A LEARJET 25B ON LNDG ROLLOUT EXITED THE RWY TO THE L AND STOPPED 800 FT BEFORE THE END OF THE RWY. CAUSED BY ANTISKID SYS FAILURE AND POOR PREFLT PLANNING.

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AT ABOUT XA00 ON BASE TO FINAL; I HAD COMPLETED THE FINAL CHKLIST. CALLED OUT VREF ON FINAL WHICH WAS COMPUTED BY THE CAPT PF. VREF 122 KIAS. SHORT FINAL; I CALLED OUT VREF +8 KTS (130 KIAS). NO ACTION TAKEN BY CAPT. TOUCHDOWN WITH FAIRLY HIGH SPD. SPOILERS AND THRUST REVERSERS EMPLOYED AS SOON AS MAIN GEAR ON GND. AT ABOUT 80 KIAS; I WAS READY TO CALL OUT 80 KTS BUT THE AIRPLANE STARTED DRIFTING TO THE L. I TRIED TO COUNTERACT AND USED R RUDDER AND BRAKE WITH NO EFFECT. NO ACTIONS TAKEN BY CAPT AT THIS POINT OR COMMENTS. NO COM. THEN WE WENT OFF THE RWY AT ABOUT 800 FT BEFORE THE END TO THE L; WHERE WE SKIDDED ONTO THE GRASS AND TOOK 1 RWY LIGHT OUT. AIRPLANE CAME TO A STOP IN A SIDEWAYS POS. CAPT REMAINED SILENT. THEN HE SHUT DOWN BOTH ENGS AND REALIZED THAT THE ANTISKID LIGHT WAS ON; WHICH INDICATED A FAILURE OF THE SYS. NOBODY HURT; NO DAMAGE TO ACFT. FAA SHOWED UP AND INTERVIEWED BOTH ME AND THE CAPT. CONCLUSION: WT AND BAL PERFORMED BY CAPT WAS WRONG; USED NOT ACTUAL WTS OF PAX (6) -- HE GUESSED. CONCLUSION: LNDG DISTANCE WAS TOO SHORT FOR OUR WT AND FAR PART 135. NO ACTIONS TAKEN YET THROUGH FAA PERSONNEL. THIS WAS MY FIRST FLT WITH THIS CAPT AND FIRST PART 135 FLT SINCE TRAINING IN THE LEARJET. CAPT STATED THAT ALL PLANNING INCLUDING WT AND BAL; FUEL; LNDG AND TKOF DATA; WAS COMPUTED PRIOR TO FLT; WHICH TURNED OUT TO BE WRONG. HE STARTED DOING THIS ENRTE ON THIS FLT TO MY SURPRISE.