The flight instructor reported that he and the pilot receiving instruction departed normally and climbed to an altitude of 1,000′ above ground level. The pilot completed the after-takeoff checklist and attempted to reduce engine power to a climb power setting, however the Piper PA28’s engine was unresponsive to throttle inputs.
The pilots returned to the airport in San Carlos, California, pulled the mixture control to the idle cut-off position when the airplane was over the runway threshold, and landed uneventfully.
Examination of the throttle control revealed that the cable had broken. Metallurgical examination showed that the cable was fractured due to fatigue near the input end. Fatigue regions on the wires covered about half the cross-section, consistent with a relatively high stress load on the cable.
An elastomeric boot covers the swivel joints at each end of the cable assembly, however examination revealed that the boot at the input end swivel joint had displaced onto the swivel tube. The boot at the input end lacked the same deformation as the boot at the output end, which indicated that the input end boot had not been installed on the swivel joint for some time before the event.
As this boot is used to dampen vibrational loads on the cable assembly, it is likely that the cable fractured from fatigue cracks that initiated and grew due to the excessive vibrations that resulted from the missing boot on the swivel joint.
Probable cause: The failure of maintenance personnel to properly install and secure the input swivel boot on the swivel joint, which resulted in excessive vibration that led to fatigue cracks and failure of the throttle cable.
NTSB Identification: WPR16IA100
This April 2016 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.