AIR TAXI CREW LANDS ON RWY THAT WAS DETERMINED LATER TO BE TOO SHORT FOR THEIR OP.

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DURING THE ARR; WE RECEIVED THE ATIS AND DISCUSSED LNDG OPTIONS. AT THE TIME OF ARR; THE WINDS WERE RPTED TO BE 290 DEGS AT 20 KTS WITH 10 MI VISIBILITY. ALSO AT THE TIME OF ARR; THE NOTAMS RPTED SEVERAL TXWY CLOSURES AND THAT THE ARPT WAS RUNNING AN EMER DRILL THAT AFTERNOON AT XA00. AT THIS TIME; THE FO AND I DISCUSSED OUR OPTIONS FOR LNDG AT HPN. I ASKED THE FO FOR THE RWY INFO (IE; LENGTH AND WIDTH). WE CHKED THE PERFORMANCE CHART FOR OUR LNDG WT AND FOUND THAT WE MET THE PART 135 LNDG REQUIREMENT FOR A 4451 FT RWY. AT THIS TIME; WE REQUESTED IF RWY 29 WERE AVAILABLE AND WE WERE CLRED FOR THE VISUAL TO RWY 29. THE APCH; LNDG AND ROLLOUT WERE NORMAL. AFTER THE TRIP WAS COMPLETED IT WAS BROUGHT TO OUR ATTN THAT RWY 29 AT HPN HAS A DISPLACED THRESHOLD AND THAT THE LNDG DISTANCE AVAILABLE IS 3154 FT THE FACTORED LNDG DISTANCE REQUIRED FOR OUR LNDG WT WAS 3890 FT WITH A 450 FT REDUCTION FOR THE 20 KT HEADWIND REQUIRING US TO HAVE 3440 FT AVAILABLE. I HAVE REVIEWED THE ARPT DIAGRAM AT HPN AND CAN SEE HOW THE DISPLACED THRESHOLD CAN BE MISSED DUE TO ITS CLOSE PROX TO THE RWY INTXN. I THINK THAT THE DISPLACED THRESHOLD MARKER ON THE PICTORIAL DIAGRAM SHOULD BE MORE WELL DEFINED. THE RWY LENGTH SHOULD REPRESENT THE TOTAL DISTANCE AND THE DISTANCE FROM THE DISPLACED THRESHOLD THIS WOULD ALLOW PLTS TO MORE EASILY INTERP THE ARPT DIAGRAMS. SUPPLEMENTAL INFO FROM ACN 450549: FO FAILED TO REALIZE AND DID NOT RELAY TO CAPT THAT RWY 29 AT HPN HAD A DISPLACED THRESHOLD AND ONLY 3154 FT AVAILABLE. THIS WAS INADEQUATE FOR PROPOSED OPS. A MORE SUITABLE RWY SHOULD HAVE BEEN SELECTED. FACTORS AFFECTING FO'S JUDGEMENT; DECISION MAKING; INCLUDING PREOCCUPATION WITH NUMEROUS VFR TFC; INABILITY TO CONFIRM GND TRANSPORTATION WITH FBO FOR PAX; PUTTING PAX CONVENIENCE OVER OPS OF ACFT ON APCH AND LNDG. IN THE FUTURE; MORE PRECISE INTERP OF APCH.