The weather when the non-instrument-rated helicopter pilot initiated the flight included a ceiling of 250 ft mean sea level (msl) and visibility of ½ mile, which corresponded to instrument meteorological conditions. After flying for 15 minutes, the helicopter “entered a wall of fog.” The pilot attempted to turn the helicopter around and return to an area of better visibility; however, the tail rotor impacted trees. The pilot did not realize that the helicopter was in a right turn and reduced engine power. The helicopter descended to ground in a bank, which resulted in substantial damage to the fuselage, tailboom, and rotor blades. The pilot reported no preimpact mechanical malfunctions or failures with the helicopter that would have precluded normal operation. According to the helicopter owner’s maintenance and flight manual, “instrument flight [is] prohibited.”