THE HELICOPTER PLT WAS ON A RECONNAISSANCE FLT TO SURVEY A HILLTOP LANDING SITE IN MOUNTAINOUS TERRAIN. WHEN HE ARRIVED, HE MADE A HIGH RECONNAISSANCE PASS, THEN BEGAN A LOW RECONNAISSANCE APCH. HE ESTIMATED THE WIND WAS FROM 210 DEG AT 10 TO 15 KTS WITH LIGHT TURBULENCE. HIS APCH WAS MADE ON A HEADING OF 200 DEG. HE BEGAN HIS APCH AT APRX 40 KTS & WITH A DESCENT RATE OF 200 FT/MIN. WHEN THE HELICOPTER WAS ABOUT 150 FT FROM THE HILLTOP & APRX 50 FT ABOVE IT, THE AIRSPEED DROPPED TO ZERO & THE RATE OF DESCENT INCREASED. THE PLT APPLIED FULL POWER & TURNED DOWN A VALLEY TO THE RIGHT, BUT COULD NOT STOP THE DESCENT. THE HELICOPTER THEN HIT A TREE, TOUCHED DOWN ON SLOPING TERRAIN & ROLLED OVER. THE ELEVATION & DENSITY ALT WERE ABOUT 9200 & 10,040 FT, RESPECTIVELY. THE HELICOPTER'S HOVER CEILING OUT OF GROUND EFFECT WAS ABOUT 8500 FT; IN GROUND EFFECT, THE HOVER CEILING WAS APRX 11,800 FT.