Which statement correctly contrasts an AD and an SB?

• A. SB is a mandatory change per the FAA; AD is a recommended modification per the manufacturer.
• B. AD is a mandatory change per the FAA; SB is a recommended modification per the manufacturer.
• C. AD and SB are both optional changes.
• D. AD is a software patch; SB is a hardware upgrade.

Answer: (B)

An Airworthiness Directive is a legally enforceable action issued by aviation authorities (like the FAA) that requires operators to fix a safety issue on affected aircraft. It becomes part of the aircraft’s airworthiness obligations and must be complied with by a specific deadline. A Service Bulletin, issued by the aircraft’s manufacturer, provides recommended maintenance actions to address an issue, but it isn’t compulsory on its own unless the regulator or the operator’s maintenance program requires it.

So the statement that AD is mandatory and SB is a recommended modification aligns with how these documents function in practice. The other options misstate the relationship: ADs aren’t optional or merely software/hardware changes, and Service Bulletins aren’t mandatory by themselves.