On June 24, 2006, a Luscombe 8A (registration N2890K) was involved in an aviation accident near Bowling Green, MO. Investigators recorded the probable cause as: The loss of engine power due to fatigue fracture of the number two cylinder exhaust valve during initial climb after takeoff. A contributing factor was the unsuitable terrain (cornfield) encountered during the forced landing. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
An airplane sustained substantial damage during a forced landing to a cornfield after losing engine power following takeoff. The pilot suffered serious injuries. Postaccident examination revealed a fatigue fracture of the number two cylinder exhaust valve, with clearance measurements exceeding service limits.
Accident Summary
The airplane experienced a loss of engine power after takeoff, necessitating a forced landing to a cornfield. The aircraft sustained substantial damage during the landing, and the pilot received serious injuries.
Postaccident Examination
Examination of the engine after the accident found that the number two cylinder exhaust valve had fractured at the transition radius between the valve stem and the valve head. The fracture surface displayed arrest lines and ratchet marks, which are consistent with a fatigue fracture initiating from multiple origins. Fatigue features had progressed across approximately 95 percent of the stem diameter.
Clearance Measurements
Measurements of the valve stem diameter and the valve guide inside diameter showed that the running clearance was 0.015 to 0.017 inch at the inboard end and 0.007 to 0.013 inch at the outboard end. The maximum serviceable limit for valve stem to valve guide clearance is 0.007 inch.