Mooney Aircraft Corp. M20 N304MA accident near Lakeway, Texas

Probable cause

A total loss of engine power due to detonation and preignition in the No. 6 cylinder.

NTSB analysis

Before purchasing the airplane, the pilot had an inspection performed. One of the findings on the inspection stated that, “spark plugs are worn past limits.” There were no indications that the magneto timing was checked during the inspection. The pilot reported that the airplane had been run with a lean mixture in the past.

After the pilot purchased the airplane, he flew the airplane on a cross-country flight to relocate it to his home base. While in flight, the engine had a low manifold pressure reading and then the engine sustained a total loss of power. The pilot attempted to fly to the nearest airport, but instead performed a forced landing to a golf course. During the forced landing sequence, the airplane impacted trees and a wood fence, and came to rest near a diesel generator. The airplane sustained substantial damage to the fuselage and to both wings.

Examination of the airframe revealed no mechanical anomalies. Examination of the engine revealed a hole in the corner of the No. 6 piston; however, the No. 6 cylinder was intact. The No. 6 piston had eroded crown material and a darkened periphery. The engine case was found intact. The top spark plugs from the Nos. 3, 5, and 6 cylinders and the bottom spark plugs from the Nos. 3 and 5 cylinders were found in a “worn out - normal condition.” The right magneto timing was found at 22°, when it is supposed to be at 20°. The left magneto timing was found at 15°, when it is supposed to be at 20°. The oil breather tube was found intact, and the slot was not blocked. Engine oil was found splattered in and around the oil breather tube area, including traveling rearward on the area underneath the oil breather tube. No anomalies were noted with the turbocharger system or the propeller.

It is likely a total loss of engine power occurred when the No. 6 piston developed a hole from burn through, consistent with detonation and preignition signatures consisting of combustion deposits and excessive temperature exposure. The hole in the piston likely allowed exhaust gases, caused by the detonation and preignition, to force engine oil out of the oil breather tube.

The combination of the engine being previously run with a lean mixture, worn spark plugs, and incorrect magneto timing, likely lead to the preignition and detonation and a subsequent total loss of engine power.

Text from the NTSB. Open the NTSB docket for CEN23LA107 for the full record.