PWRED EXPERIMENTAL ACFT TEST RUN ENDS UP OFF THE END OF THE RWY.

Narrative

THE DECISION WAS MADE TO MAKE A HIGH SPD TAXI RUN THE LENGTH OF RWY 26 AT FXE TO MEASURE THE MIXTURE CTL/RPM AVAILABLE AT TKOF PWR SETTINGS. THIS WAS DONE AT THE REQUEST OF AN ENG MECH/OVERHAULER DUE TO SOME ROUGH RUNNING ENG OPS EXPERIENCED OCCASIONALLY ON CLBOUTS. THE MECH RECOMMENDED THAT THE TEST BE DONE ON A COLD ENG SO WE ELECTED TO NOT DO A RUNUP. THE ASSISTANT I CHOSE (ANOTHER PLT) AND I BOTH FELT WE WOULD BE BETTER OFF DETERMINING THE NUMBERS QUICKLY ON THE RWY DURING A HIGH SPD TAXI AND STOPPING ON THE RWY OR SETTLING THE ACFT BACK DOWN ON THE RWY IN THE EVENT OF A LIFTOFF THINKING IT WOULD BE SAFER THAN CHANGING THE MIXTURE TO A 'ROUGH RUNNING POINT' IN THE AIR AT LESS THAN PATTERN ALTS. THE RUN WAS COMMENCED AND THE MEASUREMENTS WERE QUICKLY TAKEN. I PULLED BACK THE PWR AND SET THE ACFT BACK DOWN ON THE RWY. WE WERE TOO FAST AND TOO FAR DOWN THE RWY FOR THE SPD WE HAD OBTAINED. THE GO/NO-GO DECISION THAT I MADE REGARDING REMAINING RWY WAS WRONG. WE LEFT THE END OF THE RWY BTWN THE LIGHTS AT SLIGHTLY LESS THAN FLYING SPD (APPROX 40-50 MPH) AND SLOWED TO A STOP IN THE GRASS JUST BEYOND THE RWY. NO DAMAGE WAS DONE TO EITHER THE ACFT OR THE ARPT EQUIP. WE CONTINUED TO TAXI THE ACFT ACROSS THE GRASS TO THE HANGAR JUST TO THE S OF THE AREA WE WERE IN. I HAD FELT THAT I SHOULD NOT FLY SINCE WE DID NOT DO A RUNUP AND WERE TESTING A PROB AT THE VERY PWR SETTINGS WE WERE GOING TO OPERATE AT. EVEN THOUGH WE DID NOT DO THE RUNUP; WE SHOULD HAVE PLANNED FOR A GAR IN THE EVENT OF INSUFFICIENT RWY/POOR REACTION TIME/ETC. WE HAD ONLY SPOKEN OF STOPPING AND I WAS UNPREPARED FOR THE EVENT THAT DID OCCUR. THE ACFT IS AN EXPERIMENTAL AND I AM FULLY AWARE THAT MANY HIGH SPD TAXI TESTS END UP IN PROBS DUE TO THE ABOVE SCENARIO BUT I STILL 'WANTED' TO STAY ON THE RWY. BETTER PLANNING AND BRIEFING NEEDED. STUPID MISTAKE THAT WILL NOT BE MADE AGAIN.