LGT ENCOUNTERS GUSTS AND WIND SHEAR ON LNDG. TAIL SKID STRIKE.

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ARRIVING ON THE EXPRESSWAY VISUAL RWY 31 APCH AT LGA THE WIND RPT WAS 290 DEGS AT 22 GUSTS 35 KTS. WE WERE ASKED TO EXTEND DOWNWIND FOR DEP SO WE DID. IT WAS BUMPY AND THERE WAS VIRGA IN THE AREA. WE TURNED FINAL AND DSNDED NORMALLY AT ABOUT VREF AND 15 KTS. AT ABOUT 50 FT THE ACFT STARTED TO SINK RAPIDLY AND THE CAPT ADDED PWR TO CORRECT. HE CONTINUED ADDING PWR - WITH NO CHANGE IN SINK RATE - TO ALMOST GAR THRUST; WHILE MAINTAINING A NORMAL YOKE POS. THE ACFT CONTINUED TO SINK UNTIL THE THRUST FINALLY BECAME EFFECTIVE AT ABOUT THE TIME THE ACFT TOUCHED DOWN ON THE RWY AT VREF. THE ACFT DID NOT BOUNCE NOR WAS THE LNDG HARD. THE CAPT RETARDED THE THROTTLES AND SLOWED THE ACFT. AN ACFT AT THE DEP END RPTED THAT WE HIT THE TAIL SKID. I SUSPECT THE COMBINATION OF THE WIND COMING OVER THE TERMINALS; THE GENERAL INSTABILITY OF THE ATMOSPHERE; AND THE THRUST FROM THE DEPARTING MD-80; ALL FLOWING OVER THE DIKE AT THE ARR END OF THE RWY CAUSED A LOSS OF LIFT FROM A WIND SHEAR AND A LOSS OF AIRSPD PUSHED THE ACFT BEHIND THE PWR CURVE.