On November 4, 1996, at 0823 hours mountain standard time, the pilot of a McDonnell Douglas MD-600 (NOTAR) experimental helicopter, N630N, experienced a tail boom separation during a landing at Flagstaff, Arizona. Visual meteorological conditions existed at the time and no flight plan was filed for the local test flight. The aircraft was substantially damaged and the pilot was not injured. The aircraft is owned and operated by McDonnell Douglas Helicopter in Mesa, Arizona, and was being operated under an experimental certificate. The six-bladed main rotor helicopter can seat six to seven persons and is equipped with a 600-shaft horsepower engine. This was a recertification flight test and the pilot was performing a series of height velocity landings. The helicopter was being flown at 4,100 pounds maximum gross weight and was being monitored by onboard instrumentation and telemetry. The pilot indicated this was the 10th landing in the test profile. With a target data entry point of 60 knots indicated airspeed and an altitude of 15 feet, the aircraft touched down at 30 knots. During the 3.5 second and 200-foot slide on the skids, he felt the aircraft shudder, followed by a separation of the tail boom from contact with the main rotor blades. The winds were from 210 degrees at 2 knots and the runway was dry. The pilot reported no mechanical malfunctions or problems with the aircraft prior to the accident. A videotape of the accident sequence was taken by McDonnell Douglas (MD) ground personnel. The tape indicated a normal autorotative approach and touchdown. During the slide down the runway, the main rotor blades contacted the tail boom and severed it. The aircraft came to a full stop and the pilot exited the aircraft. Recorded engineering test data indicated the aircraft touched down at 1.5 g's with a 2.6 foot per second rate of descent. The position of the collective control during the landing and the ground slide was at near the 100 percent up position at touchdown and during the ground slide. In a discussion with MD test engineers, they described the main rotor blade contact with the tail boom as to have been a result of forward velocity and low/decaying main rotor rpm (advanced ratio) due to a full up collective position during the ground run out phase following the autorotative touchdown. In the condition of a high advance ratio, due to the low/decaying main rotor rpm and forward speed, a "blowback" of the main rotor disk occurs. They described this condition as the forward portion of the main rotor disk being displaced upward, while the rear portion of the disk displaces downward. This "blowback" condition is compounded by the high angle pitch setting which causes blade stall over a large portion of the rotor disk. This resulted in an excessive "blowback" that quickly allows tail boom contact by the main rotor blades. In a further discussion with the MD engineers, they explained that this "blowback" condition exists in all helicopters, but is more apparent in this model due a greater gross weight, reduced flare/deceleration capabilities because of tail boom length and installation angle, and the increased surface of the additional main rotor blade resulting in a more rapid decay of main rotor rpm. McDonnell Douglas Helicopter Company has claimed an exemption from public disclosure of the engineering test data and the video associated with this accident as proprietary and confidential information.