RWY EXCURSION.

Narrative

THE FLT ORIGINATED IN DAL; TX; WITH DEST OF PLK; MO. 2 PAX WERE ON BOARD. THE ACFT WAS A KING AIR 200. FLT FROM DAL-PLK WAS NORMAL THROUGHOUT THE FLT AND DSCNT INTO PLK. AS WE APCHED THE RWY I STARTED SLOWING AIRSPD AND CROSSED END OF RWY AT APPROX 100 KTS AND ON NORMAL FLT PATH. THROTTLES WERE PULLED TO IDLE; ROUND OUT ACCOMPLISHED AND TOUCHDOWN ACCOMPLISHED. AT TOUCHDOWN THE FO PLACED THE PROP LEVERS FULL FORWARD AND I IMMEDIATELY WENT INTO REVERSE WITH THE THROTTLES AND STARTED BRAKING. DURING MY LNDG ROLLOUT; I REALIZED MY RWY REMAINING WAS BECOMING CRITICAL; THUS I WENT TO FULL REVERSE AND MAX BRAKING. WITH THE MAX STOPPING EFFORT; IT APPEARED THE ACFT WAS GOING TO STOP ON THE RWY; BUT IT ROLLED UP TO THE END OF THE RWY AND SLOWLY OFF THE RWY AND DOWN AN EMBANKMENT AND CAME TO REST JUST PRIOR TO THE BOTTOM OF THE SLOPE OF THE EMBANKMENT. THE 2 PAX EVACED OUT THE REAR EXIT (ENTRY DOOR) FOLLOWED BY THE FO; AND AS SOON AS I TURNED OFF THE NECESSARY SWITCHES; I ALSO EVACED OUT THE REAR EXIT. DURING MY RECONSTRUCTION OF THIS INCIDENT I ESTIMATED MY TOUCHDOWN POINT AT APPROX 1500-1800 FT DOWN THE RWY. SPRINGFIELD APCH CTL HAD GIVEN US SURFACE WINDS OF 170 DEGS AT 6 KTS AND WX AS 3500 FT BROKEN. NO GUSTS OR WIND CHANGES HAD BEEN RPTED; HOWEVER; A QUALIFIED WX INDIVIDUAL WAS STANDING IN A HANGAR WITH OPEN DOORS ON EACH SIDE; AND APPROX 500 FT FROM MY TOUCHDOWN POINT. HE STATED TO ME THAT AS WE WERE LNDG THEY EXPERIENCED A GUST OF WIND THROUGH THAT HANGAR THAT WAS VERY FORCEFUL AND HE ESTIMATED AT 20-25 KTS. HE STATED THAT GUST WOULD HAVE BEEN BLOWING DIRECTLY AS A TAILWIND TO ME.