Incident Summary
On April 29, 2016, at about 1535 Pacific daylight time, a Piper PA-28-200 (registration N200KR) completed a precautionary landing at San Carlos Airport (SQL) in 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 conducted under 14 Code of Federal Regulations Part 91; no flight plan was filed. The CFI stated that the intended destination was Byron, California.
According to the CFI, during the initial climb after departure from runway 30, he attempted to reduce power but could only achieve a slight reduction. After determining that no further power reduction was possible, the pilot flew a right-hand traffic pattern back to runway 30. On final approach over the runway threshold, he fully retarded the mixture control to stop the engine. After landing, the airplane rolled out with sufficient momentum to exit the runway at an intersecting taxiway.
Throttle Cable Examination
A subsequent inspection of the throttle control revealed that the cable had broken. The cable was recovered by a Federal Aviation Administration aviation safety inspector and sent to the National Transportation Safety Board's Materials Laboratory in Washington, D.C., for examination and analysis.
The NTSB Materials Engineer reported the following findings: The cable was fractured near the input end. Damage to the conduit showed two areas of wear contact. One area had a flat wear pattern consistent with contact with a smooth surface; the other had a ridged pattern indicating contact with another cable. Individual cable wires were fractured within approximately one-half of a lay length. Fracture surfaces on outer wires exhibited relatively smooth features oriented perpendicular to the outer surface with curving boundaries, consistent with fatigue. The fatigue origin for each wire was on the same side relative to the overall cable, extending across about half the diameter in each wire. The core wire fracture features appeared rubbed.
The wall of the sleeve was deformed into an oblong shape. The swivel joint showed features consistent with bending deformation, including a curving deformation mark on one side of the swivel tube from contact with the sleeve wall. The conduit axis was bent relative to the sleeve axis. Adjacent to the sleeve, the blue outer sheath was depressed inward at the outside of the bend, visible also in a radiograph. Another radiograph showed the sleeve crimped onto the conduit, with variations in coil inner diameter and spacing along the sleeve.
Cable Assembly Conditions
As assembled, an elastomeric boot covers the swivel joints at each end of the cable assembly. The boot at the input end swivel joint was displaced onto the swivel tube; the boot at the output end was covering the joint and secured with a hose clamp, which was missing at the input end. During examination, the hose clamp was removed from the output end boot, revealing a flared end and a clamp impression that remained visible more than two months later. In contrast, the input end boot had a uniform outer diameter with no flared end or clamp impression. The input swivel tube had a date stamp 6 days after the accident, showing similar boot features without hose-clamp evidence. According to the cable manufacturer, the swivel joint boot is important to prevent contamination and dampen vibration loads on the cable assembly.