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.