PVT PLT IN A BEECH TURBOPROP LOSES CTL OF ACFT ON LNDG. RWY EXCURSION AS ACFT GOES OFF SIDE OF RWY; STRIKING PROPS ON A DIRT EMBANKMENT. ACFT HAD A PREVIOUS PROB WITH BRAKES AND OVERSPD CTL OF BOTH PROPS SO PIC DID NOT USE HVY BRAKING OR REVERSE.

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ACFT TOUCHED DOWN 110 FT FROM RWY NUMBER AT APPROX 100 KTS AIRSPD. HEADWIND; RWY DIRECTION; 17 KTS GUSTY 25 KTS. TOUCHDOWN WAS SMOOTH; UNIFORM AND UNEVENTFUL. INITIAL ROLL WAS SMOOTH. I PULLED THE PWR LEVERS INTO BETA AND BEGAN TO APPLY LIGHT BRAKING ACTION. THE AIRPLANE BEGAN TO YAW AGGRESSIVELY AND I COULD NOT CORRECT IT. THE AIRPLANE ROLLED OFF L SIDE OF RWY THROUGH GRASS AND THE PROPS STRUCK A DIRT EMBANKMENT. THE PROB: APPEARS TO BE BRAKES WHICH WERE SERIOUSLY DRAGGING; GRABBING; AND NOT RELEASING. TIRE TRACKS INDICATE WERE NOT LOCKED UP BUT WERE EXERTING CONSIDERABLE UNCOMMANDED BRAKING ACTION. IN JULY 1996; AT A 150 HR INSPECTION IT WAS DISCOVERED THAT THE BRAKE PADS SHOWED EXTREMELY MORE WEAR THAN THEY SHOULD HAVE. THE ACFT JUST HAD THE L PROP; L OVERSPD GOVERNOR; AND R PRIMARY GOVERNOR OVERHAULED. UPON TEST FLT WE FOUND PROBS WITH THE L PROP ON ENG. REPAIR SHOP CHKED RIGGING; PROP AND GOVERNOR SETTINGS; ETC; ALL SEEMED CORRECT. DISCUSSIONS WITH BEECH AND OVERHAUL SHOP YIELDED NOTHING NEW. ULTIMATELY THE NEW OVERSPD GOVERNOR WAS REMOVED AND RETURNED TO OVERHAUL SHOP. AT THE OVERHAUL FACILITY FOUND THE GOVERNOR TO BE FAULTY. ANOTHER OVERSPD GOVERNOR WAS INSTALLED AND PROBS IMPROVED BUT WEREN'T FULLY ELIMINATED. PROP ANGLES NEEDED TO BE CHANGED AND ENGS RE-RIGGED TO MAKE NORMAL. THIS WAS COMPLETED JUST BEFORE THE FEB/THU/97 FLT. ON FEB/THU/97; THE APCH AND TOUCHDOWN WERE EXCELLENT AND ALLOWED A DECISION TO BE MADE WHICH MAY HAVE ADVERSELY AFFECTED THE OUTCOME. THE ACFT WAS IN VERY GOOD SHAPE FOR A SMOOTH GRADUAL SPD REDUCTION AND DID NOT REQUIRE ANY AGGRESSIVE BRAKING OR PROP REVERSAL. I RECALL THINKING NO NEED TO RISK A POSSIBLE PROP REVERSE MALFUNCTION BY PULLING INTO FULL REVERSE SO I ONLY WENT INTO BETA. I ALSO APPLIED ONLY LIGHT BRAKE PRESSURE FOR ESSENTIALLY THE SAME REASON -- NO NEED TO RISK SQUIRRELY BRAKES -- WE HAVE LOTS OF TIME AND RWY TO STOP ACFT AND WE'RE ALREADY SLOW. (HINDSIGHT NOW TELLS ME I HADN'T DEVELOPED FULL CONFIDENCE IN EITHER SYS.) WHEN THE ACFT BEGAN TO YAW SEVERELY I DIDN'T KNOW WHETHER IT WAS CAUSED BY THE BRAKES OR THE PROPS AND I THEREFORE DIDN'T FEEL SAFE IN DOING ANYTHING MORE AGGRESSIVE WITH EITHER. THE BRAKES HAD DISPLAYED A RECURRENCE OF A PROB WHICH WAS DEEMED AT LEAST 4 TIMES TO BE CORRECTED BUT THE UNDERLYING ACTUAL CAUSE WAS NEVER FOUND OR DETERMINED -- THEY JUST SEEMED TO BE WORKING OK AFTER SOMETHING WAS DONE TO THEM. I SHOULD HAVE FOUND THE ROOT CAUSE BUT I SHOULD HAVE BEEN UNWILLING TO ACCEPT THEM AND FLY THE ACFT WITHOUT FINDING THAT CAUSE. IN THE FUTURE I WILL REFUSE.