AN AGRICULTURAL HELI PLT MADE AN UNINTENTIONAL LNDG AND DAMAGED HIS ACFT. THERE WAS A FIRE.

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WAS SPRAYING CANADIAN THISTLE (SPOT APPLICATION). LAST LOAD; SECOND PASS. 1/2 FUEL. INITIATED STANDARD AGRICULTURAL TURN TO MAKE SECOND APPLICATION ON TARGET. ENCOUNTERED A GUST OF WIND SHEAR. EXPERIENCED LOSS OF ALT AND THE START OF SETTLING WITH PWR SCENARIO. PULLED MORE PITCH AND MAINTAINED NOSE DOWN ATTITUDE TO TRY TO GAIN AIRSPD AND MOVE INTO CLEAN; UNDISTURBED AIR. ROTOR RPM AND MAN PRESSURE LOW. TURBO STARTED TO BUILD ABOUT THE TIME THE GND CAME TO CRITICAL PROX. ROTATED ACFT AND HIT DUMP GATES. PULLED PITCH. HIT GND (RISING TERRAIN) IN LEVEL ATTITUDE. LNDG GEAR COLLAPSED WITH IMPACT AND FORWARD SPD APPROX 15-20 MPH. ROTOR STRUCK ACFT. CAME TO REST 90 DEGS TO DIRECTION OF FLT. I EXITED ACFT. FUEL WAS LEAKING FROM FUEL TANK'S CROSS FLOW TUBE ONTO TURBO. FIRE STARTED. USED 2 1/2 LBS HALON FIRE EXTINGUISHER WITH LITTLE EFFECT. UTILIZED WATER TRUCK TO MINIMIZE FIRE. PUT OUT FIRE. REMOVED HELI FROM FIELD. THE PROB OCCURRED DURING FLT IN GRAY AREA OF HT/VELOCITY CHART AT A POINT OF MOST VULNERABILITY (THE TOP OF AGRICULTURAL TURN). AT THIS POINT; LOW PWR SETTINGS; '0' AIRSPD AND SUDDEN BURST OF TAILWIND INITIATED THE SETTLING. DSCNT RATE WAS GREATER THAN NORMAL AND CTL BUFFETING WERE THE INDICATIONS. INITIATED RECOVERY. COULDN'T DROP PWR AND COLLECTING DUE TO HT ABOVE GND. TRIED TO GET TO CLEAN AIR BY POCKETING TAILWIND AND PULLING PWR/PITCH. WAS NOT EFFECTIVE AT THE ELEVATION I WAS AT. THE RECOGNITION OF THE PROB WAS NOT THE TROUBLE AS IT WAS IMMEDIATE. THE DEG OF SERIOUSNESS WAS NOT AS PERCEIVABLE AND A QUICKER INITIATION OF RECOVERY COULD HAVE BEEN MADE. ALSO; A PICKLING OF THE LOAD COULD HAVE BEEN MADE AT A SOONER POINT IN TIME. HUMAN FACTORS WERE GOOD. THE SPRAY JOB WAS THE FIRST THAT DAY. STRESS WAS LOW; MORE THAN ENOUGH SLEEP. TIME PRESSURE WASN'T A PROB. ATTITUDE WAS GOOD AS WELL AS FLYING TECHNIQUE. AMOUNT FLYING AND SPRAYING EXPERIENCE -- 2000 HRS AS TURN WAS STANDARD. BUT PROB COULD HAVE BEEN PREVENTED BY UTILIZING ANOTHER STYLE OF TURN. THERE WAS NO WARNING ON THE GUST OF WIND. ALWAYS EXPECT THE UNEXPECTED.