ERA16LA016

A total loss of engine power due to fatigue cracking and separation of the No. 3 cylinder exhaust valve stem.

The pilot reported that, after flying the airplane for about 40 minutes on a personal flight, the engine suddenly lost power and ran rough. The pilot could not maintain level flight and performed a forced landing to a road. After touchdown on a roadway, it turned to the left, and the airplane departed the pavement, struck a drainage ditch, and then struck a fence. The airplane’s wingtips, landing gear, and stabilator were substantially damaged. Examination of the engine revealed the No. 3 cylinder had incurred significant internal damage. Numerous metal fragments were present, the piston had broken up, and the exhaust valve head had separated from the stem. Further examination revealed multiple initiation fatigue cracking of the exhaust valve stem, which progressed until failure. Research indicated that fatigue cracking of exhaust valves in similar engines had occurred in the past. Investigations by multiple countries revealed that valve stem separation due to fatigue, in addition to stem cracking of other valves in engines that did ultimately separate, had occurred. This evidence also indicated that overheating of the valves was at least a factor in those engine failures.

HISTORY OF FLIGHT On October 15, 2015, about 1300 eastern daylight time, an amateur-built Titan Tornado airplane, N4070F, was substantially damaged when it was involved in an accident near Mitchells, Virginia. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, after performing a preflight inspection on the airplane and receiving the weather via the Warrenton-Fauquier Airport (HWY), Warrenton, Virginia, automated weather observation system, he departed from runway 33 about 1235. After departure, he turned the airplane to 240° and climbed to about 2,000 ft mean sea level (msl). He flew the airplane outbound from the airport for about 35 minutes and then turned around to return to HWY. He climbed the airplane to 2,500 feet msl, and about 5 minutes after turning back to the airport, the engine suddenly "lost power, ran rough," and he could not maintain level flight. The Coffeewood Correctional Center was at his "4 o'clock" position and he believed that it presented the best available landing site, so he turned towards it. He then maneuvered the airplane to line it up with the entrance road to the prison. An automobile was entering the prison on the road in front of the airplane, so the pilot glided the airplane over the car and then made a "very quick descent" to the road surface to avoid striking a set of powerlines that crossed the road. After touchdown, he attempted to stay on the roadway but the airplane was unable to follow "a dogleg" of the roadway to the left. The airplane departed the pavement on the right side of the road, struck a shallow drainage ditch, struck the prison fence with its right wingtip, spun 180°, and came to rest with the left wing against the fence. AIRCRAFT INFORMATION The airplane was equipped with an air cooled, 4-cylinder, naturally aspirated, 85 horsepower, Jabiru 2200A engine mounted in a pusher configuration, driving a Prince Aircraft 2-bladed carbon fiber propeller. The airplane's most recent condition inspection was completed on June 28, 2015. At the time of the accident, the airplane had accrued 388 total hours of operation, and the engine had accrued 136.3 total hours of operation. WRECKAGE AND IMPACT INFORMATION Examination of the accident site revealed that the airplane traveled about 223 ft after leaving the roadway on a heading of about 108° before striking the perimeter fence of the correctional center and coming to rest. Postaccident examination of the airframe revealed that it was substantially damaged. The wingtips displayed impact damage, the right main landing gear was separated from its mounting location, the stabilator was bent, and the wheel pant for the nosewheel had been separated from its mounting location. Postaccident examination of the engine revealed that the No. 3 cylinder had internal significant damage, as the cylinder head dome displayed multiple areas of impact damage, and the exhaust valve head was missing. Examination of the No. 3 cylinder also revealed numerous metal fragments and that the piston had broken up and pieces of it were lying in the bottom of the cylinder. The pieces displayed evidence of impact damage and fracturing. The No. 3 connecting rod was visibly connected to the crankshaft but displayed areas of bending. Examination of the exhaust system also revealed that part of the piston ring had been captured in the muffler. Additional examination of the No. 3 cylinder revealed that the exhaust valve had separated at the stem. The remaining exhaust valve stem appeared straight, and no scoring was observed on the outer diameter (OD) surfaces. Progressive crack arrest marks were visible on the fracture surface, consistent with fatigue cracking, and had progressed through most of the valve stem prior to separation. Numerous thumbnail-shaped patterns with accompanying ratchet marks were present around the diameter of the valve stem, which were consistent with multiple-initiation fatigue cracking that likely occurred as the valve continued to operate after the main fatigue cracking had begun to progress. No defects or pre-existing damage were observed at the initiation site of the main fatigue cracking. After removal of the rocker shaft, both valves moved freely inside their respective valve guides. The intake valve stem was bent. Cracking was visible on the OD of the stem of the intake valve. The material around the cracking appeared to have flaked off and shiny silver metal was visible under the darkened outer layer of the intake valve. Similar cracking was observed in the head end of the exhaust valve near the fracture surface. However, the cracking in the exhaust valve was not accompanied by similar material flaking observed with the intake valve cracking. Metallurgical cross-sections through the exhaust valve revealed numerous cracks in the exhaust valve stem near the fracture surface, one of which extended through almost half of the exhaust valve diameter. Most of the cracks had oxidation products visible inside, which was consistent with the cracks being present during engine operation at temperatures sufficient to cause oxide scale growth. A distinct layer was also observed on the external surface of the exhaust valve. The layer was present on both the exhaust valve stem and tip. Some shallow cracking was observed in the layer near the tip. A distinct layer like that on the exhaust valve was observed on the intake valve. Shallow cracking was also observed on the intake valve. The cracking was observed at both the head and tip which were comprised of different materials. Some shallow cracking was observed in the layer near the tip and occurred more frequently than on the exhaust valve. The cracking was mostly confined to within the distinct layer, but there was at least one crack that had extended into the base material. Some of the cracking was in conjunction with areas where the distinct layer had separated, which was consistent with the flaking observed macroscopically. According to Jabiru, the valves were manufactured from 214N stainless steel with no additional coatings. 214N steel is a nitrogen-strengthened stainless steel. Semi-quantitative energy dispersive spectroscopy (EDS) spectra showed the base metal of the valves was iron-rich, with a high level of chromium, a moderate amount of manganese, some nickel, and a trace amount of silicon, which was consistent with 214N steel. Wavelength dispersive x-ray spectroscopy (WDS) line scans of the valve cross-sections resulted in elevated levels of nitrogen in the distinct layer compared to the base metal. The microstructure of the base metal of the valve stems was austenitic stainless steel containing ferrite stringers. The average core hardness of the intake and exhaust valves measured 47.5 and 48.3 HK, respectively, with a 50g load. The recovered pieces of the No. 3 piston had extensive secondary damage. The undamaged areas of the fracture surfaces had features consistent with overstress. One of the No. 3 sparkplugs was intact but had some impact damage. The insulator on the other sparkplug had separated at the threaded end. That same sparkplug had the center electrode tilted off-center and the ground electrode bent upright. The damage to the surfaces of the No. 3 cylinder head, the piston, and the sparkplug were likely secondary to the separation of the exhaust valve. ADDITIONAL INFORMATION Accident Airplane Engine History The engine involved in the accident (S/N 22A2371) was manufactured in Queensland, Australia, on September 3, 2006, and sold into the United States. According to the pilot, he had purchased the engine second-hand to replace his previous Jabiru 2200 engine (S/N 22A959), which had incurred a valve failure at 269.7 total hours of operation and resulted in an emergency landing in a field. The pilot installed the accident engine on the accident airplane on May 4, 2013. The engine had previous