Text in force at CS25-28: from CS25-0, effective 17 Oct 2003.
The following is required miscellaneous equipment:
(a) Reserved (b) Two or more independent sources of electrical energy. (c) Electrical protective devices, as prescribed in this CS–25. (d) Two systems for two-way radio communications, with controls for each accessible from each pilot station, designed and installed so that failure of one system will not preclude operation of the other system. The use of a common antenna system is acceptable if adequate reliability is shown. (e) Two systems for radio navigation, with controls for each accessible from each pilot station, designed and installed so that failure of one system will not preclude operation of the other system. The use of a common antenna system is acceptable if adequate reliability is shown.
ED Decision 2003/002/RM · Source