Amateur-built LANCAIR IV-P N10UU accident near Shawano, Wisconsin

Probable cause

A total loss of engine power due to a dual magneto failure, as a result of an exceedance of the engine’s operating limitations while maneuvering.

NTSB analysis

While practicing an emergency descent during a dual training flight, the pilot receiving instruction entered the airplane into a steep dive, which resulted in a 0.5-G load factor for about 5 seconds and a transient drop in engine oil pressure of about 15 psi. As the descent continued, engine speed steadily increased beyond the maximum operating limit of 2,700 rpm. While still in the dive, the pilot initiated a left turn with a 3-G load factor, during which the engine oversped to 3,390 rpm. The engine immediately began to run rough, and subsequently experienced a total loss of power. The pilot executed a forced landing to a field with the landing gear and flaps retracted, during which the airplane struck a rock wall and tumbled.

Postaccident examination revealed that 8 teeth on the left magneto distributor drive gear and 16 teeth on the right gear had fractured. No evidence of progressive damage or material anomaly was observed with the distributor drive gear teeth. The nylon gear teeth were most likely damaged by the abnormal shock loads on the gear train during the engine overspeed. The damage to the gear teeth resulted in a dual magneto failure and subsequent loss of engine power.

The flight profile of a low-G pushover to a steep dive, which was accompanied by a drop in engine oil pressure, may have led to the propeller governor not supplying adequate oil pressure to the propeller, which subsequently contributed to the engine overspeed and the failure of the magnetos.

Text from the NTSB. Open the NTSB docket for CEN16LA043 for the full record.