On June 9, 2022, about 1045 mountain daylight time, an experimental, amateur-built Rans Designs S-21 airplane, N46JH, was substantially damaged when it was involved in an accident near Fairfield, Utah. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot who also performed the maintenance on the airplane reported that the accident flight was a test flight following recent maintenance, during which a turbocharger air duct elbow, new fuel pumps, and fuel filters were installed. An engine run-up indicated that all systems, including fuel pressure, were normal. During the takeoff, about 150 ft above ground level, the engine lost power. The pilot maneuvered the airplane and landed hard on airport property during the forced landing. The left landing gear strut collapsed and the left elevator and forward fuselage sustained substantial damage. Postaccident examination of the airframe and engine revealed that there was no restricted bypass line between the inlet (pressure side of the fuel pump module) and the return line and no vent line from the header tank. The inlet b-nut fitting to the fuel filter assembly was finger loose. Fuel dripped from the fitting during the initial turns of the fitting. A 40-micron in-line low flow fuel filter was attached to the high-pressure side of the fuel system after the fuel pump. The filter was removed and disassembled. Small amounts of contaminants, consistent with the O-ring sealant, was noted in the filter cavity. Airflow was restricted when blown air was applied to the powdered sintered material filter. Borescope examination of the engine cylinders revealed damage consistent with detonation. No other damage or anomalies were noted during the borescope examination. Data downloaded from onboard avionics revealed a fuel flow that was not consistent with the fuel pressure during throttle movement. Per the Rotax Installation Manual, the purpose of the bypass line is to allow venting of the fuel system after a potential inclusion of air. A restricted bypass line may be installed between the inlet (pressure side of the fuel pump module) and the return line. In this bypass line, a restricted jet should be installed to achieve an ideal balance between short venting time and minimum fuel flow rate. The positioning and dimensioning of the jet are up to the aircraft manufacturer. If no bypass line is installed, the pressure release in the fuel lines is very slow. This should be noted accordingly in the manuals of the aircraft manufacturer (fuel lines are possibly under full operating pressure even after engine stop).