Accident Narrative
On December 14, 2023, about 2015 eastern standard time, a Diamond Aircraft Industries DA 40 NG airplane, registration N907L, sustained substantial damage during an accident near Asheville, North Carolina. The flight instructor received minor injuries, and the pilot receiving instruction was seriously injured. The flight was conducted as a Title 14 Code of Federal Regulations Part 91 instructional flight.
The flight instructor and the pilot receiving instruction were performing a cross-country flight at night when they perceived a shudder, followed by a loss of engine power. This loss of power was accompanied by a loss of oil pressure and dual engine control unit (ECU) failure. The flight instructor assumed control and attempted to restart the engine using the checklist. After several unsuccessful restart attempts, the instructor executed a forced landing onto an interstate highway. Just prior to touchdown, 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.
Data recovered from the ECUs indicated that the loss of engine power occurred approximately 30 minutes into the flight. The data showed failures in the boost pressure control system and oil pressure system, followed by a rapid decline in engine rpm and oil pressure. Multiple restart attempts were recorded but were unsuccessful. Subsequently, engine oil pressure dropped to 1 psi, and propeller speed decayed to zero.
Engine Examination
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, as both were missing their respective connecting rods. Due to the extensive postimpact fire, no fuel or fluids were recovered from the engine.
Metallurgical Findings
The NTSB Materials Laboratory examined the recovered engine components. Metallurgical examination revealed signatures consistent with overstress separation, mechanical damage, and heat tinting due to lack of lubrication and postimpact fire, except for fractured bolt fragments from the “unidentified” connecting rod and the No. 4 inner main bearing cap screw. The two fractured bolt fragments 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 (part number E4A-10-100-201). The fracture face of the screw exhibited beach marks typical of fatigue cracking, emanating 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 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. According to International Organization for Standardization (ISO) standards, an 8.8 strength screw with 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. Per ASTM International standards, this hardness value is extremely low and cannot be reliably converted to an approximate tensile strength. 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 (800–950 N/mm² per manufacturer specifications).
Of the remaining nine undamaged main bearing cap screws recovered from the wreckage, five 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.
Fuel Injector Analysis
To test for possible deviations in fuel delivery, the engine’s four fuel injectors were shipped to Austro Engine for bench testing per manufacturer’s test instructions. The No. 1 injector showed a slight increase in pre-injection quantity; the other three tested normally. Austro Engine reported that the increase in peak pressure from the No. 1 injector could increase combustion peak pressure and associated load on the main bearing cap screws. Slightly reduced injection quantities were measured for the Nos. 3 and 4 injectors at the full load point, but Austro Engine stated that this would not have had a negative effect.
All four injectors were disassembled, and corrosion, contamination, and wear were observed on several parts, 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 likely due to contaminated fuel particles. The throttles of all four injectors exhibited no significant signs of cavitation erosion, but some rough spots were observed on the outer ring where the ball valve contacted the throttle surface. Austro Engine noted that such wear may have been due to the ball not properly seating during normal operation, possibly from fuel contamination or corrosion. If the ball valve does not seat completely, fuel can continue to enter the combustion chamber, resulting in uncontrolled injection timing, potentially leading to higher combustion chamber temperature and greater thermal load on the pistons. The Nos. 1, 3, and 4 pistons were sent for hardness testing, which revealed no significant loss of strength due to injector performance.
The accident airplane operator conducted a fleet-wide inspection of aircraft, fuel trucks, and fuel farms and found no evidence of water contamination. The operator reported using an anti-microbial fuel additive (Biobor JF, approved by the engine manufacturer) at 25-hour intervals to prevent corrosion and contamination.
Regulatory Actions
In response to the main bearing cap screw failures, Austro Engine issued Mandatory Service Bulletin (MSB) dated January 31, 2024, stating that certain batches of inner main bearing cap screws were produced at the lower end of material strength tolerance for Class 8.8 screws. The MSB required replacement of the inner main bearing cap screws with screws having the highest safety factor.
EASA issued Airworthiness Directive (AD) 2024-0037R1 on February 6, 2024, enforcing Austro Engine MSB-E4-042, which mandated immediate replacement of the 8.8 strength screws with 12.9 strength screws. On February 7, 2024, Diamond Aircraft Industries issued Service Information Letter SI40NG-090 in response. On May 5, 2024, the FAA issued AD 2024-05-01, mandating the same replacement for E4 model engines operating in the United States. Austro Engine reported no failures of the 12.9 strength screws since the ADs were issued.