N4139A

On August 29, 2018, about 1840 mountain standard time, a Piper PA-28-181, N4139A, was substantially damaged when it was involved in an accident in Tat Momoli, Arizona. The private pilot and pilot-rated passenger were not injured. The airplane was operated by the pilot-rated passenger as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that they departed Nogales International Airport (OLS), Nogales, Arizona about 1710, with full fuel after completing an engine run-up. About 25 minutes into the flight, and about 100 nautical miles (nm) northwest of OLS, the engine started shaking violently and sputtering. Neither pilot observed an illumination of any annunciator lights. The pilot turned the ignition to the OFF position and retarded the mixture control to the idle setting. The pilot selected a road and made a turn to execute an emergency landing, when the engine exploded, and oil began to leak from the top of the cowling. The airplane touched down uneventfully, but during the landing roll, the right wing impacted a barbed wire fence before it departed the right side of the road and came to rest. According to the pilot, the airplane's flight control capabilities were uninhibited during the accident flight.

Postaccident photos furnished by local law enforcement revealed an approximate 3 ft long depression to the outboard right wing. Additionally, the top of the engine cowl was breached and displayed a streak of oil that terminated at the windshield.

According to Federal Aviation Administration (FAA) records, the airplane was manufactured in 1998 and was powered by a Lycoming O-360-A4M, 225-horsepower, air cooled, reciprocating engine. A review of the airplane's maintenance records revealed that the most recent 100-hour inspection was performed on March 1, 2018, at 5,510 hours total time in service about 42 flight hours prior to the accident. At the time of the inspection, the engine had accumulated a total of about 1,564 flight hours. An engine overhaul was completed on January 20, 2018, about 42 flight hours prior to the accident. Maintenance records indicated that the engine was disassembled, cleaned and inspected per "Overhaul Manual 60294-70", a publication by Lycoming Engines, and re-assembled after numerous items were repaired, inspected or replaced.

Among the items that were overhauled were the connecting rods and lifters. A work order showed that the connecting rods were sent to a facility where they were magnafluxed, rebushed, inspected, and balanced in accordance with the overhaul manual. They were then returned to the overhaul facility and installed on the engine.

The engine was examined at a facility in Phoenix, Arizona by a certified airframe and powerplant mechanic, with oversight provided by a representative of the FAA. Photographs taken after the accident revealed a large hole in the top of the engine case at the cylinder no. 4 position, and a large tear at the top of the cylinder. Each of the accessories were in good visual condition and did not exhibit any abnormal wear when inspected; however, propeller to accessory gear continuity could not be achieved as the engine was seized.

The crankshaft showed no signs of heating, oil starvation or scoring outside of minor fretting shown in the cylinder no. 4 journal. A large amount of metallic debris was observed in the oil sump that had originated from the no. 4 cylinder, which displayed abnormal wear. The other cylinders, associated rods and pistons did not exhibit any abnormal wear. The piston to cylinder no. 4 was locked in the upper half of the cylinder and displayed a tear in the barrel from the connecting rod. Further, the piston was displaced from the connecting rod and piston pin, which had separated. The connecting rod was twisted, and the bushing, which was found in the engine case, was deformed. The no. 4 exhaust valve keeper was missing, and the cylinder had a piece of the piston ring in the exhaust and intake valve ports.

A metallurgical examination of the no. 4 piston, connecting rod, and associated hardware was performed by the NTSB materials laboratory. The crankshaft side of the connecting rod assembly was generally intact, while the pin side of the rod had broken into multiple pieces, and the neck of the rod was bent. The piston pin bore surface remnant section of the connecting rod (opposite the crankshaft side) displayed a bend in the surface. Further examination showed that the bore surface facing the pin and internal bushing was heavily gouged, battered, and scraped, consistent with post-fracture impact damage with some of the adjacent parts.

A close-up view of one of the fractured sides of the pin bore showed gouging in multiple directions, batter of the fractured corner, and material flow from heavy deformation of the pin bore face. It was noted that any fracture features that may have been present were obliterated by post-fracture impact with the adjacent parts.

One of the undamaged fracture surfaces of the connecting rod pin bore walls exhibited crack arrest marks that were examined using a scanning electron microscope. The fracture displayed fatigue striations that were consistent with fatigue crack propagation. These striations exhibited varied spacing consistent with spectrum loading in high cycle applications, typical of reciprocating engines. The initiation area of the pin bore wall remnant included visual features and fatigue striations consistent with propagation from the inner face of the pin bore corner of the connecting rod.

The bore remnant was sectioned about 0.25 inches away from and oriented parallel to the fatigue fracture surface. The connecting rod microstructure exhibited a structure consistent with tempered martensite with an area that had not been tempered. Energy dispersive x-ray spectroscopy and x-ray fluorescence were used to examine the part chemistry of the connecting rod composition, which was consistent with alloy steel. It was noted that the tempered martensite microstructure is common for this type of steel alloy.

The bushing remnant that was present in the connecting rod pin bore exhibited severe plastic deformation along with gouges and impact marks. This damage was consistent with repeated impact with adjacent components.