Amateur-built LONG EZ N444MR accident near Medicine Bow, Wyoming

Probable cause

A partial loss of engine power due to the fatigue fracture of the No. 4 exhaust valve head.

NTSB analysis

According to the pilot, he and a passenger were on a cross-country flight when, at approximately 7,500 feet above mean sea level (msl), the aircraft began aggressively shaking. The pilot initially flew toward a nearby airport but decided to land on a highway. The airplane landed hard, veered off the road, and impacted terrain. Subsequently, the airplane nosed over onto a fence and sustained substantial damage to the fuselage and wings.

During the postaccident examination of the engine, excessive oil and carbon were discovered on the No. 4 cylinder valve springs, guide, stem, and retainers, which is consistent with exhaust blow-by. In addition, approximately 0.8 inches of the No. 4 exhaust valve stem and valve head were not recovered from the cylinder and engine. The remaining valve stem had excessive play within the valve guide. The fracture of the stem portion of the exhaust valve was the result of a fatigue crack that emanated from severe corrosion pits at the outer surface of the stem, which resulted in a loss of engine power.

The engine's logbook entries revealed that it was last overhauled 41 years before the accident, exceeding Lycoming's recommendation of 12 years. It is likely that the No. 4 exhaust valve deteriorated from age and likely fractured from a fatigue crack.

Text from the NTSB. Open the NTSB docket for WPR22LA334 for the full record.