Incident Overview
A student pilot and flight instructor were performing maneuvers when the engine began to lose power. The flight instructor assumed control of the airplane and executed a forced landing in an open field with uneven terrain. During the ground roll, the airplane encountered rising terrain, causing the nose landing gear to collapse rearward.
Engine Examination
Post-incident examination of the engine revealed that the number two exhaust valve was stuck. The head of the valve stem was missing, and multiple impact marks were found on the top of the corresponding piston. Portions of the valve head were recovered in the exhaust and intake manifolds.
Maintenance History
Records indicate that the number two cylinder had been replaced 172 hours prior to the incident. The number 1, 3, and 4 cylinders had been replaced 51 hours prior to the incident.