In the context of aviation and aerospace, an Advisory Circular (AC) is a document issued by a national aviation authority, such as the Federal Aviation Administration (FAA) in the United States, to provide guidance and information on a specific topic or regulation.

Advisory Circulars are used to:

1. Interpret and explain regulatory requirements: ACs help to clarify the meaning and application of aviation regulations, making it easier for individuals and organizations to understand and comply with them.
2. Provide best practices and recommendations: ACs often contain guidance on industry best practices, recommended procedures, and techniques for achieving compliance with regulations.
3. Offer guidance on new technologies or procedures: ACs can provide information on the use of new technologies, materials, or procedures in aviation, helping to promote innovation and safety.
4. Support the development of industry standards: ACs can contribute to the development of industry standards and guidelines, which can be used to ensure consistency and safety in aviation operations.

Advisory Circulars are typically non-regulatory, meaning they do not have the force of law. However, they are often considered authoritative guidance and can be used as a reference by regulatory inspectors, auditors, and the aviation industry as a whole.

Some common topics covered by Advisory Circulars include:

• Aircraft design, certification, and maintenance
• Pilot training and licensing
• Airworthiness standards
• Operational procedures (e.g., navigation, communication, and emergency procedures)
• Safety management systems
• Environmental issues (e.g., noise, emissions)

Overall, Advisory Circulars play an important role in promoting safety, efficiency, and compliance in the aviation and aerospace industries.