AN A319 HARD LANDING OCCURRED AFTER THE ACFT DROPPED DURING AN APPARENTLY NORMAL LNDG FLARE. APCH AND LNDG SPDS WERE OBSERVED.

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FIRST LNDG WITH THIS ACFT: I LANDED IN ZZZ1 USING MANAGED SPD AND AUTOTHRUST. I NOTED THAT THE ACFT SEEMED TO SUDDENLY 'PAY OFF EARLY' IN THE FLARE TO LNDG. I PULLED BACK A LITTLE MORE ON THE STICK TO AVERT A HARD LNDG; RESULTING IN A SLIGHTLY HIGHER THAN NORMAL TOUCHDOWN ATTITUDE AND A SLIGHTLY FIRMER THAN NORMAL TOUCHDOWN. I WOULDN'T HAVE PULLED BACK ANY MORE FOR FEAR OF A TAIL STRIKE. I REMARKED TO THE FO THAT THE HOT DAY AND THE LIGHT; BUT HIGHLY VARIABLE WINDS; OR PERHAPS THE ACFT LOADING; MUST HAVE CONTRIBUTED SOMEHOW TO THE EARLY PAYOFF. SECOND LNDG WITH THIS ACFT: THE FO'S LNDG IN ZZZ2. I RECOMMENDED TO HIM DURING THE APCH BRIEFING TO BE ALERT FOR AN 'EARLY PAYOFF;' JUST IN CASE. HE FLEW A TEXTBOOK APCH; PULLED THE PWR OFF FOR TOUCHDOWN AND THE ACFT JUST FELL OUT FROM UNDER US. I DON'T BELIEVE THAT THE TOUCHDOWN WAS QUITE HARD ENOUGH TO REQUIRE A MECHANICAL INSPECTION; BUT IT WAS PRETTY CLOSE. I CALLED MAINT CTL AND OUTLINED THE SITUATION; STATING THAT I WASN'T CERTAIN WHETHER IT WAS PLT TECHNIQUE OR SOME UNKNOWN PROB WITH THE ACFT. I DID STATE THAT IF IT APPEARED TO ME THAT THE ACFT WAS MISBEHAVING IN ANY WAY ON OUR NEXT APCH; THEN I WOULD ENTER SOME TYPE OF HARD WRITE-UP IN THE ACFT LOGBOOK USING OUR BEST OBSERVATIONS. THIRD LNDG WITH THIS ACFT: MY LNDG BACK IN ZZZ1. I NOTED THAT THE 'BOX' SPDS WERE: VAPP=131 AND VLS=126; WHICH SEEMED REASONABLE. I NOTED THAT THE PFD SPDS ON BOTH SIDES WERE: VAPP=131 AND VLS (TOP OF 'FISHHOOK')=129. I FLEW THE LNDG USING SELECTED SPD ABOUT 135 AND MANUAL THRUST. THIS LNDG APPEARED NORMAL. I ENTERED A HARD WRITE-UP IN THE ACFT LOGBOOK OUTLINING OUR OBSERVATIONS. FLT CREW IDENT/NARRATIVE: THE ACFT 'PAID OFF EARLY' TWICE IN A ROW WITH 2 DIFFERENT PLTS AT 2 DIFFERENT ARPTS CAUSED US TO DEVOTE AN EXTRA AMOUNT OF ATTN TO POSSIBLE REASONS. A POSSIBLE REASON WAS UNCOVERED; WITH ADDITIONAL TECHNICAL WORK NECESSARY TO VERIFY. CONSULTED WITH MAINT; OVERRODE AUTOMATION ON NEXT LNDG; AND ENTERED HARD WRITE-UP IN ACFT LOGBOOK. THERE APPEARS TO BE A TECHNICAL MALFUNCTION OF UNKNOWN ORIGIN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT SEVERAL DAYS AFTER THIS EVENT THE AIRCRAFT WAS STILL IN SERVICE BUT MAINTENANCE CONTINUED SEARCHING FOR POSSIBLE CAUSES. THE BEST THEORY ABOUT THE CAUSE WAS A COMPUTER ERROR GENERATED BY AN ANGLE OF ATTACK VANE RIGGING ERROR. THE AIRBUS LNDG SPD CALCULATIONS ARE PRODUCED IN TWO DIFFERENT COMPUTERS AND IN A NO WIND CONDITION A 5 KNOT DIFFERENCE IS CALCULATED BETWEEN THE APCH SPD AND THE LANDING SPD. IF THE ANGLE OF ATTACK VANE IS MIS-RIGGED; THE ACFT IS HEAVIER THAN DISPLAYED OR A COMPUTER ERROR IS MADE THE SAME SPD CALCULATION ERROR CAN OCCUR WITH THE END RESULT BEING THE 'BOTTOM DROPPING OUT' DURING THE LANDING FLARE.