If the pilot RTU has failed, which method allows changing VHF frequencies?

• A. Backup Tuning Unit
• B. RTU inhibit switch light on the pilot RTU and the 1/2 switch on the F/O RTU
• C. The 3/4 switch on the F/O RTU
• D. Use the VHF frequency knob on the ACP

Answer: (B)

When a pilot’s RTU fails, you still need a way to retune the VHF radios, and that’s done by transferring control to the remaining RTU. Engaging the RTU inhibit function on the pilot’s RTU signals that it should stop driving the radios, and flipping the 1/2 switch on the FO’s RTU hands tuning duties to the FO side. With the FO RTU in control, you can change the VHF frequencies as needed. The other options don’t provide a valid path to retune in this situation: the backup tuning unit isn’t the active path for routine retuning, the 3/4 switch on the FO RTU isn’t the method used to take over control, and the VHF knob on the ACP isn’t the interface used to adjust frequencies when an RTU failure has occurred.