When are manufacturer's service bulletins considered mandatory during annual inspections?

Manufacturer's service bulletins become mandatory during annual inspections when they are incorporated into 14 CFR Part 39, which pertains to airworthiness directives (ADs). Airworthiness directives are legally enforceable regulations that require the alteration or repair of an aircraft, and they often include information from service bulletins that have been deemed critical for safety. Therefore, if a service bulletin is included in Part 39, it signals that the measures outlined in that bulletin must be acted upon and followed as part of the maintenance program, making it a legal requirement for compliance during inspections.

Incorporation into the maintenance manual does not automatically make a service bulletin mandatory, nor are all service bulletins considered mandatory without explicit regulatory backing. Similarly, while notations by maintenance technicians may indicate a best practice or recommendation for adherence to a service bulletin, such notations do not confer the same legal necessity as when the bulletins are made part of the airworthiness directive framework.