ERA24LA066

Fatigue failure of the No. 4 inner main bearing cap screw and connecting rod bolts, which resulted in a total loss of engine power. Contributing to the failure were the low material strength of the inner main bearing cap screw and the increased loads on the screw due to wear in the No. 1 fuel injector.

The airplane had a sudden, total loss of engine power in flight, which resulted in the pilots conducting a forced landing to an interstate highway. The airplane struck an energized power line before touchdown and caught fire. Metallurgical examination of the diesel engine revealed the No. 4 inner main bearing cap screw and two connecting rod bolts for either the No. 1 or No. 2 cylinder fractured due to fatigue cracking. The engine manufacturer reported a history of inner main bearing cap screw failures on the accident airplane’s engine model due to certain batches of screws that were produced at the lower end of the material strength tolerance for class 8.8 strength screws. The strength of the fractured inner main bearing cap screw was tested and found to be well below tolerance. The engine manufacturer said a lower strength screw is susceptible to failure when abnormal operating conditions are encountered, such as fuel quality issues or significant deviations from the fuel system requirements. Testing of the engine’s four fuel injectors revealed the No. 1 injector had an increased injection load, most likely due to wear resulting from corrosion or fuel contamination. According to the engine manufacturer, this increase can elevate combustion peak pressure and load on the inner main bearing cap screws. Disassembly of all four injectors revealed corrosion, contamination, and wear observed on several parts of the injectors, including armature guides, control pistons, and injector housings. A fleet-wide inspection of the operator’s aircraft, fuel trucks, and fuel farms revealed no evidence of water contamination. The operator reported that they used an engine manufacturer-approved, anti-microbial fuel additive at 25-hour intervals to prevent corrosion and contamination, since diesel engines and fuel are susceptible to water and microbial contamination. The investigation was unable to determine the source of the wear found in the fuel injectors. Fatigue failure of the connecting rod bolts in this accident appears to be an isolated occurrence; the engine manufacturer reported no previous fatigue failures of connecting rod bolts for this engine model. Examination of the fractured main bearing cap screw and connecting rod bolts did not determine which fatigue crack event occurred first. However, their fracture most likely led to the complete failure of the engine. In response to the main bearing cap screw failures, the engine manufacturer issued a mandatory service bulletin (MSB) a month after the accident for the 8.8 strength main bearing cap screws to be replaced with stronger 12.9 strength screws on this engine model. In response, the European Union Aviation Safety Agency (EASA) and the FAA issued separate airworthiness directives (ADs) that mandated the replacement of the inner main bearing cap screws to the higher strength screws. According to the engine manufacturer, there have been no further occurrences of main bearing cap screw failures since the ADs were issued.

On December 14, 2023, about 2015 eastern standard time, a Diamond Aircraft Industries DA 40 NG airplane, N907L, was substantially damaged when it was involved in an accident near Asheville, North Carolina. The flight instructor sustained minor injuries, and the pilot receiving instruction was seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor and pilot receiving instruction were conducting a cross-country flight at night when they felt a “shudder,” followed by a loss of engine power. The loss of power was accompanied by a loss of oil pressure and dual engine control unit (ECU) failure. The flight instructor took control of the airplane and used the checklist to restart the engine. The flight instructor was unable to restart the engine after several attempts and made a forced landing to an interstate highway. Just before landing, the airplane struck an energized power line, then impacted the ground and caught fire. The composite airframe and the engine sustained extensive postimpact fire damage. According to recovered ECU data, the loss of engine power occurred about 30 minutes into the flight. The data showed failures in the boost pressure control system and oil pressure system, followed by rapid decline of engine rpm and oil pressure. Multiple restart attempts were logged but were unsuccessful. The engine’s oil pressure subsequently dropped to 1 psi, and propeller speed decayed to zero. The airplane was equipped with a 4-cylinder E4 diesel engine manufactured by Austro Engine GmbH, Austria. Postaccident examination revealed multiple holes in the engine block walls at cylinder Nos. 1 through 3. The Nos. 1, 3, and 4 piston assemblies exhibited heat and mechanical damage consistent with internal mechanical failure. The No. 2 piston was missing and not located. A connecting rod had separated from the engine and was found on the interstate highway where the airplane impacted the ground. However, the investigation could not determine if the rod belonged to the No. 1 or No. 2 piston, which were both missing their respective connecting rods. Due to the extensive postimpact fire, no fuel or fluids were recovered from the engine. The NTSB Materials Laboratory further examined the recovered engine components. Metallurgical examination of the components revealed signatures consistent with overstress separation, mechanical damage, and heat tinting due to lack of lubrication and postimpact fire, except for the fractured bolt fragments from the “unidentified” connecting rod and the No. 4 inner main bearing cap screw. The two fractured bolt fragments recovered from the connecting arms of the unidentified connecting rod exhibited fatigue cracks emanating from multiple origins at the root thread. The No. 4 inner main bearing cap screw had remained attached to the flat land portion of the engine block at the No. 2 main journal, and was identified as an 8.8 strength screw (P/N E4A-10-100-201). The fracture face of the screw exhibited evidence of beach marks, typical of fatigue cracking, that emanated from multiple origins at the transition radius between the head and non-threaded shank, on one side of the screw. Austro Engine reported a history of inner main bearing cap screw failures on the accident airplane’s engine model due to certain batches of screws that were produced at the lower end of the material strength tolerance for class 8.8 strength screws. The screw’s chemical composition and strength were tested to determine if it met industry and manufacturer specifications. According to International Organization for Standardization (ISO) standards, an 8.8 strength screw, which has a diameter =16 mm, should fall between 22–32 on the Rockwell Hardness “C” Scale (HRC). Rockwell “C” hardness testing of the fractured screw revealed an HRC of 19 near the fracture surface. According to ASTM International standards, this hardness value is extremely low on the HRC scale, and it cannot be reliably converted to an approximate tensile strength. Rockwell “A” hardness testing covers hardness values that are lower than 20 HRC and can be converted to an approximate tensile strength value. The Rockwell “A” hardness testing (HRA) of the fractured screw produced an average hardness of 60 HRA, which converted to about 110,000 psi (758 N/mm²), less than the minimum tensile strength for the screw, which is 800–950 N/mm² per manufacturer specifications. Of the remaining nine undamaged main bearing cap screws recovered from the airplane wreckage, five of the screws tested within hardness standards, and four tested just below standards at 21 HRC. The chemical composition of all 10 screws tested within the elemental limits specified by the engine manufacturer. Examination of the fractured main bearing cap screw and connecting rod bolts did not determine which fatigue crack event occurred first. Austro Engine reported no previous fatigue failures of connecting rod bolts on E4 model engines. To test for possible deviations in fuel delivery to the engine that may have contributed to an abnormal operating condition and failure of the No. 4 inner main bearing cap screw, the engine’s four fuel injectors were shipped to Austro Engine for testing. The four fuel injectors were placed on a test bench and flow tested electrically and hydraulically according to the manufacturer’s test instructions. The No. 1 injector showed a slight increase in pre-injection quantity, and the other three injectors tested normally. Austro Engine reported that the increase in peak pressure on the No. 1 injector could increase combustion peak pressure and the associated load on the main bearing cap screws. Slightly reduced injection quantities were measured for the No. 3 and No. 4 injectors at the full load point; however, Austro Engine stated that this slight reduction would not have had a negative effect on the operation and service life of the engine. All four injectors were disassembled, and corrosion, contamination, and wear were observed on several parts of the injectors, including armature guides, control pistons, and injector housings. According to Austro Engine, a small indentation on the conical surface of the No. 1 injector was most likely due to contaminated fuel particles. The throttles of all 4 injectors exhibited no significant signs of cavitation erosion. However, some rough spots were observed on each of the injector throttles on the outer ring where the ball valve contacted the throttle surface. According to Austro Engine, this type of wear may have been due to the ball not properly seating during normal engine operation, possibly due to fuel contamination or corrosion. If the ball valve does not seat completely inside the injector, fuel can continue to enter the combustion chamber and result in uncontrolled start and end of injection. This can lead to a higher combustion chamber temperature, thereby subjecting the pistons to a greater thermal load, as the firing cycle lasts longer. To determine if the corrosion and wear found in the injectors affected the structural integrity of the pistons, the Nos. 1, 3 and 4 pistons were sent to Austro Engine for hardness testing. Testing revealed no significant loss of strength due to injector performance. The accident airplane operator conducted a fleet-wide inspection of their aircraft, fuel trucks, and fuel farms and found no evidence of water contamination. The operator reported that they used an anti-microbial fuel additive (Biobor JF, which was approved by the engine manufacturer) at 25-hour intervals to prevent corrosion and contamination, since diesel engines and fuel are susceptible to water and microbial contamination. In response to the main bearing cap screw failures, Austro Engine issued an MSB dated January 31, 2024, which stated: Occurrences of E4 series engines have been reported recently. Investigati