RWY EXCURSION AFTER AN APCH AND LNDG AT MINIMUMS WHEN A G-2 FLT CREW DRIFT OFF THE R SIDE OF RWY 23 ON THE ROLLOUT.

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I WAS R SEAT IN THE G2 AND WE SHOT THE ILS APCH TO RWY 23 AT MMU. THE APCH WAS UNEVENTFUL. WE BROKE OUT JUST ABOVE MINIMUMS AND LANDED ON THE CTR OF THE RWY. WE STARTED TO DRIFT R OF CTRLINE AND I TOLD THE CAPT; YOUR DRIFTING R. PLANE KEPT GOING R AND I YELLED 'YOU'RE DRIFTING.' AT THAT POINT THE R MAIN AND NOSE WERE IN THE GRASS AND MUD. I YELLED AT THE CAPT TO GET THE ACFT BACK ON THE RWY AND I HAD FULL L RUDDER AND BRAKING TO HELP ASSIST IN GETTING ACFT BACK ON RWY. NO DAMAGE TO THE ACFT OR THE ARPT PROPERTY. SUPPLEMENTAL INFO FROM ACN 599265: THIS WAS A PART 91 FLT WITH THE OWNERS OF THE ACFT ON BOARD. THE WX AT THE ARPT WAS BEING RPTED AT MINIMUMS AS PUBLISHED ON THE APCH CHARTS. THE APCH WAS NORMAL AND AFTER SEEING THE RWY LIGHTS AT MINIMUMS WE CONTINUED TO THE SURFACE AND MADE A NORMAL TOUCHDOWN. AFTER TOUCHDOWN THE ACFT STARTED TO DRIFT TO THE R AND I MADE WHAT I THOUGHT TO BE A NECESSARY CORRECTION WITH THE RUDDER TO THE L. THE ACFT CONTINUED TO DIVERT TO THE R AND DRIFTED OFF THE HARD SURFACE INTO THE GRASS WITH THE R MAIN WHEEL. CONTINUING TO HOLD L RUDDER; THE ACFT CAME BACK ONTO THE HARD SURFACE AND A NORMAL DECELERATION WAS MADE TO TAXI SPD AND THE ACFT RESPONDED NORMAL TO ALL BRAKE; RUDDER; THRUST REVERSER AND TILLER INPUTS. AFTER INSPECTING THE ACFT; NO DAMAGE WAS FOUND. AFTER NOTIFYING THE MORRISTOWN TWR AND OPS; THEY FOUND NO DAMAGE TO THE RWY LIGHTS OR SIGNAGE. THE OPS PEOPLE AT MORRISTOWN SAID BEING THAT THERE WAS NO DAMAGE TO EITHER THE ACFT OR THE ARPT PROPERTY; THEY WERE GOING TO ELECT TO NOT FILE A RPT ON THE SUBJECT.