December 16, 2020 – Textron Aviation has issued a service letter requiring inspection of certain Cessna 404, 411, and 421 aircraft with Continental 520 and 550 series engines. Affected engines were manufactured, rebuilt, or modified with cross-flow cylinder replacements beginning in November 2014.

The service letter was prompted by field reports that indicated the potential for fracture beginning at the radius edge of cylinder heads in aircraft with more than 500 hours of operation. Affected cylinders can be identified by a top fin flange area above the exhaust port.

The letter requires inspection at or before the next 100-hour or annual inspection for aircraft with less than 500 hours and within the next 50 hours for aircraft with more than 500 hours. The inspection will confirm compliance with Continental Motors SB18-08. Aircraft that have not complied must be serviced according to the instructions in that service bulletin, which involves smoothing and blending of the radius edge.

Download the service letter for a list of affected serial numbers and full instructions for compliance.