North American NAVION A N969TR accident near Stonewall, Mississippi

Probable cause

The fatigue failure of the No. 3 cylinder exhaust valve, which resulted in a fractured camshaft and a total loss of engine power.

NTSB analysis

The pilot stated that, while en route to his destination, the airplane's engine sputtered and then lost total power. During the subsequent forced landing to a highway, the airplane landed hard, and the landing gear collapsed; the airplane sustained substantial damage to the fuselage. A postaccident examination of the engine revealed the No. 3 cylinder exhaust valve fractured where the stem transitioned to the valve head seat. The valve head impacted the No. 3 piston and became lodged between the crankcase and the camshaft, which resulted in the fracture of the camshaft. Fatigue cracking, which initiated at multiple sites, propagated through the valve stem. The presence of a tapered stem near the fracture surface, high temperature oxide deposits, and fatigue striations on the fracture surface suggested that the stem underwent spallation or erosion of the oxides formed during high temperature exposure, which would reduce the stem cross-section over time, increasing the likelihood of initiating fatigue cracking. Additionally, a localized area of wear on the top face of the valve stem was most likely due to the valve not rotating during operation, which could create hot spots or improper thermal dissipation in the hottest parts of the valve and lead to valve damage or failure.

Text from the NTSB. Open the NTSB docket for ERA19LA008 for the full record.