14 CFR § 21.99

When an Airworthiness Directive is issued under Part 39 the holder of the type certificate for the product concerned must—

If the FAA finds that design changes are necessary to correct the unsafe condition of the product, and upon his request, submit appropriate design changes for approval; and

Upon approval of the design changes, make available the descriptive data covering the changes to all operators of products previously certificated under the type certificate.

In a case where there are no current unsafe conditions, but the FAA or the holder of the type certificate finds through service experience that changes in type design will contribute to the safety of the product, the holder of the type certificate may submit appropriate design changes for approval. Upon approval of the changes, the manufacturer must make information on the design changes available to all operators of the same type of product.