Accident Overview
A private pilot was conducting a personal cross-country flight when, during cruise at 6,000 ft mean sea level, a crew alerting system oil pressure message appeared, followed by a total loss of engine power. An air traffic controller provided vectors to a local airport, but the pilot reported the airplane would not reach the runway. The pilot did not attempt to restart the engine. He feathered the propeller, set the power lever to idle, and the condition lever to cut off. He then performed a forced landing to a sports field with the landing gear and flaps retracted. The airplane collided with trees and the ground before coming to rest upright.
Engine Examination
Examination of the engine revealed contact signatures on internal components and evidence of ingested unburned organic debris, consistent with the engine likely being unpowered and the gas generator and power sections windmilling at impact. No preimpact mechanical anomalies or malfunctions were found that would have precluded normal operation.
Flight Data Analysis
Recorded GPS flight track and systems data showed that the loss of engine power was preceded by about 5 minutes of flight on a constant heading and altitude with an excessive lateral g-force of about 0.17 g and a bank angle between about 8 and 10 degrees, consistent with a side-slip condition. The airplane then entered a right turn with the autopilot engaged, and lost power at the end of the turn. Data indicated that, even though the autopilot was engaged, the lateral g-forces increased as the airplane leveled off and accelerated, suggesting the automatic rudder trim feature of the yaw damper system was not engaged. The yaw damper system operated normally after the flight, so it is likely the pilot inadvertently and unknowingly disengaged the yaw damper during flight with the autopilot engaged.
Pilot Actions and Fuel System
During a postaccident interview, the pilot stated he was not aware of a side-slip condition before the loss of engine power. Although the fuel tank system was designed to prevent unporting of fuel lines during momentary uncoordinated flight, it was not intended for extended periods of uncoordinated flight. Therefore, the fuel tank feed line likely unported during the prolonged uncoordinated flight, resulting in the subsequent loss of engine power. If the pilot had recognized the side-slip condition, he could have returned to coordinated flight and prevented the engine power loss. Also, once the airplane returned to coordinated flight, an engine restart would have been possible.