Accident Overview
On June 24, 2016, at approximately 1015 mountain daylight time, a North American Aviation Navion airplane, registered as N91137, executed a forced landing near Alamogordo, New Mexico. The pilot and one passenger were not injured, but the airplane sustained substantial damage. The flight was operated by the pilot under 14 Code of Federal Regulations Part 91 as a personal cross-country trip. Visual meteorological conditions were present, and no flight plan had been filed.
The flight originated from Sierra Blanca Regional Airport (SRR) in Ruidoso, New Mexico, at 0938. According to the pilot, the purpose of the flight was a planned proficiency competition involving several aircraft. The objective was to predict the time en route and total fuel consumption for a predetermined four-leg route and then fly as close as possible to those predictions.
Accident Sequence
While cruising at 7,500 feet mean sea level (msl), the pilot noticed the engine running rough. At 0958, cylinder head temperature (CHT) and exhaust gas temperature (EGT) readings indicated that the No. 1 and No. 5 cylinders were significantly cooler than the others: No. 1 CHT was 350°F and EGT 990°F; No. 5 CHT was 305°F and EGT 1320°F, while No. 6 CHT was 398°F. At 1010, the aircraft had climbed to 8,000 feet msl over mountainous terrain when an acrid metallic smell was detected in the cabin. The pilot made a distress call and turned right toward an airport.
About 13 nautical miles southeast of Alamogordo-White Sands Regional Airport (ALM), the engine became very rough, and a metallic clanking sound was heard. The pilot reduced power and began a descent. Approximately 5 to 6 nautical miles from ALM, the engine continued operating with reduced power, but the airplane kept descending. The pilot attempted an emergency landing on a residential road, but with no engine power remaining, the airplane landed in a field about 100 yards short of the road. It then entered a drainage culvert and came to rest on the road. The airplane suffered substantial damage to the engine mounts, firewall, wing spar, fuselage, and empennage.
Postaccident Examination
An FAA inspector examined the aircraft at the accident site alongside the pilot. The engine cowling was removed to inspect the Continental Motors IO-520-B engine. The No. 5 cylinder appeared damaged, and the engine crankcase was cracked in that area. A detailed postaccident examination conducted by the NTSB and Continental Motors revealed that the No. 5 cylinder rocker box cover had been previously removed by the FAA inspector and was found in the engine compartment. Inside it were an inner exhaust valve spring, an outer exhaust valve spring, a damaged valve guide, portions of a roto coil assembly, damaged valve spring keys, and six machine screws and lock washers. The rocker box cover had a hole directly over the exhaust valve assembly. The crankcase was cracked about 2 inches at the base of the No. 5 cylinder. The No. 5 top spark plug exhibited mechanical damage; other spark plugs showed normal to worn-out normal patterns. The oil filter safety wire was attached to the fuel pump hose fitting. Upon opening, the oil filter contained metal debris. The oil sump held a large amount of metal debris and damaged engine components, including the No. 5 piston, piston rings, exhaust guide, and exhaust valve head. The No. 5 cylinder was removed, revealing damage to the cylinder head, cylinder wall, intake push rod, and piston connecting rod; the rod was twisted and its pin deformed on both ends. The rod bearing showed normal wear. The No. 5 cylinder and associated parts were sent to Continental Motors for further examination.
Component Disassembly
At Continental Motors' Analytical Department, the No. 5 cylinder was disassembled. The exhaust valve guide was fragmented, with some pieces unrecovered. A fragment of the guide was marked "FAA-PMA," indicating it was a third-party part, not from Continental Motors. The valve guide bore was worn beyond the specified size, suggesting movement of the guide. The exhaust valve head hardness met specification. Only one valve key spring was found, and it was worn.
Maintenance History
Review of the aircraft maintenance logbooks showed that on January 20, 2006, at an engine total time (TT) of 358.3 hours, the No. 5 cylinder was replaced with an overhauled Engine Components International Division (ECi) cylinder. A 100-hour inspection was completed on March 23, 2016, at an engine TT of 1,089.9 hours. At the time of the accident, the engine had accumulated 1,099 hours TT.