THE HELICOPTER COLLIDED WITH TREES AND THE GROUND IN AN ATTEMPT TO REVERSE DIRECTIONS AT LOW ALT & AT NIGHT AFTER ENCOUNTERING WEATHER. THE PLT HAD RECEIVED A WEATHER BRIEFING AND KNEW THE WEATHER WAS BAD WITH LOW CEILINGS AND FOG. HERADIOED HE COULD MAKE IT BY FOLLOWING THE HIGHWAY AT LOW ALT. THE FLT PURPOSE WAS TO TRANSPORT AN 890 LB VALVE. THE PLT WAS IN CONTACT WITH ELLINGTON TOWER AND A POLICE HELICOPTER DURING THE INITIAL PART OF THE FLT, DISCUSSING THE WEATHER AND HIS INTENTIONS. THERE WAS NO RADIO CONTACT AFTER THE HELICOPTER LEFT THE HOUSTON AREA. THE WRECKAGE WAS FOUND ABOUT 135 MILES FROM HOUSTON ORIENTED ALONG A HEADING OF ABOUT 075 DEGREES FOR A DISTANCE OF ABOUT 500 FT FROM THEINITIAL POINT OF CONTACT WITH ABOUT A 75 FT HIGH TREE. THERE WAS NO EVIDENCE OF FLT CONTROL OR ENGINE MALFUNCTION PRIOR TO IMPACT. THE CARGO VALVE WAS FOUND 30 FT FROM THE MAIN WRECKAGE. CABIN STRUCTURE SHOWED EVIDENCE THE VALVE WAS PROPELLED THROUGH THE RIGHT SIDE OF CABIN/DOOR/CUSHION AFTER IMPACT. NO TIE DOWN CARGO STRAPS WERE FOUND.