HISTORY OF FLIGHTOn December 17, 2017, about 1300 mountain standard time, an experimental amateur-built Rotorway Exec-90 series helicopter, N716JB, landed hard following a loss of engine power at Caldwell Industrial Airport, Caldwell, Idaho. The private pilot/builder was not injured, and the helicopter sustained substantial damage to the lower fuselage structure. The helicopter was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed, and no flight plan had been filed. The local flight departed Caldwell about 1230. The pilot stated that he planned to perform a series of hover-taxi and air-taxi maneuvers on taxiway A as practice, after a two-week period of bad weather. After taxiing around the airport for about 30 minutes, he heard an unusual sound and the engine began to lose power. He was initiating a turn at the time, and as the helicopter descended to 10 ft, the engine lost all power. He performed a forced landing, and the helicopter landed hard. Examination of the helicopter revealed that oil was leaking from below the engine compartment, and a trail of oil was present on the ground in the areas that the helicopter had been taxing. Following the accident, the pilot recovered the helicopter to his hangar and removed the body panels around the engine. He stated that the sump contained about 2 quarts of oil (capacity 5 quarts), and the engine appeared to have seized. An inspector from the Federal Aviation Administration (FAA) performed an external inspection and found that all the visible major oil lines were intact and tight at their fittings, and there did not appear to be any catastrophic breaches or damage to the engine crankcase. A few months after the accident the pilot began to disassemble the helicopter for repair, and discovered that the oil pressure gauge line, which was attached to the oil filter housing by a brass compression fitting, had failed and separated from the fitting. He stated that the line was made of nylon tubing, which he purchased from a local automobile parts store. He had installed the tubing about one year before the accident, after the original kit-supplied plastic line had degraded and was leaking oil into the cabin. The pilot discarded the oil line before it could be examined by the FAA or NTSB. According to FAA Advisory Circular AC 20-27G, Certification and Operation of Amateur-Built Aircraft, "Amateur builders are free to develop their own designs or build from existing designs. We do not approve these designs and it would be impractical to develop design standards for the wide variety of design configurations, created by designers, kit manufacturers, and amateur builders." Photo 1 – Helicopter at Accident Site Photo 2 – Oil on Ramp Previous Accidents The pilot was involved in three previous accidents in the accident helicopter, all involving a loss of power: On October 19, 2016, the helicopter lost power during the initial takeoff climb (accident number: WPR17LA009). The NTSB determined the probable cause to be, "The pilot's failure to properly tighten the tension bolts during the installation of the alternator belt, which resulted in inadequate voltage to sustain ignition and the subsequent partial loss of engine power." On September 5, 2002, the helicopter lost power shortly after takeoff (accident number: FTW02LA250). The NTSB determined the probable cause to be, "The pilot's failure to refuel the helicopter, which resulted in fuel exhaustion." On October 1, 1999, the helicopter lost power while in the traffic pattern (accident number: DEN00LA001). The NTSB determined the probable cause to be, "The pilot inadvertently allowing main rotor rpm to decay while avoiding obstacles during an emergency autorotation following a loss of power for reasons undetermined. Factors were houses and powerlines."