For example, in the United States, when an arriving “heavy” (H) aircraft—defined as one with a maximum takeoff weight (MTOW) greater than 255,000 lbs—is immediately followed by an arriving “small” (S) aircraft (MTOW < 41,000 lbs), the required separation between them, at the instant when H is about to touch down on the runway, is 6 nautical miles ( ∼10.9 km)