What is the maximum validity period for using an ATC transponder after inspection?

The maximum validity period for using an ATC transponder after inspection is 24 months. This timeframe is established by aviation regulations to ensure that transponders, which play a crucial role in aircraft identification and tracking, remain accurate and reliable. Regular inspections are mandated to verify the functionality and compliance of the transponder with operational standards, and a 24-month validity period allows for thorough checks while ensuring ongoing safety in air traffic management.

The other options suggest shorter inspection intervals, which do not align with current aviation regulations. A 12-month validity would be more frequent than necessary, while a 90-day or 30-day validity period would be impractically short for transponder inspections, requiring an excessive frequency of checks that could disrupt operational efficiency. Thus, the 24-month period strikes a practical balance between ensuring safety and maintaining operational readiness.