Accident Description
While in cruise flight, the engine RPMs unexpectedly dropped from 2,350 to 1,700, causing the airplane to begin descending. The pilot executed a forced landing in an open, uneven field. The landing gear was sheared off during the landing, and the fuselage sustained buckling damage.
Postaccident Examination
A postaccident examination of the engine revealed that the No. 4 cylinder exhaust valve was jammed in the open position. The valve stem and guide were coated with 'gummy black deposits.'
Prior Incident
The pilot experienced a similar accident on July 22, 1995 (referenced as FTW 95-L-A312). In that occurrence, the jammed exhaust valve was attributed to excessive carbon deposits resulting from the use of high octane aviation gasoline in a low compression engine.