Executive Order 12909
Order of Succession of Officers To Act as Secretary of the Air Force
This executive order establishes the line of succession for who may act as Secretary of the Air Force in cases of death, permanent disability, resignation, absence, or temporary disability. It designates the Under Secretary, then Assistant Secretaries and General Counsel (by seniority), then the Chief of Staff as successors, while clarifying that such succession is temporary and does not vacate the successor's original statutory position.
Federal Register 59 FR 21909