Mooney M20C N6422U accident near Elma, Washington

Probable cause

A total loss of engine power during cruise flight due to the fatigue failure of the number 4 cylinder connecting rod.

NTSB analysis

The airplane was returning to its home airport following an aerial observation mission. A witness reported that the airplane was making a clattering noise as it flew over her home and commented that it sounded as if it was running out of oil. The witness estimated the airplane to be 100-150 feet above ground level. A second witness reported that the airplane descended into the tree canopy and collided with power transmission lines. Post accident examination of the engine revealed extensive thermal and impact related damage. The main bearings exhibited erosion and copper was visible. A large hole was observed in the engine crankcase above the number four cylinder bay and mechanical damage was observed to the area surrounding the cylinder. The number four connecting rod remained attached to the piston; however, the connecting rod was separated from the crankshaft at the base of the yoke. Fragments of the associated connecting rod cap and corresponding bolt were found within the engine oil sump; only residual oil (less than one quart) was observed within the sump along with debris, which was consistent with main bearing material and had obscured the oil pickup screen. Two fractures were found on the connecting rod near the end of the rod. The first noted was a fatigue fracture that initiated at the crankshaft end connecting rod journal surface just below the boss for the connecting rod bolt/nut. On the opposite side of the connecting rod, in a similar location, a second fracture was identified. The fracture was the result of mechanical overload and initiated at the connecting rod journal surface. The connecting rod bolt fracture surface exhibit features consistent with mechanical overload. The lack of engine oil found in the crankcase is believed to have contributed to the main bearing erosion; however, it was not determined if the bearing erosion was the initiating event.

Text from the NTSB. Open the NTSB docket for WPR09LA362 for the full record.