On June 22, 2006, about 0130 central daylight time, a skid-equipped Messerschmitt-Bolkow-Blohm (MBB) BK 117A-3 helicopter, N913EF, sustained substantial damage after colliding with trees while landing at an off-airport site, about 10 miles north of Ponce de Leon, Florida. The helicopter was being operated as a visual flight rules (VFR) emergency medical services (EMS) positioning flight under Title 14, CFR Part 91, when the accident occurred. Of the three occupants aboard, the certificated commercial helicopter pilot and one medical crewmember were not injured. The remaining medical crewmember sustained minor injuries. The helicopter was operated by CJ Systems Aviation Group, of West Mifflin, Pennsylvania. The medical facility, Doctor's Memorial Hospital, based in Bonifay, Florida, was utilizing CJ Systems Aviation Group to provide EMS helicopter transport services. The helicopter was en route to a prearranged landing site to meet ambulance and fire department personnel caring for a patient. Dark night, visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight originated about 0110, from the Tri-County Airport, Bonifay. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on June 22, the pilot reported that he was transporting two flight paramedics to a rural community located in Holmes County to pickup a maternity patient. He said that pilots use a number of prearranged landing sites in the area, which have been previously inspected for obstruction clearance, as well as the overall site condition. According to the pilot, these prearranged landing site locations are stored in the helicopter's onboard GPS navigation system. The pilot said that during the outbound flight, fire department personnel contacted him via radio and requested that the landing site be changed to another prearranged landing site, a sod-covered open field. He agreed to the change, selected the new landing site from coordinates previously stored in the GPS, and went to the new site. He reported that after arriving overhead of what he thought was a sod-covered field, he used an externally mounted searchlight to aid in seeing the ground and obstructions during the landing approach. As the helicopter approached the open field, and transitioned from forward flight to a hover, the pilot said he encountered a substantial amount of blowing, dry sand and dust created by the helicopter's main rotor downwash, which momentarily reduced his visibility. He stated he lost all visual reference with the surface of the field, and while attempting to regain a visual reference, the helicopter's main rotor struck a large tree, and began to spin. The helicopter's tail boom assembly subsequently struck the tree, and the helicopter impacted terrain. The helicopter sustained substantial damage to the main rotor drive system, tail boom assembly, and fuselage. The pilot reported that there were no preaccident mechanical anomalies with the helicopter. A postaccident inspection of the landing site disclosed that the sod had recently been removed from the field, leaving a dry, sand-covered field.