FTW97LA077

The pilot reported that while in cruise flight the airplane shuddered and began to lose power. She changed the fuel selector from the right to left tank; checked the fuel mixture, the exhaust gas temperature, and the instrument panel. The pilot reported that the engine performance continued to decay and she initiated a forced landing to a pasture. Examination of the engine revealed that the number two cylinder exhaust rocker arm retaining studs had separated. Examination of the studs by a metallurgist revealed that over torquing of the attachment nuts during installation caused plastic deformation of the studs and probable cracking in the threads which led to a fatigue crack in at least one of the studs. The fatigue crack led to the subsequent failure of the exhaust rocker arm retaining studs.

On December 31, 1996, approximately 1350 mountain standard time, a Piper PA-28R-201T, N40437, was substantially damaged during a forced landing after a power loss near Raton, New Mexico. The private pilot and her three passengers were not injured. The airplane was registered to and operated by a private owner under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the cross country flight which originated from Midland, Texas, with the intended destination of Trinidad, Colorado. No flight plan had been filed and the flight had begun at 1210.

The pilot reported in the NTSB Pilot/Operator Aircraft Accident Report that while in cruise flight "the airplane shuddered" and began to lose power. She changed the fuel selector from the right to left tank; checked the fuel mixture, the exhaust gas temperature, and the instrument panel. The pilot reported that the engine performance continued to decay and she began looking for a place to land. She initiated a forced landing to a "pasture." Subsequently, during the landing roll, the landing gear separated from the airplane, the wings were damaged, and both wing spars were fractured.

Examination and tear down of the engine revealed that the number two cylinder exhaust rocker arm retaining studs had separated. Examination of the studs by a metallurgist revealed evidence of plastic deformation and a fatigue crack which was associated with "small cracks in the threads" (see enclosed metallurgist report).