WPR24LA060 · Mooney M20J

About 2 hours into the visual flight rules cross-country flight, the airplane climbed from 11,500 ft to 13,500 ft mean sea level (msl). The pilot configured the airplane’s trim, fuel mixture, and power setting to maintain cruise flight while operating with the fuel selector in the left tank position. About 20 minutes later, the pilot heard what he referred to as a “loud clunk” sound from the engine, followed by strong vibrations and the smell of engine oil. The pilot turned on the fuel boost pump, moved the fuel selector to the right tank position, and made a left turn to the northwest while continuing to troubleshoot the anomaly; he declared an emergency with the air route traffic control center (ARTCC) controller. Moments later, the engine lost all power. The pilot attempted to restart the engine to no avail, selected the nearest suitable landing site, and made an off-airport, gear-up, forced landing in mountainous terrain. Postaccident examination of the engine’s ignition system revealed a malfunction of the single-drive dual magneto. The magneto did not produce spark at any of the ignition leads and was removed for further examination. The magneto distributor housing was removed and neither of the contact assemblies was found to be functioning. The magneto drive was rotated by hand after supplying power and neither the left nor right contact assembly cam follower engaged the main breaker point frame bracket. After manually separating the main breaker point from the retard breaker point, the right magneto produced an audible tone and the green light illuminated; however, the left magneto did not. The magneto was shipped to the manufacturer for bench testing. The bench test revealed that the right magneto produced a spark at each of the four ignition leads. The left magneto did not produce a spark at any of the leads. The magneto distributor housing was removed for further examination, which revealed that the right magneto cam follower was engaging the cam, allowing the contact assembly breaker points to open when the cam lobe lifted the cam follower, contact assembly frame bracket, and breaker point. However, the left magneto cam follower was not engaging the cam. The engine manufacturer’s High Tension Magneto Service Support Manual provided maintenance instructions for the main and retard breaker contact assembly, with instructions to adjust the assembly frame bracket for breaker point clearance. The left magneto main contact assembly was adjusted to increase the breaker point clearance, according to the manual, and the dual magneto was reassembled and bench tested. During the second bench test, the right and left magneto produced electrical impulses to each of the ignition leads in firing order. A review of the airplane’s maintenance logs showed that during the last annual inspection the magneto-to-engine timing was checked. However, a definitive magneto service history could not be determined. Intermittent operation of the magneto would result in, at minimum, the strong vibrations experienced by the pilot, and was the likely origin of the inflight total loss of engine power.

Verbatim NTSB analysis and probable cause from the NTSB dataset