Cessna 210-5(205) accident near Darby, Montana, July 22, 2011

The NTSB's narrative final · quoted from the NTSB record

The pilot reported that, while in a cruise flight, the engine began to run rough and eventually lost power. The pilot subsequently initiated a forced landing to a gravel road, and the airplane collided with a tree during the landing roll. Postaccident examination of the engine found a 4-inch by 2-inch hole and material loss on the bottom side of the aluminum air intake duct assembly. The circumference of the hole was an irregular shape with both jagged and smooth edges/fracture surfaces. The fractures were consistent with fatigue and were possibly the result of vibration. Internal borescope examination of the cylinders revealed mechanical etching to the No. 4 and No. 6 combustion chambers and associated valve heads. The corresponding spark plugs were removed and mechanical damage was noted. The ground electrodes were bent down onto the insulators and center electrodes. No other mechanical failures were noted with the engine or airframe. It is likely that the missing aluminum duct material was ingested into the cylinders, which resulted in a loss of engine power. Although there was evidence that a weld repair had been performed on the air intake duct near the area of the hole, it could not be determined if it contributed to the hole and loss of material. A review of the airplane’s maintenance records showed no recent modifications or repairs to the engine air intake duct.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.