ACR SMT LANDED ON RWY BEFORE WDB ACR BECAME AIRBORNE RESULTING IN LESS THAN STANDARD SEPARATION.

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THE LAX TWR CTLR CLRD WDB Y FOR DEP ON RWY 25L WHEN WE WERE CLRD TO LAND AND LESS THAN A MI TO THE THRESHOLD. AT THE TIME OF HIS CLRNC; WDB Y WAS PERPENDICULAR TO THE RWY ON THE ADJOINING TXWY. HE WAS NOT COMPLETELY IN POSITION; POWER UP; AND ROLLING UNTIL WE PASSED THE HALF MI TO TOUCHDOWN MARK. AS WE CROSSED THE APCH LIGHTS AT APPROX 150' AGL THE CTLR ASKED IF WE PREFERRED 25R INSTEAD; OBVIOUSLY TOO LATE. THE F/O RESPONDED 'NEGATIVE'. THE F/O THEN SUGGESTED A GO AROUND TO THE CAPT BY STATING 'READY FOR GO AROUND'. CAPT CHOSE TO CONTINUE THE APCH AND LANDED PRIOR TO WDB Y'S ROTATION POINT BUT WE WERE STILL ROCKED CONSIDERABLY IN THE #2 ENGINE'S JET WASH. A FIRM LNDG RESULTED. THIS OCCURRENCE COULD HAVE BEEN DISASTROUS AND COULD HAVE BEEN PREVENTED BY ANY ONE OF THE FOLLOWING: CTLR HOLDING ON CLEARING A LOADED WDB FOR DEP WITH ANOTHER ACFT ON SHORT FINAL LESS THAN 30 SECONDS TO TOUCHDOWN ESPECIALLY SINCE THE ACFT LNDG WAS MUCH SMALLER AND LIGHTER THAN THE WDB. THE CREW OF THE WDB SEEING THE LNDG ACFT (LIGHTS ON) ON SHORT FINAL AND HEARING IT CALLED SHORT FINAL SHOULD HAVE REFUSED THE TKOF CLRNC. OR THE CAPT OF THE LNDG SMT ACFT MAKING A GO AROUND AS THE F/O SUGGESTED.