Ayres Corporation S2R G6 N3300U accident near Tivoli, Texas

Probable cause

The failure of the fuel control unit due to the worn stub shaft spline, which resulted in an engine overspeed. Contributing to the accident was maintenance personnel's inadequate inspection of the fuel control unit.

NTSB analysis

The pilot reported that, during the accident flight, which was the twelfth aerial application flight of the day, the engine made an uncommanded power acceleration. The pilot retarded the throttle lever, but the engine did not respond. The pilot subsequently made a forced landing into a field, and the airplane nosed over and then came to rest inverted.

A postaccident examination of the airplane revealed no anomalies with the engine rigging. Examination of the fuel control unit revealed that the stub shaft spline was worn beyond the allowable limits. Such wear can allow the fuel control underspeed governor to be driven at a lower speed than the engine speed, which can increase fuel flow and engine speed and is consistent with the uncommanded power acceleration that occurred during the accident flight. A review of the airplane's maintenance records revealed that, 2 months before the accident flight, the fuel control unit was inspected for wear and damage in accordance with an airworthiness directive (AD). It is unlikely that the wear observed during the postaccident examination could have occurred between the time of the AD inspection and the accident. It is likely that the AD was either not accomplished as noted in the maintenance records or that maintenance personnel inadequately conducted the inspection.

Text from the NTSB. Open the NTSB docket for CEN15LA239 for the full record.