Discussion about MGS5
Discussion about MGS5
Under the missions section there should be an option labeled "return to motherbase" when operating at the Ariel Command Center, also known as the helicopter.
The choices in the iDroid appear to shift based on the situation. It might just mean you should finish the next mission first or something similar. If you're traveling in Afghanistan, you'll likely need to request a pickup and return to the helicopter before the Mother Base option shows up.