Accident Overview
On August 9, 2019, about 1820 Pacific daylight time, a Cessna 206, registration N5152U, sustained substantial damage during an accident near Prospect, Oregon. The aircraft was operated by an unnamed company under Title 14 Code of Federal Regulations Part 135 as a fire reconnaissance mission flight. The pilot and one passenger were not injured.
Pilot's Account
According to the pilot, during takeoff and climb, all engine gauges indicated normal readings, and he continued to monitor them every 10-15 minutes during the flight. Approximately 2.8 hours into the flight, while over Rustler Peak at an estimated altitude of 1,500 to 2,000 feet above ground level (agl), a loud bang was heard followed by violent shaking of the airplane. The pilot retarded the propeller control to extend glide distance and searched for a landing area. He radioed the air attack unit, which relayed a mayday call.
Locating an open area, the pilot activated the emergency locator transmitter (ELT), switched the radio to 121.5 MHz, and made a mayday call. The chosen landing area was a swamp; upon touchdown, the occupants were thrown forward against their seatbelts. They exited the airplane and awaited rescue.
Engine Examination
A complete engine teardown inspection was performed following the accident. The oil pan contained numerous metal pieces and shavings, including a broken counterweight, crankshaft counterweight attachment, counterweight retainer pin, damaged retainer pin plates, damaged retainer plate spring clips, and engine casing pieces. One damaged retainer pin plate displayed a deep groove approximately the diameter of a counterweight pin.
Extensive damage was observed to the No. 1 cylinder, piston, and connecting rod. The No. 1 cylinder valve guide rods had broken through the crankcase and exhibited damage. The rear counterweight was separated from the crankshaft, and the counterweight lobe was broken free from the crankshaft.
Maintenance History
Examination of the engine logbooks revealed that the engine was last overhauled in April 1999. During that overhaul, the crankshaft was removed and replaced with the one involved in the accident. An entry dated March 5, 2010, indicated the engine was disassembled on that date and components were inspected. According to the entry, the crankshaft assembly was magnafluxed and dimensionally inspected before being returned as serviceable. The associated work order indicated the crankshaft had been overhauled by ECi in June 2009. The crankshaft assembly had accumulated 1,380.7 hours since that overhaul, as of the last maintenance entry made three days before the accident.