Accident Overview
During a sightseeing flight in the Mount St. Helens National Volcanic Monument area, an aircraft crashed on the mud flats of the North Fork of the Toutle River. The terrain at the impact site was relatively flat and open. Prior to the crash, there were indications of a massive oil leak originating from the engine area in flight.
Engine Examination
Post-crash examination of the engine revealed the No. 3 cylinder had become demounted. Hold-down bolts were missing from one 1/2-inch stud, one 1/2-inch throughbolt, and two 3/8-inch studs. There was no evidence of tensile overloading or stripping on the remaining hardware. One hold-down nut of each size (1/2-inch and 3/8-inch) was found unattached and captured in the exterior engine baffling. The No. 3 connecting rod cap was missing. The engine had accrued 446 hours since overhaul and 24 hours since the last inspection.
Witness and Pilot Actions
The wreckage was discovered near a level, straight road free of shoulder obstructions. A surviving passenger stated that a forced landing on a road was intended. However, a steep turn was made over the road, and the aircraft subsequently "fell." The owner's manual specifies that a pilot should be at 1,000 feet above ground level at the downwind position for a normal approach to a power-off landing. The stall warning system was reported as inoperative.
Conclusion
The accident investigation identified a massive oil leak in flight due to the separation of the No. 3 cylinder, with missing hold-down bolts and a missing connecting rod cap. No probable cause is stated in the source.