Wsk Pzl Mielec M-18A N92640 accident near Anderson, Indiana

Probable cause

The fatigue failure of the propeller governor speeder spring, which resulted in the pilot’s inability to control engine output power.

NTSB analysis

The pilot was marking a field in preparation for an aerial application when he experienced what he thought was an engine failure. He stated that the engine was still running, but it was not producing power. The pilot pushed the power lever forward; the engine power did not increase, but the engine temperature increased. He stated that the engine was idling fast and that he forced the airplane into a bean field about 150 to 200 yards ahead of a tree line. The pilot ground looped the airplane before contacting the trees. A postaccident examination of the engine did not reveal any anomalies. A bench test of the propeller governor revealed that the maximum speed was above the nominal limit and the minimum speed was below the nominal limit. The control lever position was set higher than the specified position, which indicated that the governor had been adjusted in an attempt to regulate the propeller speed. X-rays and a teardown revealed that the propeller governor speeder spring was fractured. A metallurgical examination revealed a preexisting fatigue crack that, over time, resulted in the failure of the spring. The failed speeder spring would have resulted in a loss of oil pressure to the propeller piston, which allowed the propeller blades to rotate to a high blade angle, which then decreased the engine speed and overall power output of the engine. Maintenance records showed the propeller governor was overhauled on May 19, 2009. The speeder springs did not have serial numbers, and, therefore, the maintenance history of the speeder spring could not be determined.

Text from the NTSB. Open the NTSB docket for CEN11LA512 for the full record.