Key distribution can be done manually at the factory where the plane is built. Which can also keep a keystore holding the key used for each plane. But we don't

Key distribution can be done manually at the factory where the plane is built. Which can also keep a keystore holding the key used for each plane. But we don't need to delve deep in this scenario, as ultimately op's point was that, by introducing crypto, you risk adding failure modes to the plane.