N907L

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. Investigation of the occurrences has shown in some cases the failure of one inner main bearing cap screw. Subsequent investigations have determined that certain batches of inner main bearing cap screws were produced at the lower end of the material strength tolerance for Class 8.8 screws. Depending on the magnitude of the screw’s strength properties, the potential for screw failure, leading to engine failure, exists in cases where abnormal operating conditions are experienced such as fuel quality issues or significant deviations from the fuel system requirements. To address this possible unsafe condition, this Service Bulletin requires the replacement of the inner main bearing cap screws, with screws having highest safety factor.

The replacement screw specified in the MSB is a class 12.9 screw, having P/N E4A-10-100-202, where the head portion of the replacement screw is identified by the imprint “12.9.”

EASA, which is the regulatory authority for Austro Engines, issued AD 2024-0037R1 on February 6, 2024. The AD enforced Austro Engine MSB-E4-042, which required immediate replacement of the 8.8 strength main bearing cap screws with the 12.9 strength screws.

On February 7, 2024, Diamond Aircraft Industries issued Service Information Letter SI40NG-090 in response to Austro Engine MSB-E4-042.

On May 5, 2024, the FAA issued AD 2024-05-01, which mandated the replacement of the 8.8 strength main bearing cap screws to the 12.9 strength screws for E4 model engines operating in the United States.

Austro Engine reported that there have been no reported failures of the 12.9 strength main bearing cap screws since the ADs were issued.