N7502C

On June 27, 1998, approximately 0630 Pacific daylight time, a Schweizer G-164B, N7502C, impacted a ditch during an aborted takeoff from a private agricultural airstrip near Oakdale, Washington. The commercial pilot, who was the sole occupant, was not injured, but the aircraft, which was owned and operated by Darrell Fender, of Colfax, Washington, sustained substantial damage. The attempted takeoff, which took place during visual meteorological condition, was for the purpose of a 14 CFR Part 137 aerial application flight. No flight plan had been filed, and there was no report of an ELT activation.

According to the pilot, during takeoff, approximately 10 feet above the ground, the aircraft started backfiring and losing power. He therefore aborted the takeoff, but was unable to get the aircraft stopped before impacting a ditch just off the departure end of the airstrip. A post-accident inspection of the engine by an FAA Airworthiness Inspector found that a portion of the number three cylinder head casting through which the exhaust valve rocker pin passes had fractured. This allowed the number three cylinder exhaust valve to remain in the closed position during the time that it should have been open, resulting in a loss of power.

As part of the investigation, the rocker box casting was sent to the NTSB metallurgical laboratory for further analysis. That examination revealed that the fracture in the casting was the result of a fatigue crack that originated in the wall of the hole in the casting through which the rocker arm pivot bolt passed. It could not be determined what caused the fatigue crack to initiate.