On April 14, 1997, approximately 2100 central daylight time, a Hughes OH-6A helicopter, N6638P, registered to and operated by the U.S. Border Patrol as a public use aircraft, was substantially damaged during a practice night touchdown autorotation in Marfa, Texas. Neither the certificated designated examiner pilot nor the commercial pilot-in-command receiving instruction were injured. Visual meteorological conditions prevailed for the night check ride conducted under Title 14 CFR Part 91. A flight plan was not filed for the local flight that originated at Marfa Municipal Airport, Marfa, Texas at 2030. The instructor pilot, a FAA designated flight examiner, was giving a "semi-annual helicopter checkride" to the commercial pilot. According to the instructor, the flight was planned for a "before dark departure... for a more gradual dark adaptation for planned PM full touchdown autorotations." The commercial pilot performed "several hovering autorotations and a Run-On landing before complete sunset." The commercial pilot's first practice autorotation was initiated from approximately 700 feet AGL, during dusk light conditions, and all indications were "normal." At 65 feet, he initiated a flare and "held the flare to an approximate altitude of 10 feet, where he began the initial pitch pull and began to level the aircraft." The aircraft touched down prior to reaching a "full level attitude and prior to applying full up collective." The helicopter "made initial contact on the rear portion of the skids causing the main rotor blades to flex down and sever the tailboom" approximately 2 1/2 feet forward of the tail rotor gearbox.