CEN22LA334

The pilot of the airplane stated that after departure he heard a “very loud pop” from the engine and then saw smoke enter the cockpit. After the smoke cleared, he decided to perform an off-airport landing even though the engine continued to operate. He stated that he did not check the engine gauges to evaluate any anomalies. The pilot reported that the engine was operating after he heard the “loud pop” noise. During the approach to the field, the pilot added engine power to avoid hitting power lines. The airplane impacted a soccer goal, a streetlight, and a portable toilet, which resulted in substantial damage to the fuselage and right wing. Postaccident examination of the engine found the spark plug wire boot for the No. 6 cylinder was not connected to the spark plug and was found resting on the exhaust pipe. The rubber boot likely sustained thermal damage when it contacted the exhaust pipe; there were no other mechanical anomalies that would have precluded normal operation. A review of the maintenance records showed that 79.7 hours had accumulated on the engine since the last inspection, which was a condition inspection. It possible that the spark plug wire boot was not secured during the last maintenance work performed. Based on the available records, investigators were not able to determine why the spark plug wire boot was unsecured.

On July 22, 2022, about 1647 central daylight time, a Van’s Aircraft RV-10 airplane, N4400K, sustained substantial damage when it was involved in an accident near Beaver Dam, Wisconsin. The pilot and one passenger sustained no injuries and one passenger sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight. The pilot reported that after take off from the Dodge County Airport (UNU), Juneau, Wisconsin, he heard a “very loud pop” from the experimental engine. Smoke entered the cockpit, but then the smoke cleared. The amount of smoke in the cockpit did not inhibit the pilot from performing visual functions. The pilot decided to perform an off-airport landing to a field. During the landing, the pilot was able to add power to the engine to avoid impacting power lines. After landing, the right wing impacted a soccer goal post. The airplane continued with the rollout and impacted a streetlight and a portable toilet. The airplane came to rest upright, and all three occupants were able to egress from the airplane without further incident. The airplane sustained substantial damage to the fuselage and the right wing. After the accident, the pilot reported that the engine was operating after he heard the “very loud pop;” however, he did not confirm this with the cockpit gauges or try to maintain level flight. A postaccident examination of the engine found that the lower spark plug wire boot for the No. 6 cylinder was not connected to the spark plug. The rubber boot had signatures consistent with the boot being burned on the outside of the 90° bend on the boot, and at the end of the boot, where the boot likely contacted the exhaust pipe it was resting on. The boot was burned enough so the electrode was able to contact the exhaust pipe. There were no other areas in the engine that showed signs of burning. Airframe to engine control continuity was established. Except for the burned spark plug wire boot, there were no preimpact mechanical malfunctions or failures that would have precluded normal operation of the engine. A review of the maintenance records showed that on March 22, 2022, a condition inspection was performed on the airplane. For the engine, a compression check was satisfactorily performed by a mechanic. For both the airframe and the engine, the entries stated that the condition inspection was performed in accordance with 14 CFR Part 43 Maintenance, Preventive Maintenance, Rebuilding, and Alteration, and the “manufacturer’s maintenance and inspection procedures.” According to information provided by the pilot, the engine had accumulated 79.7 hours since the inspection work.