ON RWY 9R. SUSPECT THE JET BLAST FROM THE B747 AS IT TAXIED ONTO RWY 9L AND THE DIRECTION OF THE PREVAILING WIND CAUSED A 'MINI SHEAR' OVER THE LNDG AREA OF RWY 12. THIS PRACTICE OF CLRING HVY ACFT INTO POS IN THESE WIND CONDITIONS MAY NEED TO BE INVESTIGATED AT MIA TO AVOID POTENTIALLY HAZARDOUS CONDITIONS TO LNDG ACFT.