What forms can actions required by an Airworthiness Directive (AD) take?

Airworthiness Directives (ADs) are legally enforceable rules issued by aviation authorities to address potential safety issues in aircraft, engines, propellers, and other aeronautical products. The actions required by an AD can take several forms depending on the nature of the safety concern being addressed.

The correct answer includes a variety of measures: part replacements, design modifications, inspections, and changes in operating procedures. Part replacements may be mandated if a specific component has been identified as unsafe. Design modifications could be required to rectify a design flaw that poses safety risks. Inspections can be stipulated to ensure that the maintenance of an aircraft meets safety standards and to catch any issues before they lead to failures. Changes in operating procedures might be necessary if existing practices are found to be contributing to safety risks.

This comprehensive approach ensures that all aspects of aircraft safety are covered, making it a crucial component of aviation safety management. The other options are too narrow in scope and do not reflect the broad range of actions that can be specified by an AD.