Which of the following is NOT a secondary power source in the T-6?

The option related to the 12 VDC, 20 ampere-hour battery is not considered a secondary power source in the T-6. In aircraft systems, a secondary power source usually refers to systems that provide backup or supplemental power to the primary systems, which are typically supported by a specific voltage and capacity.

The T-6 uses a 24 VDC battery, so the referent power source must match this specification to be classified as secondary. A 12 VDC battery deviates from the standard operating voltage for the aircraft’s systems, making it unsuitable for providing the required power levels. Moreover, the capacities of 5 ampere-hour and 42 ampere-hour batteries listed in the other options are related to batteries that meet aircraft specifications for secondary power. The external power receptacle is also used in conjunction with these systems to support them when ground power is available. Thus, the 12 VDC battery stands out as not conforming to the necessary voltage or typically accepted power levels for a secondary power source in the T-6 context.