The PIC of an aircraft shall ensure that before the aircraft enters a controlled airspace, two-way radio contact is established with the responsible ATSU on the designated radio frequency, and shall ensure, while the aircraft is within, and until it leaves, the controlled airspace, that continuous radio watch is maintained and that such further two-way radio communication as such ATSU may require, is established: Provided that—
(a) the ATSU may permit an aircraft not capable of maintaining continuous two-way radio communication, to fly in the control area, TMA, control zone or aerodrome traffic zone for which it is responsible, if traffic conditions permit, in which case the flight shall be subject to such conditions as such ATSU deems necessary to ensure the safety of other air traffic; and
(b) in the case of radio failure, a flight for which an air traffic service flight plan was filed and activated by the ATSU on receipt of a departure time, may continue in controlled airspace if the communication failure procedures specified in Document SA-CATS 91 are complied with.