D.C. Code § 9-921
The exercise of the powers granted by this chapter shall be in all respects for the benefit of the inhabitants of the District for the increase of their commerce, and for the promotion of their safety, health, welfare, convenience, and prosperity, and as the operation and maintenance of the airport by the Authority will constitute the performance of essential governmental functions, the Authority shall not be required to pay any taxes or assessments upon the airports or any property acquired or used by the Authority under the provisions of this chapter or upon the income therefrom; and the bonds issued under the provisions of this chapter, their transfer and the income from the bonds, including any profit made on the sale of the bonds, shall at all times be free and exempt from taxation by the District.
D.C. Code § 9-921