On May 16, 1996, approximately 1500 central daylight time, a McDonnell Douglas MD-520(NOTAR) helicopter, N520DD, owned by U.T.E. Management Corporation, and operated by the Federal Aviation Administration (FAA) as a Title 14 CFR Part 91 public use training flight, sustained substantial damage while executing shut down procedures at Wiley Post Airport, Bethany, Oklahoma. Visual meteorological conditions prevailed and no flight plan was filed. The airline transport rated pilot-in-command and the pilot receiving the training were not injured. Both occupants were FAA Aviation Safety Inspectors. The flight was conducted locally and had just landed for final shut down and parking. At 1400 the helicopter departed to conduct an evaluation/currency flight for an FAA Aviation Safety Inspector and to familiarize him with the NOTAR helicopter. The pilot-in-command (instructor) was an FAA Aviation Safety Inspector qualified in the helicopter. Subsequent to one hour of uneventful flight, the aircraft landed at the east ramp of the airport. During the approach to landing, the Wiley Post tower reported (to the pilots) winds from 180 degrees at 19 knots, gusting to 22 knots. After the landing, the instructor lifted the helicopter into a hover and demonstrated an autorotation from a hover with no difficulty. Upon completion of the maneuver, the instructor hover taxied to near the original parking spot from which the flight commenced. The nose was oriented to 360 degrees and the helicopter was landed in front of a hangar building (tail facing toward the building). The instructor stated that he selected the landing spot in front of the building because he thought it would block much of the wind. Prior to shut down the instructor demonstrated how to center the rotor disc utilizing outside reference points. With the helicopter stable on the ground, the flight controls were frictioned down and normal shut down procedures were started. Passing through 19% rotor RPM, the rotor brake was applied. To demonstrate that the rotor would continue turning without rotor brake application, the instructor released the brake at 10% RPM and allowed the rotors to turn down to 9% RPM. The rotor brake was then re-applied, and while the rotor was decelerating, "two or three thumps were heard", and the instructor observed the rotors "jerk and come to a stop." The instructor stated that wind being redirected around the building may have caused the rotors to flap down into the tail boom. According to the manufacturer's flight manual, the shut down sequence and rotor brake application were conducted in accordance with current published procedures. Examination of the aircraft by a FAA inspector revealed that at least two of the three main rotors had struck the tail boom, resulting in structural damage to the rotors. No mechanical anomalies or pre-existing defects were found in the flight control system or rotor system (including the rotor brake system).