On December 12, 2019, about 1900 Pacific standard time, a Cessna 182P airplane, N182EF, sustained substantial damage when it was involved in an accident near Carlsbad, California. The private pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the engine started to run rough during the approach to McClellan-Palomar Airport (CRQ), Carlsbad, California. About a minute later, despite the pilot’s troubleshooting, the engine roughness increased. The pilot noticed the engine monitor showed normal operating indications; however, the alternator voltage light illuminated, and the engine continued to run rough. The pilot stated he saw two “flash explosions” from the left side of the engine and propeller area followed by the smell of oil in the cabin, and the engine lost total power. The pilot subsequently performed a forced landing to the southbound side of Interstate 5. The airplane landed hard, and the nose landing gear collapsed. Postaccident examination of the airplane revealed that the aft fuselage was buckled near the cabin door area, and oil covered the belly of the airplane. There were two holes in the engine crankcase between Nos. 5 and 6 cylinders, and it was noted that a supercharger was attached to the accessory section of the engine. Review of the engine logbook showed that the Continental O-470-U engine was overhauled and converted to a PPA O-470-50 engine per engine Supplemental Type Certificate (STC) SE4985NM by P. Ponk Aviation on December 20, 2018. The conversion changed the cylinders to those used on Continental IO-520 engines and increased engine displacement by 50 cubic inches. The PPA O-470-50 engine was shipped to the owner and installed on the airplane on February 15, 2019. At the time of the accident, the engine had accumulated about 150 hours since the overhaul and conversion. An engine logbook entry dated September 4, 2019, noted that the engine oil and supercharger fluid were changed about 50 hours of operation before the accident, and an entry dated November 4, 2019, noted that the engine oil was changed and the supercharger belt was checked about 2 hours of operation before the accident. Review of STC SE4985NM showed that a supercharger installation was not part of the modification, nor were maintenance procedures outlined for one. Additionally, no logbook entries or documentation regarding the installation of the supercharger were found. Disassembly of the engine revealed that the Nos. 5 and 6 connecting rods were separated from the crankshaft. Connecting rod parts and metal fragments were found in the oil sump. The crankshaft was found separated into three sections. One of the separations was at the crankshaft cheek forward and adjacent to the No. 3 bearing journal, and the separation surfaces displayed beach marks consistent with fatigue cracking. According to a metallurgical exam conducted by Continental Aerospace Technologies (CAT), the fracture appeared to initiate below the outer nitride layer near an oxide inclusion within the metal.