WPR17LA050

The loss of engine oil pressure during cruise flight, which resulted in partial loss of engine power and necessitated a forced landing. The loss of oil pressure was the result of a failed engine piston, likely initiated by foreign object damage, which resulted in the pressurization of the crankcase and expulsion of the engine oil.

The pilot reported that, during cruise flight, the airframe began to shake, and he received a failure annunciation indication from both engine control units, followed by indications of an increase in oil temperature and a loss of oil pressure. He decided to perform a forced landing into a field, during which both wings impacted vegetation and the airplane sustained substantial damage. Subsequent examination revealed that a large quantity of engine oil had been expelled out of the engine breather tube and onto the belly of the airplane. Disassembly of the engine revealed heavy pitting to the piston crown and combustion chamber surfaces of the No. 1 cylinder. The damage appeared to be the result of a fatigue crack in the piston, which allowed combustion gases to create a gas path, which eventually tunneled through the piston. The path ultimately reached the lowest piston ring, allowing pressurization of the engine crankcase, resulting in the observed expulsion of oil. The crack appeared to have been initiated by a nick in the piston crown, possibly caused by ingestion of a foreign wire-like object, which was harder than the aluminum material that comprised the piston and cylinder head. A determination of the foreign object type could not be made, but there are multiple possibilities regarding the source. The induction side of the turbocharger had experienced a failure about 234 flight hours prior, and the induction-to-intake plenum was replaced 192 hours prior to the accident. A fragment of material, such as safety wire, could have been inadvertently left inside the unfiltered side of the induction system during those maintenance events and been ingested into the engine. The design of the air induction system incorporated filters for both normal and alternate air operations, and, as such, foreign object ingestion during operation was unlikely. The engine’s oil analysis history did not indicate any significant deviations throughout its service history. Such damage would likely have resulted in oil contamination; therefore, the crack likely began to propagate during the 90-hour period between the last oil analysis and the accident flight. Multiple similar piston failures have occurred to other engines in the series, seemingly initiated by either foreign object ingestion and damage or thermal overload due to an out-of-tolerance fuel injector, or a combination of both. The manufacturer released a mandatory service bulletin to address the fuel injector defects and a service letter to address the potential for foreign object ingestion following maintenance events. Although not contributing to the accident, during the investigation, a series of cracks were observed in the cylinder head of both the accident engine and a series of other similar engines. In one event, a crack led to a loss of coolant after the cooling system became over-pressurized. In all cases, the cracks had occurred before the engines had reached their service life. The FAA-certified engine was originally designed and manufactured for automobile use, and subsequently modified by the manufacturer for aviation use. As such, the engine was not operating in the manner (continuous high-power operation) for which it was originally designed. The significance of this finding could not be determined.

The airplane was manufactured in 2015 and was used exclusively for flight training. It was equipped with a liquid-cooled, four-cylinder, turbocharged, AE300 (E4-series) diesel-fuel engine manufactured by Austro Engines. It utilized a chain-driven, double overhead cam system with four valves per cylinder and incorporated a full authority digital engine control (FADEC) system, which controlled both the engine parameters and propeller governor. The engine’s air induction system comprised an inlet air plenum mounted to the cowling and routed via a hose to an air filter housing. The housing contained both the standard air filter and an alternate air inlet, which was protected with a fine mesh foreign object damage (FOD) screen. A hose directly connected the outlet of the housing to the inlet of the turbocharger compressor housing. The outlet of the compressor then routed to the engine’s intake system via an intercooler. The engine was originally manufactured by Daimler AG., as the Mercedes-Benz OM640 automobile engine, and was modified for aviation use by Austro Engines. The design was type-certified by the Federal Aviation Administration (FAA) with a time between overhaul (TBO) interval of 1,800 hours. The airplane was inspected under a progressive inspection program; the most recent inspection was completed on December 13, 2016, 40 flight hours before the accident. At the time of the accident, the engine had accrued 1,342 total flight hours. Maintenance records indicated that on October 14, 2016, 192 flight hours before the accident, an inspection of the engine induction system resulted in the replacement of the induction-to-intake plenum tube after a tear was found on its inner surface. On September 19, 2016, 234 flight hours before the accident, the turbocharger assembly was replaced after it seized during flight, resulting in a total loss of power and loss of oil. The event did not result in an accident and was ultimately reviewed by representatives from Austro Engines. It was determined that the turbocharger housing was damaged at an undetermined time, eventually causing the separation of the compressor wheel bolt. The bolt then struck the compressor wheel, resulting in seizure of the turbocharger. The engine was repaired, and the airplane was returned to service. On December 29, 2016, about 1230 mountain standard time, a Diamond Aircraft DA40 NG, N605SJ, was involved in an accident near Kingman, Arizona. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, during level cruise flight about 9,500 ft mean sea level (msl), he felt the airframe shake, and a few minutes later, he received an engine control unit (ECU) A and B failure annunciation. While performing the emergency checklist, he noticed that the engine oil temperature was rising. He reduced engine power and the oil temperature began to drop; however, a short time later, the oil pressure dropped to zero. Having descended to 2,500 ft above ground level, he decided to perform a forced landing into a field. During the landing roll, both wings impacted vegetation, resulting in substantial damage. Subsequent examination revealed that the belly of the airplane was covered in a black-colored oil from the engine cowling to the tail skid. Initial examination revealed that the engine had not seized. Residual quantities of oil remained in the sump and the oil filter was clear. The engine was removed from the airplane and shipped to the facilities of Austro Engines in Austria, where a complete examination was performed under the supervision of the Austrian Civil Aviation Safety Investigation Authority. A series of examination reports were prepared by Austro Engines and independent testing facilities. These reports were reviewed by a specialist from the NTSB Materials Laboratory, who concurred with the findings. External examination revealed that the outer surfaces of the crankcase at the exhaust port of cylinder No. 1 were coated in oil, and oil had also pooled in the fuel injector well just below the oil separator and breather tube assembly. The engine was dissembled, revealing damage to the No. 1 piston and its associated cylinder bore. The piston crown was covered in oil and carbon and exhibited pitting and peening damage, concentrated around its outer circumference (see figure 1). Similar pitting damage to the piston crown was observed around the circumference of the accompanying cylinder head combustion surface (see figure 2). Figure 1. No. 1 piston crown, with hole and crack. Figure 2. No. 1 cylinder head. A 5-mm-wide hole was present on the wall of the piston crown bowl, and a crack emanating from the hole, traversing aft along the piston pin axis (see figure 1). The crack continued down the side of the piston skirt, crossing the three piston ring grooves and the upper face of the piston pin bore. The left and right sides of the piston skirt exhibited vertical score marks, corresponding to similar abrasions on the adjacent areas of the cylinder bores. The intake and exhaust valves were intact and undamaged. Removal of the piston from cylinder No. 1 revealed that the 1st and 2nd combustion rings exhibited material transfer and abrasions on their sealing faces. The oil scraper ring exhibited similar damage along with carbon buildup. A section of the scraper ring, about 1/6th of the total circumference, had detached revealing the inner spring. About 1/3 of the spring had detached, and a series of spring fragments were located in the oil sump. Piston Examination Examination of the crack revealed two fatigue fracture areas with different fracture origins. One fracture origin was located at the surface of the piston crown close to the edge of the bowl and appeared to begin at a notch in the surface. The notch was similar in scale and definition as the pitting observed around the crown (see figure 3). From the notch, the crack propagated toward the back side of the piston, where it reached the ring grooves and the cooling bore. In this area, a void had developed, connecting the hole in the piston dome to the cooling bore and the two lower ring groves. The walls of the void were granular in appearance and appeared consistent with the formation of a “gas channel” by hot combustion gasses penetrating the crack and eroding the piston material (see figure 4). The second fatigue fracture area was located between the void and the back side of the piston, where it propagated to the piston pin bore (see figures 3 and 5). The fracture origin had been obliterated by the progression of the gas channel. No foreign particles were located in the combustion chamber, and a detailed review of the imprints in the piston crown and cylinder head revealed multiple notches with similar sharp edge surface features. Two pitted imprints, along with exemplar undamaged aluminum piston surface material, were examined utilizing scanning electron microscopy with energy dispersive x-ray analysis. The exemplar areas were primarily composed of aluminum. No definitive material type was identified within the imprints; however, carbon, silicon, sodium, and iron were present in the pits at higher concentrations than the surroundings. Figure 3. First fatigue crack, with notch. Propagation direction indicated by arrows. Figure 4. Piston No. 1 cross section. Figure 5. Second fatigue crack with propagation direction indicated by arrows. Small circle indicates presumed initiation point. Oval indicates end of first crack at boundary with gas path. Fuel Injector Examination The fuel injectors were examined and tested at an independent facility. All appeared undamaged and exhibited light carbon buildup at their tips which did not impeded fuel flow. All injectors displayed similar and nominal fuel delivery, and disassembly did not reveal any mechanical anomalies or contamination. Oil Analysis The engine ha