The callout comes from the Ground Proximity Warning System fitted to Boeing airliners, which uses a radio altimeter to announce height above ground during approach and landing. As flight simulation and cockpit videos became popular online, short recordings of these automated warnings were clipped and shared by aviation enthusiasts. The twenty callout is especially familiar because it marks the final height announcement before touchdown, making it a shorthand for a landing in edits, memes and simulator streams, where a single word instantly signals that the aircraft is almost on the ground.