What should be done when permanently closing a runway?

When permanently closing a runway, the proper procedure requires obliterating the designation numbers at both ends of the runway and marking an 'X' on the runway centerline, ensuring that the markings do not exceed 1,000 feet in length. This is crucial for safety and clarity, signaling to pilots that the runway is no longer in use. The obliteration of the designation numbers prevents any confusion regarding the runway's identification, which is vital for both navigation and air traffic control. The 'X' marking serves as a universal indicator of a closed runway, clearly communicating to pilots that it is not available for landing or takeoff.

The other options, while potentially tangentially relevant to runway maintenance or construction practices, do not meet the specific requirements for permanently closing a runway. Painting over all runway markings with white paint, for example, could cause confusion rather than clarity, while simply putting up barriers might not adequately communicate the closure to all pilots. Covering the runway with grass and landscaping it would not provide the necessary visual indicators that a runway is closed, potentially leading to dangerous situations if pilots misinterpret the runway's status.