Which inspection is required after a specific number of flight hours or the expiration of a calendar period?

Periodic Inspection is required after a specific number of flight hours or the expiration of a calendar period because it is designed to ensure that the aircraft remains safe and airworthy over time. This type of inspection is scheduled based on usage and maintenance intervals, which helps identify potential issues before they affect the aircraft's performance or safety. The timing of a periodic inspection can be critical in maintaining compliance with aviation regulations and operational standards, as it adheres to manufacturer recommendations and regulatory requirements.

In contrast, pre-flight inspections are conducted before each flight to ensure the aircraft is ready for operation. Routine maintenance inspections typically focus on more frequent, less extensive checks to address minor issues or wear and tear. Comprehensive inspections often refer to a more thorough evaluation that might happen less frequently but is more in-depth than a periodic inspection. Therefore, the nature and frequency dictated by periodic inspections distinguish them as essential in maintaining aircraft integrity over time.