FAA Federal Aviation Regulations (FARS, 14 CFR)

The holder of an air traffic control tower operator certificate may not perform any duties under that certificate unless --

(a) He has served for at least three of the preceding 6 months as an air traffic control tower operator at the control tower to which his facility rating applies, or at the operating positions for which he has qualified; or

(b) He has shown that he meets the requirements for his certificate and facility rating at the control tower concerned, or for operating at positions for which he has previously qualified.