On July 9, 2023, about 0947 central daylight time, a Robinson R-44 II helicopter, N163RK, was substantially damaged when it was involved in an accident near Marble Falls, Texas. The pilot and two passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight. The pilot reported that he was conducting a power pole inspection flight along with two non-pilot crewmembers. During the third day of the mission and while about 250 ft above ground level (agl) and about 50 knots indicated airspeed, the engine lost total power. The pilot entered an autorotation and turned right to an area suitable for a forced landing. During the descent, he saw a set of power distribution lines that crossed his path. He increased the collective control to clear the power lines and after he passed over the power lines, he felt a slight shudder. The helicopter touched down on slightly sloped terrain. During the touchdown, the helicopter rocked forward and aft. The lower vertical stabilizer contacted the ground, resulting in substantial damage. The pilot and crew exited unassisted. Examination of the helicopter revealed that both fuel tanks and the gascolator contained fuel along with a reddish-brown fluid that tested positive for water. According to a mechanic who performed postaccident maintenance on the helicopter, both fuel cells were coated on the inside with the red substance, and the electric fuel pump was clogged with the red substance. He stated that the red substance could only be dissolved with a methyl ethyl ketone (MEK) equivalent cleaner. The fuel samples were shipped to the NTSB Materials Laboratory for further examination. Further testing of the contaminated fuel samples revealed that the sample was consistent with a polyamide; however, a spectral library search did not find any strong matches to a single known substance. The pilot reported that he obtained fuel from several different airports as well as his own personal supply during the three-day mission; postaccident testing revealed no contamination in any of the fuel delivery systems.