On November 19, 2024, at 1851 eastern standard time, a Cessna 172S, N951JA, sustained substantial damage when it was involved in an accident near Chattanooga, Tennessee. The flight instructor was not injured and the private pilot sustained minor injuries. The flight was conducted as a Title 14 Code of Federal Regulations Part 91 instructional flight. The private pilot was receiving instruction in preparation for his instrument rating practical test. He conducted a preflight inspection and engine run-up, including a check of each magneto, prior to departure and no anomalies were noted. The private pilot said that shortly after takeoff from runway 20 at Lovell Field Airport (CHA), Chattanooga, Tennessee, the engine’s rpm dropped to 2,300 rpm. He declared an emergency with air traffic control (ATC) and advised they were in instrument meteorological conditions (IMC) and needed to return to land. The air traffic controller provided the pilot with radar vectors to land on runway 2, but once he and the flight instructor visually acquired the runway, the airplane was too fast, so they executed a go-around to avoid trees. The private pilot said that as the airplane climbed above the trees, the engine lost total power, and their only option was an off-airport landing to a road. During the landing, the airplane struck trees and came to rest on the side of the road resulting in substantial damage to both wings and the fuselage. There was no postimpact fire. The flight instructor said that when the private pilot declared an emergency the engine instruments were in the “green” and engine rpm was about 2200 to 2300 rpm. Air traffic control began to vector them to runway 33, but they took a wrong turn, and they were then vectored to runway 2; however, they were in IMC which made it hard to see the runway. The instructor said that when they turned on final approach to the runway, they could not see the runway environment until they were halfway down the runway. At that time, the airplane was flying too fast. She took control of the airplane, and added full power to go-around since the engine still “had some power left.” The instructor initiated a climb, but the airplane’s airspeed began to drop as she tried to turn back toward the runway. The instructor said, “this is when the engine quit” and the airplane’s airspeed dropped to 40 knots. The airplane descended nose down into trees and impacted the ground. The airplane was equipped with a Garmin G-1000 primary and multi-function flight displays, which recorded flight and engine performance data. The data revealed the airplane departed runway 20 at 1840:10, at a power setting of 2,500 rpm (maximum engine rpm for this airplane was 2,700 rpm). The cylinder head temperature (CHT) of the No. 4 cylinder began to rapidly increase after takeoff. Around 1841, engine rpm fluctuated between 2,200 to 2,300 rpm as the airplane was being vectored to runway 33. At 1849:20, as the airplane was attempting to land, engine power decreased to about 963 rpm. At 1850:00, engine rpm suddenly increased to about 2,135 rpm, the airplane began to climb, and it turned to the northwest. Over the next 46 seconds, engine rpm remained between 2,000 at 2,150 rpm; however, the airplane’s pitch attitude increased and the indicated air speed dropped to about 40 knots before the data ended at 1850:56. Postaccident examination of the engine revealed no evidence of catastrophic failure. The propeller was manually rotated, and compression/valve train continuity were established on each cylinder. Continuity was also established to the accessory section. Both magnetos were properly timed in accordance with the engine data plate. After removal of both magnetos, it was determined the right magneto impulse coupling was binding at the latching end of the pawls and stop-pin where normal force should have released the latching end of the pawl. Additionally, the nut on the magneto rotor shaft was only finger tight with the cotter pin installed. The magneto, which had been recently overhauled, was installed on the engine the same day of the accident flight. According to the installation manual, the nut on the magneto rotor shaft should have been torqued to 120-320 in/lbs. The magneto was retained and shipped to the manufacturer for testing. The body of the impulse coupling was removed, and the spring was found to be distorted. The spring was not engaged with the slot of the hub; however, it was still able to rotate normally. An exemplar spring and hub were mounted on the magneto and attached to the test bench. The magneto was tested up to 2,700 rpm with normal spark between all gaps. No anomalies were noted. The magneto was removed from the test bench and re-fitted with the accident spring and hub. There was no audible impulse action when turned by hand. The magneto was attached to the test bench and was tested up to 2,700 rpm with normal spark and no anomalies noted. The reason for the spring distortion could not be determined, and the binding of the impulse coupling could not be duplicated. Internal timing of the magneto was checked and found within specifications. The spark plugs were removed and displayed various levels of wear. A small piece of metal was found wedged between the center electrode and prongs on the No. 4 cylinder’s bottom plug. The spark plugs were examined at the NTSB Materials Laboratory, and the metal particle wedge in the No. 4 bottom plug was determined to consist primarily of lead. Resistance measurements were also taken on each of the spark plugs to see if there was a short circuit between the two electrodes. The center electrode resistance measurements were all nominally around 1 kohm. This resistance was within the manufacturer’s acceptable resistance range. The center electrode to ground electrode resistance measurements were all above 11 Gohm (the meter’s maximum reading) indicating that there was no electrical path between the electrodes. Only the bottom spark plug for cylinder No. 4 had a lower resistance of approximately 100 Mohm indicating that there was an electrical path between the two electrodes. A review of the engine maintenance log revealed that the spark plugs were last “removed, inspected/cleaned, and re-installed” on October 17, 2024, at the airplane’s last 100-hr inspection, about 87 hrs. prior to the accident. The fuel manifold, respective fuel injector lines, and fuel injector nozzles were removed and found absent of any debris. The fuel injector servo remained installed and undamaged to the bottom of the engine. The fuel injector servo was sent to the manufacturer for testing. The fuel injector servo was flow tested, and the results were within service limits. The unit was disassembled and no mechanical anomalies were observed that would have precluded normal operation. All four of the engine cylinders were removed and displayed a lack of combustion residue within their respective combustion chambers. The tops of all four pistons lacked combustion residue. During the removal of the No. 4 cylinder, the piston pin plug was difficult to remove and heat discoloration was evident on the small end of the connecting rod, piston pin, and piston pin plugs. Similar discoloration of the piston pin and small end of the connecting rod of the No. 2 cylinder was observed. The piston for the No. 4 cylinder displayed signatures of metal erosion and the oil control ring was partially seized. The engine oil filter contained minor non-metallic metal debris. The oil suction screen was absent of debris. According to the airplane’s Before Takeoff Checklist, a functional test of the ignition system is to be performed by switching between the individual magnetos from the “both” position on the ignition switch. The checklist stated to bring the throttle to 1,800 rpm and check each magneto position for a maximum drop of 150 rpm, and a maximum difference of 50 rpm between the left and right magneto. Both pilots said that they tested the magnetos prior to takeoff and both were within limitations. A review of the engine operation manual stated that to conduct a proper check of the magnetos, the pilot should increase engine power to 1,800 rpm, and “Switch from both magnetos to one and note the drop off; return to both until engine regains speed and switch to the other magneto and note the drop off, then return to both. Drop-off must not exceed 175 rpm and must not exceed 50 rpm between magnetos.” Review of the G-1000 engine data revealed that engine power was increased prior to takeoff to about 1,800 rpm; however, the data was unclear if the private pilot or instructor allowed the engine rpm to stabilize during the magneto check to get an accurate measurement.