Saginaw, MI, USA
N5806J
Cessna A185
The pilot was in cruise flight in a single-engine airplane when the engine lost power. He selected a road for the forced landing, during which the airplane impacted a power pole and electrical wires. The airplane came to rest inverted and incurred substantial damage to the airplane’s fuselage and wings. Postaccident examination revealed that the engine’s crankshaft had failed due to fatigue. The accident crankshaft was subject to a mandatory service bulletin (MSB) and subsequent Federal Aviation Administration airworthiness directive (AD) requiring inspection of the crankshaft material for metallurgical anomalies; however, this inspection was not performed on the accident airplane’s crankshaft. A detailed metallurgical examination of the crankshaft revealed that a fatigue crack had progressed through most of the crankshaft web cross section, consistent with relatively low-stress, high cycle fatigue. Because no indications of intergranular fracture or large areas of overstress fracture with the planar dimpled rupture features indicative of thermal damage on the fracture surface were noted, it is unlikely that manufacturing issues resulted in the fatigue cracking even though metallurgical testing revealed that the crankshaft material did not meet specification. Rather, the cracking likely initiated due to stray or excessive wear to the forward journal to web radius. It is further likely that a misalignment during a previous reassembly of the engine that occurred about two years prior when two cylinders were replaced resulted in the failure. Given this information, it is likely that maintenance personnel’s failure to properly align the affected crankshaft during the reassembly resulted in a low-stress high cycle fatigue failure of the crankshaft and the subsequent loss of all engine power.
On August 3, 2019, about 1000 eastern daylight time, a Cessna 185 airplane, N5806J, was substantially damaged when it was involved in an accident near Saginaw, Michigan. The pilot and three passengers received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that during cruise flight, the engine lost power. He selected a road for a forced landing; however, the airplane impacted a power pole and electrical wires during the forced landing. The airplane came to rest inverted and incurred substantial damage to the fuselage and wings. The wreckage was moved to an aircraft salvage facility near Springfield, Tennessee, where an examination was conducted by the NTSB Investigator-in-Charge (IIC) and a technical representative from the engine manufacturer. At the time of the examination, the airplane’s tachometer read 1,556.34 hours. The airplane was equipped with a factory-rebuilt Continental IO-520-D reciprocating engine, serial number 812972-R; the serial number on the crankshaft flange was F109804N. The examination noted that the engine could not be rotated by hand. Disassembly of the engine found a broken crankshaft. The broken crankshaft was sent to the NTSB Material Laboratory in Washington, DC, for a detailed examination. The web fracture surface was examined using a scanning electron microscope (SEM). The exam found fatigue striations consistent with fatigue crack propagation. Examination of the fatigue crack initiation sites revealed that many of the initiation sites were obliterated by post-fracture smearing; however, an undamaged site revealed no indications of material defects, such as inclusions, corrosion pits, or voids. There were some indications of streaks or microwear marks along the fracture surface edge. Some parallel microcracks were present in this initiation site. In April 2000, Teledyne Continental Motors issued Mandatory Service Bulletin (MSB) 00-05D to address 11 crankshaft fractures that occurred as a result of manufacturing defects. The MSB provided instructions for obtaining crankshaft core samples for metallurgical evaluation. Crankshafts that did not meet material specifications were required to be replaced with a serviceable crankshaft before further flight. The Federal Aviation Administration (FAA) subsequently issued Airworthiness Directive (AD) 2000-23-21, effective December 12, 2000, requiring compliance with the inspection specified in the MSB. The crankshaft serial number for the accident engine appeared on the MSB page list of, “Noncompliant affected service spare crankshaft by part number – no record of compliance with MSB-00-5 or [Airworthiness Directive] AD 2000-23-21”. Review of the engine maintenance records revealed an entry dated May 10, 2010, that stated, “Continental Service Bulletin MSB00-5 DNA [Does Not Apply] by serial number.” The crankshaft serial number appeared on the MSB page list of “Non-compliant affected service spare crankshaft by part number – no record of compliance with MSB-00-5 or [Airworthiness Directive] AD 2000-23-21.” Further review of the original MSB 00-5 in the table labeled; “Affected engine models by engine serial number and crankshaft serial number”, did not find any reference to the accident engine or crankshaft serial number. However, in the section labeled: “Affected crankshafts by crankshaft part number and serial number”, the accident crankshaft’s serial number was listed on page 14. The maintenance records also contained an entry dated August 11, 2017, which revealed that the Nos. 2 and 6 cylinder assemblies were replaced with new assemblies at a tachometer reading of 1,516.3 hours. A section of the crankshaft’s propeller flange was sent to Continental, for additional testing in accordance with MSB-00-5. The test sample did not pass the metallurgical exam, and if the testing had been accomplished before the accident in accordance with the MSB, the crankshaft would have been required to be removed from service.
Maintenance personnel’s failure to properly align the crankshaft during an engine reassembly which led to a fatigue failure of the crankshaft and the loss of all engine power.
Source: NTSB Aviation Accident Database
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