The special fund created and provided for in § 5.1-51 shall be so set up on the books of the Comptroller as to segregate the amounts paid into the fund as a direct tax upon aviation motor fuel purchased by operators of airplanes and aircraft, and no portion of such fund, accumulated in the manner hereinbefore provided, shall ever be used for any other purpose than for the administration of the aviation laws and the construction, maintenance and improvement of airports and landing fields, and no portion of the same shall be covered into the general fund of the state treasury, such funds derived in the manner aforesaid being hereby irrevocably appropriated for promotion of aviation in the interest of operators and of the public, as provided in this chapter, and to the extent necessary only, for the administration of this and other aviation laws.