In what scenario is an engine service bulletin not mandatory?

An engine service bulletin is considered non-mandatory in scenarios where it does not pertain to ongoing air carrier operations. This means that if the bulletin is issued for maintenance recommendations or enhancements that do not affect the operational safety or reliability of an aircraft currently in commercial use, operators are not required to comply with those recommendations.

Service bulletins are typically recommendations made by manufacturers to improve engine performance or resolve known issues, but they become mandatory only when tied directly to safety and compliance regulations that govern air carrier operations. Thus, if an operator does not have an aircraft involved in air carrier operations, they have discretion on whether to follow the bulletin or not.

In contrast, other choices reference conditions where a service bulletin might be treated differently. For instance, if a bulletin is referenced by an airworthiness directive, it becomes mandatory due to regulatory requirements. Disregarding a bulletin as a pilot does not exempt an operator from its intent, and a requirement for extensive downtime might be a consideration but does not eliminate the necessity of compliance if safety is at stake.