Incident Overview
The pilot was attempting to climb after dropping fire retardant. Advancing the throttle produced no engine response. The aircraft began losing altitude, prompting the pilot to execute a forced landing to an open plateau.
Forced Landing
During the landing, the throttle could not be retarded fully. To land safely, the pilot turned the fuel selector to the off position. As the aircraft rolled out, the left wing impacted a fence pole, bending ribs and wrinkling the aileron.
Post-Accident Examination
Examination of the aircraft revealed that the bolt normally connecting the throttle linkage clevis to the throttle arm was missing. Investigators also inspected a sister aircraft from the same provider. On that aircraft, a nylon lock nut had been installed on the clevis bolt and was loose enough to be removed without tools.
Industry Standards
The investigation noted that FAA Advisory Circular AC43.13-1B, Chapter 7, Section 4, Paragraph 7-64 states that nylon locking nuts should not be used with rotating components or assemblies. It further states that nylon and fiber nuts are highly susceptible to wear from repetitive use and high heat applications.