On August 18, 2007, an Air Tractor AT-301 (registration N69WN) was involved in an aviation accident near Alamosa, CO. Investigators recorded the probable cause as: Fatigue cracking initiated from multiple locations in a root of the head threads and propagated in an arc to the outer surface of the combustion chamber wall. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During aerial application, a cylinder head separated, covering the windscreen with oil. The pilot performed a forced landing on a dirt road, striking a sand berm and nosing over, resulting in substantial damage.
Incident Overview
An aircraft conducting aerial applications of anti-sprouting chemicals experienced an in-flight failure when the number one cylinder head separated from the engine. The resulting oil spray covered the windscreen, obscuring the pilot's forward vision.
Pilot Actions
The pilot immediately jettisoned the chemical load and initiated a forced landing onto a nearby dirt road. During the landing roll, the left wing contacted a sand berm, causing the aircraft to nosed over and come to rest inverted.
Damage and Examination
The aircraft sustained substantial damage. The pilot reported no other anomalies with the airplane's systems prior to or during the event. Post-accident examination of the fractured cylinder head revealed fatigue cracking that originated from multiple locations at the root of the head threads. The cracks propagated in an arc pattern toward the outer surface of the combustion chamber wall.
Conclusion
The investigation determined that the cylinder head failure was due to fatigue cracking initiating from the thread roots. No other contributing factors were identified.
Probable cause
Fatigue cracking initiated from multiple locations in a root of the head threads and propagated in an arc to the outer surface of the combustion chamber wall.