On August 8, 1999, about 1600 central daylight time, a Bell 47G-3B, N6712D, registered to Precision Air Services, Inc., operating as a Title 14 CFR Part 91 maintenance test flight, crashed while attempting a landing onto a trailer near Selma, Alabama. Visual meteorological conditions prevailed and no flight plan was filed. The helicopter received substantial damage, the trailer-truck helipad was damaged, and the commercially-rated pilot and a passenger were not injured. The flight originated from the same location about 20 minutes before the accident. According to the pilot, during his landing attempt, the right skid became hooked on the trailer, and the helicopter sustained a dynamic rollover, damaging the main and tail rotor blades, the main rotor mast, and the tail boom. In his statement the pilot stated, "I screwed up." According to FAA personnel, postcrash pilot interview and examination of the damaged main rotor system and rotor collision damage to the trailer and truck revealed that during the landing phase onto the helipad, the right landing skid slid off the landing platform and the pilot's control correction, (full forward and left cyclic) led to a main rotor thrust vector similar to what would be experienced in a dynamic rollover. The main rotor pitch change control rods were sent to the NTSB Materials Laboratory for failure analysis. It was determined the components were part of a main rotor mast modification approved as Supplemental Type Certificate (STC) SH2772SW. The difference between the original and the replacement mast head controls is illustrated in Figure 2 of the enclosed report titled, "NTSB Materials Laboratory Factual Report". All separation and bending overstress damage to the components appeared consistent with impact damage. The center hexagons of three of the control rods were hardness tested using a Rockwell "A" scale indenter. The equivalent tensile strength specifications on the samples were found to be under that required by drawings of the control rods; however, it is not considered a factor in this accident. For additional information, see "NTSB Materials Laboratory Factual Report."