RPTR LOSES CTL OF ACFT ON LNDG.

Narrative

MY APCH TO TWIN FALLS WAS UNEVENTFUL. VISIBILITY WAS GOOD; I HAD BEEN AIRBORNE FOR 2 HRS WITH MORE THAN 2 HRS OF FUEL REMAINING; AND THE NEW ENGS WERE RUNNING SMOOTHLY. 30 MI N; I CONTACTED TWIN FALLS APCH ON 126.7. I WAS GIVEN AN ALTIMETER SETTING; WAS ADVISED THAT WINDS WERE FROM THE S AT 10-12 KTS; GUSTING TO 20 KTS; AND WAS TOLD TO EXPECT A L BASE TO RWY 7. I CONTACTED TWIN FALLS TWR ON 118.2; 10 MI N AND AGAIN 2 MI OUT ON MY L BASE LEG; WHEN I WAS CLRED TO LAND. NEVER BEFORE HAS LNDG MY CESSNA 340A IN A XWIND PRESENTED A PROB. XWINDS AND TAILWINDS ARE THE NORM AT SUN VALLEY; WHERE MY PRACTICE HAS BEEN TO HOLD RELATIVELY HIGH PWR SETTINGS UNTIL JUST BEFORE TOUCHDOWN. THIS; HOWEVER; TENDS TO WEAR OUT BRAKES. (IN ADDITION TO THE VORTEX GENERATORS; I HAD HVY DUTY CLEVELAND BRAKES INSTALLED ON MY ACFT WHILE IT WAS IN REDMOND WAITING FOR NEW ENGS.) MY PURPOSE FOR INSTALLING VORTEX GENERATORS WAS TO ENABLE ME TO SLOW THE PLANE DOWN A LITTLE EARLIER ON ITS FINAL APCH; TO EASE IT DOWN MORE GENTLY WITHOUT LOSS OF CTL. I HAD THIS IN MIND WHEN I MADE MY L TURN FROM BASE TO FINAL FOR RWY 7 AT TWIN FALLS. THE TURN WAS FAIRLY TIGHT; BUT NOT UNUSUALLY SO. AS I CAME UP ON THE NUMBERS; MY AIRSPD HAD DECREASED TO ABOUT 100 KTS. WHEN I HAD NEARLY COMPLETED MY TURN; I REALIZED THAT I NEEDED MORE PWR TO HOLD THE NOSE OF THE ACFT ON THE RWY CTRLINE. I WAS CRABBING AGAINST THE XWIND. THEN THE FIRST OF 2 WIND GUSTS FROM THE S HIT ME AND DROVE THE ACFT OFF OVER THE WEED FIELD ON THE N SIDE OF THE RWY. AS I APPLIED MORE PWR TO GET BACK ON COURSE; A SECOND BLAST DROVE ME EVEN FARTHER OFF THE RWY. RECOVERY FROM THIS GUST INCREASED THE ANGLE OF MY CRAB TO WHAT SEEMED LIKE ABOUT 45 DEGS. WHATEVER THE ANGLE; IT WAS ENOUGH TO RISK A SERIOUS GND LOOP WHEN I TOUCHED THE PAVEMENT. IT WAS THEN THAT I DECIDED TO GAR. I ANNOUNCED THIS INTENTION TO THE TWR; WHO TOLD ME TO DO SO WITH TIGHT R TURNS. AT THIS POINT I WAS STILL ABOUT 100 FT AGL; WITH MY GEAR AND LNDG LIGHTS DOWN; AND MY FLAPS EXTENDED TO30 DEGS. I APPLIED FULL PWR; RAISED THE GEAR AND FLAPS; AND EXPECTED THE PLANE TO RESPOND. BUT I WAS ALREADY BEHIND THE PWR CURVE; AND THE PLANE CONTINUED TO SINK. WHILE THIS WAS HAPPENING; THE TWR ASKED ME IF I WANTED RWY 12. BY THEN I REALIZED THAT IT WAS TOO LATE; AND I WAS GOING DOWN. I RECALL NO RESPONSE TO THE TWR; ONLY HEARING THE STALL WARNING AND DOING MY BEST TO HOLD THE NOSE OF THE PLANE UP AS IT BELLIED INTO THE GND. THE BOTTOM OF THE ACFT HIT; SLID; AND CAME TO REST ON A VERY SOGGY FIELD. HAD THE LNDG GEAR STILL BEEN DOWN; IT DOUBTLESS WOULD HAVE DUG IN AND CAUSED THE PLANE TO CARTWHEEL WITH DISASTROUS RESULTS. AS IT WAS; I WAS ALONG IN THE ACFT; EFFECTIVELY RESTRAINED BY MY SEAT BELT; AND EXCEPT FOR MY PRIDE; UNHURT. FORTUNATELY AND UNFORTUNATELY; AVIATION IS A CONSTANT LEARNING EXPERIENCE. IN THIS CASE; THE VORTEX GENERATORS THAT I THOUGHT WOULD IMPROVE LNDG PERFORMANCE ACTUALLY CONTRIBUTED TO THE PROB. IN RELYING ON THEM TO MAINTAIN CTL AT LOWER SPDS; I WAS CAUGHT BY THE XWIND COMPONENT THAT REQUIRED MORE RATHER THAN LESS AVAILABLE APCH PWR. IT IS A LESSON I WILL NOT SOON FORGET. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THESE ADD ON AFTER MARKET MODIFICATIONS ARE GREAT -- SOMETIMES! IN THIS CASE THE CONVERSION VORTEX GENERATOR ALLOWS THE ACFT TO FLY SLOWER THAN THE DESIGNER INTENDED AND CARRY 300 MORE LBS THAN THE DESIGNER INTENDED (AND IN THE CASE OF THE $175000 TURBO SUPERCHARGED PRESSURIZED CESSNA 340 THAT 300 LBS AMOUNTS TO ABOUT A 50 PERCENT INCREASE IN CABIN LOAD WITH FULL FUEL) THE DOWN SIDE IS THE ACFT IS FLYING OUTSIDE OF THE DESIGNER'S PERFORMANCE ENVELOPE. THIS MAKES THE UNWITTING NEW OWNER OF THIS PERFORMANCE PACKAGE IN THE ABSENCE OF SPECIAL TRAINING; A TEST PLT! HE IS NOT AWARE OF IT BUT HE IS PIONEERING THE DEEPEST RECESSES OF ENERGY MGMNT; AND CTR OF GRAVITY CHARACTERISTICS. WITH THESE NEW DEVICES HE WAS LULLED INTO DEVIATING FROM THE SPDS HE NORMALLY USED ON WORSE DAYS THAN THIS IN THE OVER 500 HRS OF FLYING HE HADBEFORE INSTALLING THIS PERFORMANCE PACKAGE. FORTUNATELY; THERE WAS NO LOSS OF LIFE OR LIMB!