N.Y.C. Charter § 482: Section 482. Chief may be designated as commissioner.

The mayor may designate the chief of the fire department to serve as commissioner, and in such case he shall exercise the powers and duties of commissioner and shall continue to exercise his powers and duties as chief and shall receive the salary of the commissioner. While serving as commissioner the chief shall forfeit none of his pension rights and privileges as chief or his civil service status, and such service and the time during which he so serves shall be part of his time and service as chief. Such designation as commissioner shall be in writing filed in the office of the department and in the office of the mayor.