NTSB Identification: FTW97LA298.
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Accident occurred Sunday, August 03, 1997 in CASTLE ROCK, CO
Probable Cause Approval Date: 02/02/1998
Aircraft: Piper J3C-65, registration: N5985V
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

While in cruise flight, engine RPMs dropped from 2,350 to 1,700, and the airplane began to descend. The pilot made a forced landing in an open, uneven field. The landing gear was sheared off and the fuselage was buckled. Postaccident examination of the engine disclosed the no. 4 cylinder exhaust valve was jammed in the open position. The valve stem and guide were coated with 'gummy black deposits.' The pilot had a similar accident on July 22, 1995 (FTW 95-L-A312). In that accident, the jammed exhaust valve was attributed to excessive carbon deposits from high octane aviation gasoline usage in a low compression engine.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

A partial loss of engine power due to a cylinder exhaust valve jammed in the open position from excessive carbon deposits. A factor was the lack of suitable terrain on which to make a forced landing.

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