How many additional ATC codes are stored in the RMU?

The correct answer highlights that there is one additional ATC (Air Traffic Control) code stored in the RMU (Radio Management Unit) of the T-6 aircraft. This means that, aside from the standard codes typically used for aviation communication and navigation, the RMU includes a single extra code that can be utilized, enhancing the versatility and functionality of the unit.

The presence of this additional ATC code allows pilots to quickly switch to an emergency or alternate communication frequency if needed. This feature is significant for safety and efficiency during flight operations, as it aids in effective communication with ATC under various circumstances.

Having a value of zero or a choice of two, five, or any number apart from one does not accurately reflect the RMU's design and capabilities. The aircraft equipment configuration is such that it is optimized for ease of use, and including only one additional code aligns with typical design practices for aviation instrumentation where simplicity and reliability are paramount.