On January 14, 2022, about 1248 mountain standard time, an experimental amateur-built Zenith CH 750 (Cruzer), N145WT, was substantially damaged when it was involved in an accident near Spanish Fork, Utah. The pilot was seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that this was his first flight in the accident airplane since he was involved in a landing incident that took place in the same airplane about 18 months earlier. Before the accident flight, he fast-taxied the airplane down the runway to check the operability of the flight control system and engine controls and did not observe any anomalies. Shortly after takeoff when the airplane was about 5 nautical miles away from the airport, the pilot observed a slow decay in engine power and was suddenly unable to maintain altitude. He immediately started a left turn to return to the airport and he advanced the throttle to the full power setting but was unsuccessful in restoring power to the engine. According to a witness, the airplane entered a steep left turn at approximately 80 ft above ground level that quickly transitioned into a nose-down dive before it disappeared from view behind obstacles. The pilot stated that the airplane slowed during the turn and impacted the roof of a building. The airplane sustained substantial damage to the fuselage and both wings. The airplane was equipped with a Honda VTEC automotive engine. Postaccident examination revealed that mechanical continuity was established throughout the rotating group, reduction gearbox, engine flywheel, crankshaft and accessory section as the crankshaft was manually by hand rotated at the prop hub. Thumb compression was achieved at all four cylinders. Examination of the cylinders combustion chamber interior components using a lighted borescope revealed normal piston face and cylinder wall signatures, and no indications of a catastrophic engine failure. The ignition coils were tightly secured to their respective spark plugs and the ECU harnesses were connected to each coil. The coils were all normal in appearance and did not exhibit any debris or discoloration when visually inspected. All four of the spark plugs were gray in appearance, consistent with normal wear and the center electrodes were unremarkable. A postaccident engine run did not reveal any preimpact anomalies. An inspection of the elevator, flaperon, and rudder controls did not reveal any abnormalities. The engine was equipped with a catalytic converter that was used to convert toxic exhaust gases produced during combustion. The converter was comprised of a honeycomb ceramic substrate secured within the case that directed the exhaust gas airflow towards the tailpipe. Although the converter remained securely attached to the engine case, the bottom half of the internal ceramic substrate had broken into numerous large pieces. Figure 1. Interior catalytic converter substrate as observed through tailpipe Figure 2. Top and bottom halves of catalytic converter with exposed substrate (top is on the left side, bottom on the right side) The pilot reported that following a landing incident that occurred 18 months before the accident, he noticed gray fragments coming from the tailpipe, which was bent as a result of impact damage. The pilot subsequently repaired the tailpipe by straightening it and re-welded it back to the catalytic converter. The pilot reported he noticed small white particles come out of the tailpipe the first time he started the engine after the incident and then once again a piece that was the size of a quarter to a half-dollar came out on the second or third engine start. He did not inspect, repair, or replace the catalytic converter before the accident flight. According to a representative of the engine manufacturer who reviewed the engine examination report, as backpressure is required for the engine to function, an obstructed exhaust can affect engine backpressure and result in a partial loss of engine power. The engine kit manufacturer, and company responsible for retrofitting the automotive engine for aviation applications, also stated that an obstructed catalytic converter could prevent the engine from producing power.