Accident Summary
The airplane experienced a loss of engine power during a training flight. A passenger reported hearing a "roar" through the four-way intercom. The certified flight instructor (CFI) and the student pilot checked the oil pressure and fuel pressure, both of which were indicated as normal. The CFI pointed to the left and directed the student toward a private airstrip. During or immediately after the left turn, the CFI stated, "I've got it." The student looked back at the two passengers and gestured with his right hand that they intended to land at the strip. The last thing the passenger recalled was contact with trees; the airplane came to rest after clearing a ridge top but colliding with trees approximately one-quarter mile from the landing strip.
Investigation Findings
Examination of the engine revealed mechanical damage to the spark plug of cylinder No. 1, which contained metallic debris. The head of the exhaust valve for cylinder No. 1 was missing, and the top of the piston showed mechanical damage across its entire surface. Metallurgical analysis of the valve and cylinder was conducted. The valve stem was broken and remained in the valve guide. The stem had fractured, with numerous secondary cracks below the fracture surface. The areas around the valve seats were severely damaged. The condition of the fracture surface prevented determination of the fracture mechanism. The secondary cracks were confined to a localized area where the material was severely battered and deformed; many cracks were slightly curved or did not appear perpendicular to the surfaces, consistent with overlapping surfaces or excessive local strains.
Engine Examination and Maintenance History
No material or dimensional abnormalities that would suggest fatigue were found. The manufacturer had issued maintenance instructions describing methods for determining exhaust valve and guide condition, but this maintenance was optional for the operator. Maintenance records indicated that this procedure had not been performed. Although the exhaust valve stem diameter was slightly below specification, this would have a minor effect on overall stresses; however, a small change in diameter significantly alters the clearance between the valve stem and guide. Excessive clearance can lead to exhaust valve sticking, which can result in overstress fracture. The engine had accumulated 2,079 total hours and had not been overhauled since its manufacture in 1975. The manufacturer recommended overhaul at 2,000 hours or after 12 years of service life. Given the high operating hours and years since overhaul, it was likely that the failure resulted from exhaust valve sticking.