What happened
During a sightseeing flight within the Mount St. Helens National Volcanic Monument area, the aircraft experienced a catastrophic engine failure while in flight. Witnesses observed massive oil leaking from the engine area prior to impact. The plane subsequently crashed onto the mud flats of the North Fork of the Toutle River, located in a relatively flat and open region near a level straight road without shoulder obstructions.
A surviving passenger reported that the crew had intended to execute a forced landing on the nearby road. To align with the road, the pilot executed a steep turn. However, the aircraft appeared to lose lift and fell from the sky during this maneuver. The stall warning system was reported as inoperative at the time of the accident.
The investigation
Examination of the recovered engine revealed that cylinder number three had been demounted. Critical fasteners were missing: hold-down bolts were absent from one 1/2-inch stud, one 1/2-inch throughbolt, and two 3/8-inch studs. One hold-down nut of each size was found unattached but captured within the exterior engine baffling. Additionally, the connecting rod cap for cylinder number three was missing.
Forensic analysis showed no evidence of tensile overloading or thread stripping on the remaining fasteners. The engine had accumulated 446 hours since its last overhaul and 24 hours since its most recent inspection.
Findings
The investigation determined that the loss of engine power was caused by the disassembly of cylinder number three due to missing hold-down bolts and a connecting rod cap. This mechanical failure led to an in-flight oil leak and total power loss.
Contributing factors included:
- The pilot's decision to make a steep turn at low altitude during a forced landing attempt, which likely resulted in a stall.
- The stall warning system being inoperative, depriving the pilot of critical auditory cues regarding airspeed and angle of attack.
- Deviation from standard operating procedures, as the owner's manual specified that the aircraft should be at 1000 feet AGL on the downwind leg for a normal power-off approach.
The wreckage was found near a road, consistent with the passenger's account of an intended forced landing site. The 3 fatal outcome resulted from the high-impact crash following the loss of control.