Air Tractor Inc AT-400 N3659N accident near Nelson, Nebraska

Probable cause

Failure of the fuel control unit's spool cap bearing for reasons that could not be determined during extensive postaccident examination, and the pilot’s decision to shut the engine down without sufficient altitude and distance to reach the runway.

NTSB analysis

When the pilot was unable to reduce power normally for landing, he used the fuel cut-off lever to shut down the agricultural airplane's turbine engine. However, the pilot stated that he misjudged the airplane’s altitude and distance to the runway when he shut off fuel to the engine, and the airplane landed short of the runway. During postaccident examination, the fuel control unit failed the manufacturer's functional check. The unit was then disassembled, and the spool cap bearing and its associated hardware, including the cap containing the bearing outer race and the post with the bearing inner race, were found damaged. Metallurgical examination of the bearing races found embedded alumina particles. A historical review of the manufacturer's fuel control unit revealed a history of premature spool cap bearing failures in fuel control units installed on other turboprop engines, but not on the accident airplane’s make and model engine. These failures were attributed to high diamond content and other hard particle contamination and could result in fuel flow and power/torque fluctuations. However, no direct link could be established between the accident-related failure and previous failures in other turboprop engines. Although hard alumina particles were found, there was insufficient historical data to support that these particles had attributed to previous bearing failures in this engine. The cause of the spool cap bearing failure could not be determined.

Text from the NTSB. Open the NTSB docket for CEN11LA566 for the full record.