HS125-600 FLC FAILED TO STOP AND WENT OFF END OF RWY AFTER FAILING TO STOP ACFT AFTER LNDG AT THE 4000 FT MARKER DOWNWIND ON A WET RWY WITH NO THRUST REVERSERS.

Narrative

WHILE ACTING AS SIC ON HAWKER; THE CAPT BEGAN THE APCH ON ILS 5 AT GON ARPT; CT. THE WX WAS RPTED AS 1 1/2 MI VISIBILITY; 5000 FT RVR; 200 FT CEILING IN FOG. WIND 240 DEGS AT 8 KTS; WET RWY. THE CAPT WAS CLRED FOR THE APCH AND AT 400 FT I OBSERVED THE RWY LEAD-IN LIGHTS AND THE RWY ENVIRONMENT. I TOLD THE CAPT WHAT I SAW AND HE CONFIRMED HE HAD THE RWY AND LANDED. OUR REF SPD WAS 114 KIAS AND WE LANDED AT AROUND THE 3500-4000 FT MARK. AFTER LNDG; THE CAPT APPLIED BRAKES AND LIFT DUMP. HE THEN TOLD ME HE HAD POOR BRAKING ACTION AND ASKED FOR MY ASSISTANCE. WE BOTH APPLIED BRAKES; HOWEVER; THIS WAS INSUFFICIENT TO AVERT GOING OFF THE END OF THE RWY. WE CAME TO REST APPROX 90 YARDS OFF THE END; AND GOT OUT OF THE ACFT AND PINED THE GEAR. THERE WAS NO DAMAGE TO THE RWY NOR ITS LIGHTING. THERE WAS NO DAMAGE TO THE ACFT; HOWEVER; THERE IS A MAINT FACILITY WHICH IS CLEANING AND CARRYING OUT THE NECESSARY INSPECTIONS AND NO PAX WAS HURT NOR INJURED. I THINK THAT THE MAIN CONTRIBUTING FACTOR WAS THE POOR BRAKING FROM THE ACFT AND LNDG WITH A TAILWIND. THIS ACFT ALSO DOES NOT HAVE ANY THRUST REVERSERS AND IS COMPLETELY RELIANT ON BRAKES AND LIFT DUMP.