ERA24LA066 · Diamond Aircraft IND Inc DA 40 NG

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.

Verbatim NTSB analysis and probable cause from the NTSB dataset