Amateur-built SLING 4 TSI accident near San Pedro, California, September 25, 2023

The NTSB's narrative final · quoted from the NTSB record

The pilot and passenger departed on a local flight and were returning to land when the accident occurred. While on final approach to land, the pilot reported to air traffic control that the engine lost power. The pilot was cleared to land. A witness video recorded the airplane as it flew north trailing white smoke. About 12 seconds later the smoke ceased and the airplane began a right descending turn. The airplane then entered a steep right turn in a nose-down attitude and rapidly descended until it disappeared from view. The airplane impacted a field about 3 nm from the destination airport. Postaccident examination of the engine revealed internal catastrophic damage and extensive damage to the No. 4 cylinder. Both spark plugs were mechanically damaged and both the intake and exhaust valves had separated at the stems. The cylinder wall and piston face were mechanically damaged, and the piston head was also breached. Data from the engine control unit recorded 350 knock (detonation) events at cylinder No. 4 in the previous 3 hours and 41 minutes of engine operation. A warning in the maintenance manual states that oil residues are permissible on up to 20% of the running surface, else the valve may be at risk of failure at the weld point due to overheating. A metallurgical examination revealed discoloration from oil residue along the running surface of cylinder 4 that was three times greater than the maximum permissible tolerance prescribed by the manufacturer. The discoloration indicated the valve stem was subjected to high heat signatures and likely overheated. The damage to the cylinder indicates that the exhaust valve failed first and damaged the cylinder. Engine data suggests the No. 2 cylinder also failed when metal fragments entered it, damaging the piston and cylinder head. Cylinder No. 2 sustained less damage than cylinder No. 4, which suggests that the No. 4 cylinder failed first. Video of the accident flight also showed white smoke trailing the airplane about one minute before the accident. The presence of the white smoke was likely due to the No 4 cylinder exhaust valve overheating. Impact damage to the No 4. cylinder exhaust valve precluded determining if the valve failed in fatigue or overstress. The No. 4 cylinder exhaust valve contained oil residue on the running surface of the stem that exceeded the tolerance by 300%. In addition, the distance between the valve stem and valve guide for cylinder Nos. 3 and 4 exceeded the 100% tolerance from the manufacturer. The lack of heat dispersion likely caused the No. 4 cylinder exhaust valve to overheat and ultimately fail, which resulted in engine failure. Although the operator failed to comply with the engine manufacturer’s oil inspection and maintenance schedule required by their fuel blend of automotive fuel and 100LL aviation grade gasoline, there was insufficient evidence to determine that their fuel blend practice was responsible for accelerating the valve wear.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.