Peel the green plantains by cutting off both ends, then making a shallow slit along the length of each ridge. Pry the peel away in strips. It helps to run your thumb under the skin to release it.
Using a sharp knife or a mandoline slicer, cut the plantains into very thin rounds, about 1/16 to 1/8 inch thick. Consistent thickness is the key to chips that cook evenly and get perfectly crispy.
Place the plantain slices in a bowl. Drizzle the avocado oil and lime juice over the slices, then sprinkle with salt, garlic powder, and chili powder if using. Toss everything together gently with your hands until every slice is lightly and evenly coated.
Preheat the air fryer to 375 degrees F. Arrange the plantain slices in a single layer in the basket, making sure they do not overlap. You will need to cook these in several batches.
Air fry for 6 minutes, then shake the basket or flip the chips with tongs. Cook for another 3 to 5 minutes, checking frequently during the last couple of minutes. The chips should be golden and crisp with no soft spots.
Transfer the chips to a wire rack or plate lined with paper towels. They will continue to crisp up slightly as they cool. Sprinkle with a tiny bit of extra salt if desired. Repeat with the remaining batches and enjoy.