What is the difference between Airworthiness Directives (ADs) and Service Bulletins (SBs)?

• A. ADs are manufacturer recommendations; SBs are mandatory.
• B. ADs apply only to new airplanes; SBs apply to old ones.
• C. ADs are mandatory regulatory actions issued by aviation authorities; SBs are manufacturer recommendations not legally mandatory unless adopted.
• D. ADs and SBs are the same.

Answer: (C)

The main distinction is regulatory force. Airworthiness Directives are mandatory actions issued by aviation authorities to address an identified unsafe condition. They require operators to perform specific inspections, modifications, or part replacements within a defined timeline, and noncompliance can ground aircraft or lead to penalties.

Service Bulletins come from the manufacturer as recommended maintenance or improvement actions. They’re guidance and not legally binding unless the aviation authority adopts them into a regulation or issues an AD that requires them. So SBs inform and advise, but they don’t mandate action on their own.

A is not correct because it flips the relationship—ADs are the mandatory regulatory items, not manufacturer recommendations. B is not correct because ADs aren’t limited to new airplanes; they apply to aircraft or components with unsafe conditions, regardless of age. The essential point remains: ADs are mandatory regulatory actions; SBs are manufacturer recommendations unless adopted.