Final answer:
Compliance with an engine service bulletin is mandatory when it is included in an Airworthiness Directive issued by the FAA or similar authority.
Step-by-step explanation:
Compliance with an engine service bulletin is typically mandatory when the bulletin has been incorporated into an Airworthiness Directive (AD) issued by the Federal Aviation Administration (FAA) or a similar regulatory authority. Manufacturer service bulletins alone are considered recommendations and are not mandatory unless made so by an AD, which is legally enforceable. Safety is the primary concern, and ADs are issued when a condition is found that compromises this and requires corrective action.
Airworthiness Directives serve to notify aircraft owners and operators of a known safety defect with a particular model of an airplane engine (or another system) and mandate certain actions to correct the defect within a specified timeframe. Not all engine service bulletins culminate in an AD, but when one is issued based on a bulletin, failure to comply may result in the grounding of the aircraft or other legal actions. Compliance is essential to ensure the continued airworthiness of the affected aircraft.
Anyone involved in aircraft maintenance or operation needs to stay current with all issued ADs and understand the implications of service bulletins for the aircraft they manage or operate. In summary, a bulletin itself is a recommendation until made mandatory through an AD.