History of Flight
On July 22, 2011, at 1225 mountain daylight time, a Cessna 210-5, registration N8427Z, diverted to a forced landing near Darby, Montana, after the engine lost power. The commercial pilot and one passenger sustained minor injuries. The airplane had departed Lemhi County Airport in Salmon, Idaho, at 1155, en route to Chamberlin, Idaho. Visual meteorological conditions prevailed, and no flight plan was filed for the cross-country flight.
According to the pilot, while cruising, the engine began to run rough and subsequently lost power. The pilot maneuvered toward an open valley with a gravel road, positioning for a downwind landing. The airplane touched down approximately 200 feet beyond a cattle guard and began to skid. After reducing braking pressure, the aircraft crossed a second cattle guard. During the landing roll, the left wing struck a tree, causing the airplane to pivot about 90 degrees to the left before coming to a halt. Substantial damage was sustained to the left wing and forward fuselage.
Aircraft Information
The accident airplane, serial number 205-0427, was manufactured in 1963. It was a single-engine aircraft equipped with a Continental IO-470 S engine and a McCauley constant-speed propeller. The most recent annual inspection of the airframe, engine, and propeller was completed on January 9, 2011. At the time of the accident, the airplane had logged 7,343 total hours, with the engine accruing 475 hours since new. Approximately 56 hours had flown since the annual inspection.
Postaccident Examination
The National Transportation Safety Board (NTSB) conducted a postaccident examination of the engine and airframe after the aircraft was transported to a storage facility in Phoenix, Arizona.
Examination revealed a dark residue coating the left side of the engine accessory section, concentrated around the outboard engine air intake duct assembly, just aft of the bracket air filter. The air filter, bracket assembly, and aluminum intake duct were removed, exposing a large hole measuring approximately 4.25 inches by 1.75 inches on the bottom side of the duct. The hole had irregular edges with both jagged and smooth fracture surfaces, and dark residue was present on these surfaces. A weld repair was noted adjacent to the hole, and a section of fuel pump baffle tubing was routed nearby. Rub marks and dark residue were found on the baffle tubing, concentrated near the hole, along with material loss and deformation.
Internal borescope inspection of the piston cylinders showed mechanical etching in the No. 4 and No. 6 combustion chambers and their valve heads. The corresponding spark plugs exhibited mechanical damage, with ground electrodes bent down onto the insulators and center electrodes.
A senior NTSB metallurgist examined the intake duct and determined that the fractures were consistent with fatigue and possibly the result of vibration. Although a weld repair was present near the hole, it could not be determined whether that repair contributed to the hole and loss of material.
No additional anomalies were found in the airframe or engine. Review of maintenance records showed no recent modifications or repairs to the engine air intake duct.