On June 26, 1995, about 1020 hours Pacific daylight time, a Hughes 369D helicopter, N1089N, was substantially damaged during an emergency landing in mountainous terrain near Highland, California. The pilot and two passengers received minor injuries; a third passenger received serious injuries. Visual meteorological conditions prevailed for the 14 CFR Part 135 local survey flight. A company VFR flight plan had been filed. The flight originated at Rialto, California, at 0920 hours on the morning of the accident. The helicopter was being operated with the two left doors and the right rear door removed. The pilot stated that he observed something in his left skid mounted mirror for a fraction of a second just prior to experiencing a loss of tail rotor control. Postaccident investigation revealed that the tail rotor and half of the tail rotor gearbox were missing. Numerous ground and air searches have failed to find the rotor system components and a passenger's missing jacket. During the investigation, an abundance of 12-inch-wide nylon reinforced plastic survey marker tape was found at the accident site. PERSONNEL INFORMATION The 12,000-hour commercial instructor pilot stated that he had briefed the passengers prior to flight regarding operations with doors removed and the security of loose items. The pilot stated that they were in cruise flight heading out of the mountains and had started a gentle descent when two things happened almost simultaneously. First, "I saw something move in the field of vision of my skid-mounted convex mirror". Less than a second later, a strong "whump" was felt and heard by everyone onboard, and the helicopter yawed to the right. WRECKAGE AND IMPACT INFORMATION The loss of the tail rotor control occurred over mountainous terrain. The pilot was able to maneuver around some power lines and control the spinning descent onto a ridgeline with approximately a 10-degree down slope. After impact, the helicopter rolled downhill about 100 to 150 feet. The tailboom with the vertical and horizontal stabilizers still attached was found at the initial point of impact. The tail rotor blades, the gearbox output shaft, and half of the gearbox were missing. Subsequent searches failed to find the missing items, nor a purple sweat jacket belonging to the left rear seat passenger. ADDITIONAL INFORMATION A metallurgical analysis was performed on the remaining gearbox casting. According to the results of the analysis, a high strain rate crack growth had occurred, similar to a one-time overload.