49 CFR § 1544.305 - Security Directives and Information Circulars.
(a) TSA may issue an Information Circular to notify aircraft operators of security concerns. When TSA determines that additional security measures are necessary to respond to a threat assessment or to a specific threat against civil aviation, TSA issues a Security Directive setting forth mandatory measures.
(b) Each aircraft operator required to have an approved aircraft operator security program must comply with each Security Directive issued to the aircraft operator by TSA, within the time prescribed in the Security Directive for compliance.
(c) Each aircraft operator that receives a Security Directive must -
(1) Within the time prescribed in the Security Directive, verbally acknowledge receipt of the Security Directive to TSA.
(2) Within the time prescribed in the Security Directive, specify the method by which the measures in the Security Directive have been implemented (or will be implemented, if the Security Directive is not yet effective).
(d) In the event that the aircraft operator is unable to implement the measures in the Security Directive, the aircraft operator must submit proposed alternative measures and the basis for submitting the alternative measures to TSA for approval. The aircraft operator must submit the proposed alternative measures within the time prescribed in the Security Directive. The aircraft operator must implement any alternative measures approved by TSA.
(e) Each aircraft operator that receives a Security Directive may comment on the Security Directive by submitting data, views, or arguments in writing to TSA. TSA may amend the Security Directive based on comments received. Submission of a comment does not delay the effective date of the Security Directive.
(1) Restrict the availability of the Security Directive or Information Circular, and information contained in either document, to those persons with an operational need-to-know.
(2) Refuse to release the Security Directive or Information Circular, and information contained in either document, to persons other than those with an operational need-to-know without the prior written consent of TSA.