Mooney M20J N201HH accident near Phoenix, Arizona

Probable cause

A total loss of engine power due to oil starvation for reasons that could not be determined due to a lack of available evidence, which resulted in impact with terrain during a subsequent forced landing.

NTSB analysis

The pilot, who was also a longtime owner of the accident airplane, recently had the propeller and propeller governor overhauled by a service facility. During a subsequent attempted maintenance flight with his mechanic, they aborted the takeoff after having observed that the takeoff engine power was slightly lower than normal. After troubleshooting the discrepancy, and completing unreported adjustments to the governor, they returned to the runway to attempt another maintenance takeoff. During the initial stage of the takeoff roll, when an improvement in the engine power output was observed, the pilot elected to continue the takeoff. Subsequent to becoming airborne and after a short, uneventful flight, they decided to return to their departure airport, during which they observed a small engine overspeed. A few minutes later, they experienced catastrophic engine failure and performed a forced landing to rough terrain.

The engine was recently rebuilt by the engine manufacturer and accumulated only 170 flight hours prior to the accident. Engine monitoring data retrieved from the accident flight showed that a continuous rise in oil temperature began shortly before cylinder no. 4 stopped functioning, and the first indication of the engine failure. The other cylinders failed about 40 seconds later. This data also showed that cylinder no. 1 may have failed with cylinder no. 4, but this could not be corroborated by physical evidence. The engine examination revealed thermal and mechanical damage at the cylinder no. 3 and 4 connecting rod caps and bolts. This damage was consistent with high temperatures and an absence of lubrication during rotation.

A materials laboratory examination found that the cylinder no. 4 connecting rod cap had fractured from a fatigue crack that had initiated on the inner bearing surface of the part. The crack initiated at multiple sites along multiple parallel locations, which developed into parallel cracks. Once the largest fatigue crack had progressed through enough of the cap cross section, the remainder of the part fractured from overstress. Composition analysis of the fatigued area found an excessive, uniform layer of brominated tetraethyllead, consistent with improper engine conditions such as oil starvation in this case. While a lack of lubrication due to a clogged oil hole is most likely to have led to the conditions observed, the damage to the oil holes on the connecting rod could not be determined due to plastic deformation damage on the journals; thus, the origin of the oil starvation is unknown.

Examination of the propeller and propeller governor did not reveal any anomalies that would have contributed to the total loss of engine power. An improper gasket had been installed on the face of the propeller governor, which lacked an inlet port filter. Although the gasket may have formed a seal between the governor and adapter, the absence of the filter likely allowed metallic fragments from the engine to enter the governor, which may have affected its operability.

Text from the NTSB. Open the NTSB docket for WPR20LA144 for the full record.