On April 29, 2016, about 1535 Pacific daylight time, a Piper PA-28-200, N200KR, completed a precautionary landing at the San Carlos Airport (SQL), San Carlos, California following a power control failure. The certified flight instructor (CFI) and the pilot receiving instruction were not injured, and the airplane was not damaged. Visual meteorological conditions prevailed for the instructional flight, which was being conducted in accordance with 14 Code of Federal Regulations Part 91, and a flight plan was not filed. According to the CFI, the intended destination was Byron, California.In a telephone interview with the National Transportation Safety Board (NTSB) investigator-in-charge, the CFI reported that he attempted to reduce power during the initial climb following departure from runway 30 at SQL, but he was only able to make a slight reduction in power. After he determined that no further power reduction was possible, the pilot then flew a right-hand traffic pattern back to runway 30, and on final approach over the runway threshold he fully retarded the mixture control to stop the engine. After landing, the airplane completed its landing roll with sufficient momentum to exit the runway at an intersecting taxiway. A subsequent inspection of the throttle control revealed that the cable had broken. The cable was subsequently recovered by a Federal Aviation Administration aviation safety inspector for further examination by the NTSB. The throttle cable was sent to the National Transportation Safety Board's Materials Laboratory in Washington, D.C., for examination and analysis. Subsequent to the examination, a Senior NTSB Materials Engineer reported the following: The cable was fractured near the input end. Damage to the conduit revealed two areas of wear contact. The smallest wear contact was located in the lower area and had a flat wear pattern, which corresponded to contact with a smooth surface. The larger of the two areas had a ridged pattern, which indicated that the area made contact with another cable. Individual cable wires were fractured at locations within approximately one-half of a lay length. Portions of the fracture surface on each of the outer wires had relatively smooth fracture features oriented perpendicular to the outer surface with curving boundaries, which are features consistent with fatigue. The fatigue origin for each individual wire was located at the same side relative to the overall cable, and the fatigue region extended across approximately half the diameter in each wire. The core wire fracture features appeared rubbed. The wall of the sleeve was deformed into an oblong shape. Additionally, the overall features of the swivel joint were consistent with bending deformation. A curving deformation mark was observed on one side of the swivel tube, which corresponded to contact with the wall of the sleeve under bending loads on the joint. The axis of the conduit was angled (bent) relative to the axis of the sleeve. Adjacent to the sleeve, the blue outer sheath of the conduit was depressed inward at the outside of the bend. The depression in the outer sheath of the conduit was also visible in a radiograph of the part. Another radiograph of the conduit and sleeve revealed that the sleeve was crimped onto the conduit, and that the inner diameter of the coil and the spacing between coils varied along the length of the sleeve. As assembled, an elastomeric boot covers the swivel joints at each end of the cable assembly. As received, the boot at the input end swivel joint was displaced onto the swivel tube. In comparison, the boot at the output end was received covering the joint, and a hose clamp was in place on the boot. No hose clamp was observed at the input end. During the examination, the hose clamp was removed from the boot at the output end swivel joint, and the boot was displaced onto the output swivel tube. The portion of the boot that covered the swivel joint was flared outward relative to the other end of the boot, and an impression was observed where the hose clamp had been installed. More than two months after the hose clamp was removed, the hose-clamp impression remained visible on the surface, and a slight flare was present at the end that had been covering the swivel joint. In comparison, the boot for the input swivel joint had a uniform outer diameter with no evidence of a flared end or hose clamp impression. Additionally, the input swivel tube had a date stamp 6 days after the accident, which revealed similar features for the boot with no evidence of a hose-clamp impression or flared end. According to the cable manufacturer, the swivel joint boot is an important component to prevent contamination and dampen vibration loads on the cable assembly.