What is the primary difference between an Airworthiness Directive (AD) and a Service Bulletin (SB)?

The primary distinction between an Airworthiness Directive (AD) and a Service Bulletin (SB) lies in their regulatory nature. An AD is a legally enforceable regulation issued by the FAA that mandates specific actions to be taken to address safety concerns associated with aircraft, engines, propellers, or appliances. These directives must be complied with to ensure continued airworthiness and safety, and they often stem from findings related to safety issues, accidents, or malfunctions.

On the other hand, a Service Bulletin is typically issued by the manufacturer of the aircraft or its components. While an SB provides recommendations or suggestions to enhance safety, efficiency, or reliability, it does not carry the same mandatory enforcement as an AD. Compliance with a service bulletin is usually at the operator's discretion and may depend on a variety of factors, including the operational context and manufacturer's guidance.

In summary, the mandatory nature of an AD is what sets it apart from a service bulletin, which, while it may address important issues, does not carry the weight of legal obligation.