 In the UK's Fleet Air Arm, "landing on" is usually achieved by first lining up on the port quarter parallel to the ship's heading, then once the deck motion is deemed to be acceptable the pilot sidesteps the aircraft laterally using a white painted line (the bum line) as a reference.Shipboard landing for some helicopters is assisted though use of a haul-down device that involves attachment of a cable to a probe on the bottom of the aircraft prior to landing.Tension is maintained on the cable as the helicopter descends, assisting the pilot with accurate positioning of the aircraft on the deck; once on deck locking beams close on the probe, locking the aircraft to the flight deck.