ERA20LA327 · Mooney M20R

The pilot completed a multi-day cross-country trip on the day prior to the accident flight during which he noted an increase in the engine’s oil consumption. The purpose of the accident flight was a planned trip for a scheduled oil change, where he also intended to ask the maintainer about the oil consumption. Prior to departing on the accident flight, he performed a preflight inspection and noted that the engine oil level was about 6.5 quarts. While in cruise flight about 20 minutes after departure, the engine lost all power. The pilot performed a successful forced landing to the right shoulder of an interstate highway, where the airplane came to a stop. As the pilot and passenger prepared to disembark, a truck struck the airplane’s left wing from behind, resulting in substantial damage to the wing. The pilot reported that the oil level after the accident was between 6 and 6.5 quarts.

A review of data recorded by the airplane’s avionics system revealed that during two previous legs of the cross-country trip the oil pressure began to gradually decrease and fluctuate about midway through each flight. During the last leg, the oil pressure decreased continuously throughout the entire flight. During the accident flight, the oil pressure remained in the green range until just prior to the loss of engine power.

During the most recent annual inspection, the engine oil was noted as excessively dirty and contaminants (“gunk”) were found in the oil filter pleats. Contaminated oil is commonly caused by an excessive interval between oil changes and/or excessive wear of the piston rings, neither of which appear to have occurred in this case. The oil change intervals were consistently within the manufacturer’s specifications and there was no significant loss of cylinder compression (a potential indicator of piston ring wear) noted during the last annual inspection. There were no anomalies noted with the oil and no debris noted in the oil filter during the following oil change, which was the last change before the accident flight.

Although the damage to the engine components was consistent with thermal damage due to a lack of lubrication, given the reported oil quantity after the accident and during the previous trip, as well as the gradual decreases in the recorded oil pressure on the previous flights, it is unlikely that the damage resulted from too little oil in the sump. The gradually decreasing oil pressure during the previous flights suggests a possible problem with the oil pump, an oil leak or restriction, or excessive oil temperature, no evidence of which was found during the examination. It is possible that the overheating and subsequent damage to the bearings began during one of the previous flights when the oil pressure decreased. The reasons for the decreased oil pressure and the increased oil consumption could not be determined.

The engine had never been overhauled, and at the time of the accident had reached just over half of the recommended operating hours between overhauls. However, it was 14 years old, which is 2 years past the recommended calendar time between overhauls. Had the engine been overhauled at the recommended calendar time, it is likely that the issue with the lubricating system could have been addressed or prevented.

Verbatim NTSB analysis and probable cause from the NTSB dataset