PLT OF AN SMT TWIN INADVERTENTLY TAXIED OFF THE SIDE OF THE RAMP TXWY AREA CAUSING DAMAGE TO THE ACFT'S NOSEWHEEL AND PROPS.

Narrative

AFTER COMPLETING PREFLT AND COCKPIT CHKS; I STARTED BOTH ENGS; STARTED THE HEATER AND ENGAGED THE DEFROSTER. I THEN RECEIVED DEP CLRNC AND TAXI INSTRUCTIONS. BEFORE MOVING; I VISUALLY IDENTED THE DESIRED TXWY ENTRANCE AND ENSURED THAT I HAD CLR VISIBILITY IN ALL DIRECTIONS. I THEN PROCEEDED TO TAXI TOWARD THE DESIRED TXWY ENTRANCE; USING TAXI/LNDG LIGHTS. AS I NEARED THE TXWY ENTRANCE (A DISTANCE I PERCEIVED TO BE APPROX 50 FT) I NOTICED THAT MY FORWARD VISIBILITY WAS DECREASING. I DECIDED NOT TO CONTINUE TO THE TXWY AND CHKED THE RAMP AREA TO MY L BEFORE EXECUTING A 30 DEG L TURN. THIS MANEUVER WOULD ENSURE THAT I DID NOT BLOCK THE RAMP AREA IN WHICH ANY OTHER ACFT MIGHT ENTER OR EXIT THE TXWY. MY NEXT INTENTION WAS TO EXECUTE A SHORT RADIUS L TURN AND HALT THE ACFT SO THAT IT WOULD BE FACING THE PARKING AREA I HAD JUST DEPARTED; WHICH WAS WELL ILLUMINATED BY FLOODLIGHTS. ONCE IN THIS POS I PLANNED TO ADVISE THE TWR (WHICH WAS ALSO PERFORMING AS GND CTL) OF MY PROB AND INTENTIONS. I WANTED TO RECHK THE DEFROST AND HEATER CTL KNOBS; WHICH ARE LOCATED IN POS WHICH WOULD HAVE MADE THEM UNSAFE TO CHK WHILE TAXIING. AT THE TIME I DECIDED TO EXECUTE THIS 180 DEG L TURN; I WAS APCHING THE EDGE OF THE RAMP AT A PERPENDICULAR ANGLE AND AT A PERCEIVED DISTANCE OF 35 FT. THIS PERCEIVED DISTANCE IS WHY I PLANNED A SHORT RADIUS TURN WHICH WOULD USE LESS AREA. I THEN BEGAN THE PLANNED L TURN USING SLIGHT L BRAKE PRESSURE AND DIFFERENTIAL THROTTLE CTL. AT THIS POINT I ENCOUNTERED A SLIGHT 'BUMP' SIMILAR TO THAT EXPERIENCED WHEN TAXIING OVER A METAL DRAINAGE GRATE FOUND IN MANY RAMPS. STILL CONVINCED OF MY POS; I CONTINUED MY TURN. AT THE 90 DEG POINT OF THE TURN; IT BECAME APPARENT THAT I WAS ACTUALLY ON GRASS AND THE SLIGHT BUMP WAS MY NOSEWHEEL PASSING ACROSS THE RAMP EDGE. IN THE 2-3 SECONDS IT TOOK TO COMPLETE THE TURN I DECIDED NOT TO TRY STOPPING. SLAMMING ON THE BRAKES IN A TIGHT TURN ON GRASS WAS MORE LIKELY TO DAMAGE THE NOSE GEAR THAN XING BACK ONTO THE RAMP AT THE END OF MY TURN; WITH THE NOSEWHEEL STRAIGHT. THIS DECISION WAS BASED ON THE GENTLE BUMP I HAD EXPERIENCED GOING OFF THE RAMP. WHEN THE TURN WAS COMPLETE; I EXPERIENCED A SHARP JOLT AND SNAPPING SOUND AS I CROSSED ON THE RAMP. I CLOSED THE MIXTURE CTL KNOBS AS THE NOSE DROPPED AND THE PROPS CONTACTED THE RAMP. I THEN SHUT OFF ALL ELECTRICAL SWITCHES AND FUEL VALVES. USING A WHITE MAP LIGHT; I THEN INSPECTED THE INSIDE OF THE WINDSHIELD AND DISCOVERED THAT IT HAD FOGGED UP WHILE I WAS TAXIING. THIS FOGGING BEGAN TO CLR AS SOON AS I OPENED THE CABIN DOOR. AFTER EXITING THE ACFT; I INSPECTED THE ACFT DAMAGE AND THE RAMP EDGE. IT WAS THEN THAT I NOTICED A LAYER OF LIGHT FROST ON THE GRASS. THIS FROST REFLECTED THE LIGHT FROM THE RAMP AREA AND CAUSED THE GRASS TO LOOK VERY MUCH LIKE THE CONCRETE RAMP. USING THE FBO TELEPHONE; I CALLED THE CTL TWR TO INFORM THEM OF THE INCIDENT. A REPRESENTATIVE FROM THE ARPT OPS ARRIVED 10 MINS LATER; TOOK MY STATEMENT; AND PHOTOGRAPHED THE ACFT. THE ILLUMINATION FROM THE RAMP FLOODLIGHTS HAD BEEN DECREASING AS I TAXIED; WHILE THE FOGGING OF MY WINDSHIELD WAS INCREASING. THE LACK OF CONTRAST BTWN THE RAMP AND THE FROST-COVERED GRASS CREATED THE ILLUSION THAT THE RAMP EXTENDED FURTHER THAN IT DID IN REALITY. WHAT I HAD PERCEIVED TO BE THE EDGE OF THE RAMP WAS ACTUALLY THE BEGINNING OF A DOWNSLOPE IN THE GRASS THAT LED TO A CANAL BTWN THE RAMP AND THE PARALLEL TXWY. REFLECTIVE PAINT ALONG THE EDGE OF THE RAMP WOULD BREAK THE CONTRAST BTWN THE RAMP AND THE GRASS.