According to FAR 39, when are Airworthiness Directives (ADs) issued?

Airworthiness Directives (ADs) are issued when an unsafe condition exists that could affect the safety of the aircraft in operation. This typically occurs when there is evidence or information indicating that a particular aircraft model, or a specific component, has a defect or issue that could potentially lead to failure or unsafe operation. ADs serve as official notifications to aircraft owners and operators to take corrective action, which may include inspections, repairs, or modifications to ensure ongoing safety.

The issuance of ADs is a proactive measure taken by aviation authorities to prevent accidents and enhance aviation safety. This aligns with the regulatory intent of maintaining the operational safety of all aircraft in service. The other scenarios, such as the sale of an aircraft, the release of new models, or routine maintenance periods, do not inherently denote the existence of unsafe conditions that would warrant an AD. Thus, the correct understanding of the purpose of an AD is rooted in its response to identified safety risks.