INSTRUCTOR AND STUDENT PLTS ENCOUNTER ARPT MANAGER VEHICLE ON TURF RWY RESULTING IN A GAR. STATE THE VEHICLE WAS PURPOSELY PARKED THERE TO PREVENT THEIR USE OF THE RWY.

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I WAS GIVING MR X SOME INSTRUCTION IN SIAI MARCHETTI IN PREPARATION OF HIS PURCHASE OF THE ACFT. SINCE MR X IS PLANNING TO TAKE THE ACFT TO MT AND AK; I FELT IT WAS PRUDENT TO GIVE HIM SOME SHORT FIELD GRASS INSTRUCTION. WE WERE APCHING THE ARPT AND MADE THE APPROPRIATE ANNOUNCEMENTS THAT WE WERE GOING TO DO SOME TKOFS AND LNDGS ON RWY 16. WE OBSERVED NO ONE AT THE ARPT AND NO CARS IN THE PARKING LOT. WE SAW NO REASON WHY WE SHOULD NOT BE ABLE TO USE THE GRASS RWY. WE PROCEEDED TO MAKE AN UNEVENTFUL LNDG. AS WE WERE TAXIING BACK FOR TKOF; I OBSERVED THE ARPT MGR; MR Y; DRIVING DOWN THE TURNPIKE TOWARD THE ARPT. WHEN WE TURNED AROUND AND GOT INTO POS FOR TKOF; I SAW MR Y HAD PARKED HIS TRUCK ON THE ARPT RAMP. AS WE WERE CLBING OUT; I SAW MR Y STANDING BESIDE HIS TRUCK TAKING A PICTURE OF US. WE CONTINUED TO DEPART AND INTENDED TO COME AROUND FOR ANOTHER LNDG. WHEN WE CAME AROUND ON THE DOWNWIND; MR Y CONTACTED US ON THE UNICOM QUESTIONING THE CERTIFICATION STATUS OF THE AIRPLANE. MR X WAS TAKEN ABACK BY THE QUESTIONING AND SAID HE DIDN'T KNOW THE ANSWER. THEN MR Y DEMANDED TO KNOW WHO THE FLT INSTRUCTOR IN THE AIRPLANE WAS. WE DID NOT ANSWER. WE CONTINUED THE APCH AND WHEN WE WERE ON SHORT FINAL; MR Y DELIBERATELY DROVE HIS BLACK PICKUP TRUCK OUT ONTO THE RWY WE WERE ABOUT TO LAND ON AND PARKED THE TRUCK; CAUSING US TO MAKE A FULL-FLAP GAR. WE RECOGNIZED THIS AS A HOSTILE SITUATION AND WE DEPARTED THE AREA. MR Y INTENTIONALLY CREATED AN EXTREMELY HAZARDOUS SITUATION TO LIFE AND PROPERTY BY HIS RECKLESS ACTIONS.