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This question lies at the heart of aviation safety regulations. Consider this scenario: An aircraft receives an airworthiness directive (AD), but subsequently undergoes modifications, alterations, or repairs to the specific area addressed by the directive. Does the AD remain valid?

According to Title 14 of the Code of Federal Regulations (14 CFR) §39.15 , the answer is unequivocally yes. Even when an individual product (aircraft, engine, propeller, etc.) has been modified, altered, or repaired, the AD remains effective as long as the product is specifically identified in the directive. In other words, an AD's applicability doesn't depend on whether the product maintains its original configuration, but rather whether it falls within the directive's specified scope.

This regulation serves a critical purpose in maintaining consistent aviation safety standards. If ADs automatically became void after modifications, potential safety gaps could emerge, as modified products might still harbor the very hazards the directive was designed to address. Therefore, compliance with ADs is mandatory regardless of a product's modification status.

This doesn't necessarily mean modified products must comply with ADs in their original form. In some cases, modifications may have already resolved the issue that prompted the AD, or may require alternative compliance methods to achieve equivalent safety. Such situations demand additional engineering evaluation and regulatory approval to ensure the modified product meets all applicable safety standards. The key principle remains: While AD applicability is mandatory, specific compliance methods may require case-by-case adaptation.