Mooney M20A N6018X accident near Wikieup, Arizona

Probable cause

The manufacture and installation of a nonstandard part by unknown maintenance personnel to compensate for a bent, misaligned crankshaft propeller flange, which resulted in the improper clearance of the bearings on the crankshaft journal, a loss of oil pressure, overheating of the bearings, and the failure of a connecting rod during cruise flight.

NTSB analysis

According to air traffic control records, during the cross-country flight, the pilot reported that the engine had lost oil pressure. About 10 minutes later, he reported that the engine was running roughly and that there was smoke in the cockpit. He then stated that he was shutting down the engine. The pilot then made a forced landing in rough terrain. An examination of the wreckage site revealed black viscous fluid on the belly of the fuselage from the engine compartment to the tail skid. The ground underneath the fuselage was also stained with black fluid.

Postaccident examination of the engine determined that several of the connecting rod journals and the No. 2 connecting rod had overheated and that the No. 2 connecting rod had fractured and separated. A nonstandard shim was found installed between the propeller hub and the crankshaft propeller flange. A review of the maintenance logbooks found no documentation or entries indicating when the nonstandard shim was manufactured or installed nor who made it. Further examination revealed that the No. 1 main bearing journal and crankshaft propeller flange were misaligned relative to the Nos. 2 and 3 main bearing journals and that the crankshaft was bent. The orientation of the misalignment in both the flange and the No. 1 journal matched, indicating that they were related. The thickness variation in the shim also matched the orientation and magnitude of the propeller flange misalignment, indicating that it had been machined to compensate for the flange misalignment.

The heat tinting observed on the connecting rod journals and on the No. 2 connecting rod was consistent with overheating of the bearings typically associated with either improper clearance or insufficient oil pressure at the bearing surface. The misalignment of the crankshaft likely affected both the clearances and the oil pressures at the connecting rod bearings, which led to the overheating of the bearings during flight. The continued use of a bent crankshaft led to the secondary failure of the No. 2 connecting rod.

Text from the NTSB. Open the NTSB docket for WPR13FA169 for the full record.