((a)) When there is a vacancy in the office of chief of a bureau, or during the absence or disability of the chief of a bureau, the deputy chief of that bureau, unless otherwise directed by the President, shall perform the duties of the chief until a successor is appointed or the absence or disability ceases.

((b)) When subsection (a) cannot be complied with because of the absence or disability of the deputy chief of the bureau, the heads of the major divisions of the bureau, in the order directed by the Secretary of the Navy, shall perform the duties of the chief, unless otherwise directed by the President.