Amateur-built TORNADO N4070F accident near Mitchells, Virginia

Probable cause

A total loss of engine power due to fatigue cracking and separation of the No. 3 cylinder exhaust valve stem.

NTSB analysis

The pilot reported that, after flying the airplane for about 40 minutes on a personal flight, the engine suddenly lost power and ran rough. The pilot could not maintain level flight and performed a forced landing to a road. After touchdown on a roadway, it turned to the left, and the airplane departed the pavement, struck a drainage ditch, and then struck a fence. The airplane’s wingtips, landing gear, and stabilator were substantially damaged.

Examination of the engine revealed the No. 3 cylinder had incurred significant internal damage. Numerous metal fragments were present, the piston had broken up, and the exhaust valve head had separated from the stem. Further examination revealed multiple initiation fatigue cracking of the exhaust valve stem, which progressed until failure.

Research indicated that fatigue cracking of exhaust valves in similar engines had occurred in the past. Investigations by multiple countries revealed that valve stem separation due to fatigue, in addition to stem cracking of other valves in engines that did ultimately separate, had occurred. This evidence also indicated that overheating of the valves was at least a factor in those engine failures.

Text from the NTSB. Open the NTSB docket for ERA16LA016 for the full record.