A plane that is airworthy is safe to take off: the body, the engines, and the papers are all in order. It is a safety word, used by engineers, inspectors, and news writers, not a compliment for a pleasant flight. A comfortable old jet can fail an airworthy check; a small, noisy one can pass. Helicopters, gliders, and even drones can be called airworthy or not. People are not airworthy, and neither are birds — though actually, the pattern is the same as seaworthy for ships and roadworthy for cars: fit for that place. If a machine is not airworthy, it must stay on the ground until it is fixed and signed off. A little official; in chat you might just say the plane is safe to fly.