What does the term "airworthiness directive" (AD) refer to?

The term "airworthiness directive" (AD) refers specifically to a notification issued by aviation authorities, such as the Federal Aviation Administration (FAA) in the United States, that addresses safety concerns related to a specific aircraft model or its components. These directives are issued when there are potential safety issues that could affect the performance of an aircraft, including defects found during inspections or reports of accidents.

The purpose of an AD is to mandate corrective actions that must be taken to ensure the ongoing safety and airworthiness of affected aircraft. Compliance with these directives is legally required, and failure to adhere to an AD can result in penalties and issues with aircraft operation.

In contrast, the other options deal with different aspects of aviation regulation. While maintaining pilot licenses is crucial, it does not relate directly to airworthiness directives. Similarly, guidelines for aircraft engine maintenance and flight scheduling policies are specific operational aspects, but they do not represent the purpose and function of an AD in the aviation safety framework.