RWY EXCURSION WHEN A DA20 PVT PLT PERFORMS A DESTABILIZED APCH TO A 1400 FT GRASS STRIP ON A PVT ARPT AND GOES OFF THE END AFTER HYDROPLANING ON THE WET GRASS.

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PIC DEPARTED GSO; JUL/SAT/04; AT XA04 UTC FOR ZZZ1; VFR AT 3000 FT. MADE 2 TOUCH AND GOES ON RWY 6 AT ZZZ1 APPLYING SHORT-FIELD LNDG TECHNIQUE. DEPARTED THE PATTERN TO THE SW TOWARD A PVT LNDG FIELD APPROX 5 MI FROM ZZZ1. FLEW OVER THE FIELD IN A WESTERLY DIRECTION AT 1600 FT MSL CHKING FOR TFC AND RWY OBSTACLES. RWY WAS CLR OF TFC AND CLR OF OBSTACLES AND ACCORDINGLY; PIC ENTERED THE PATTERN FOR LNDG. NO OTHER ACFT WAS IN THE PATTERN. THE PVT FIELD TURF RWY IS APPROX 1400 FT LONG WITH 3000 FT OF OPEN FIELD PRECEDING THE THRESHOLD. ON SHORT FINAL; PIC OBSERVED THAT THE OPEN FIELD PRECEDING THE RWY WAS BEARING A CROP OF CORN; BUT FROM THE AIR WAS NOT ABLE TO DETERMINE THE HT OF THE CROP. AS PIC APCHED WITHIN 100 FT OF THE RWY THRESHOLD; THE ACFT EXPERIENCED AN UNEXPECTED SINK AND PIC APPLIED A BRIEF PULSE OF THROTTLE TO AVOID TOUCHING DOWN IN THE CORN CROP (WHICH WAS LATER DETERMINED TO BE APPROX 8 FT TALL). GLIDE PATH WAS REESTABLISHED; BUT WITH SOMEWHAT HIGHER AIRSPD THAN PLANNED. THE ACFT TOUCHED DOWN WITHIN THE FIRST 200 FT OF RWY AND THE PIC IMMEDIATELY APPLIED BRAKES FIRMLY. THE GRASS TURF; HOWEVER; APPARENTLY RETAINED MOISTURE FROM THE MORNING DEW AND THE MAIN GEAR WHEELS LOST TRACTION IMMEDIATELY. THE PIC ATTEMPTED PUMPING THE TOE BRAKES RAPIDLY; BUT WAS UNABLE TO REGAIN ADEQUATE TRACTION. THE ACFT CONTINUED IN A CTLED SLIDE STRAIGHT DOWN THE TURF RWY. WITHIN THE FINAL 75 FT OF RWY; THE PIC REALIZED THAT THE ACFT WAS NOT GOING TO STOP BEFORE RUNNING OUT OF RWY; AND APPLIED FULL R RUDDER IN AN ATTEMPT TO DIRECT THE ACFT ONTO A TURF TXWY TO THE R OF THE RWY. THE ACFT PARTIALLY MADE THE TURN; BUT PROCEEDED TO SLID SIDEWAYS; BEYOND THE R TXWY AND OFF THE END OF THE RWY. THE ACFT CAME TO REST IN A DITCH. PERFORMANCE CONSIDERATIONS: THE PIC; PRIOR TO DEP; REVIEWED THE ACFT FLT MANUAL FOR TKOF AND LNDG PERFORMANCE DATA; AND DETERMINED THAT SLIGHTLY LESS THAN 600 FT OF GND ROLL IS REQUIRED ON A DRY; PAVED SURFACE. THE AFM DOES NOT PROVIDE SPECIFIC DATA FOR SURFACES OTHER THAN DRY PAVEMENT ALTHOUGH IN A FOOTNOTE; IT DOES STATE THAT HIGH TEMP; RAIN; UNFAVORABLE WIND CONDITIONS; AND SLIPPERY RWY COULD INCREASE THE LNDG DISTANCE. A GAR WAS NOT AN OPTION POST-LNDG DUE TO ELECTRICAL PWR LINES AT THE END OF THE RWY. THE ACFT IN THIS INCIDENT ACTUALLY SLID MORE THAN 1000 FT BEFORE EXHAUSTING THE RWY. JUDGEMENT/CONSIDERATIONS: THE PRIMARY ERROR CONTRIBUTING TO THIS INCIDENT WAS THE PIC'S LACK OF KNOWLEDGE AS TO THE EFFECTS OF NON-PAVED SURFACES ON REQUIRED ROLL-OUT DISTANCES; AND TO HIS LACK OF AWARENESS OF THE POSSIBILITY OF RETAINED GND MOISTURE (DEW). DURING PVT PLT TRAINING WITH A CFI; OTHER LNDGS HAD BEEN MADE ON OTHER TURF RWYS WITH NO PROBS WITH TIRE TRACTION. HAD LOSS OF TRACTION BEEN PREVIOUSLY EXPERIENCED; OR HAD AN INSTRUCTOR CAUTIONED AS TO THAT POSSIBILITY WITH RESPECT TO TURF SURFACES; THE PIC WOULD NOT HAVE ATTEMPTED TO LAND AT THE LOCATION OF INCIDENT HAD HE BEEN AWARE OF THE ADDITIONAL RISKS.