Wake turbulence separation minimum distance must be maintained until what event occurs for all IFR aircraft?

Wake turbulence separation minimum distance must be maintained until touchdown occurs for all IFR aircraft because this is when the aircraft is physically on the runway and in a position where it can safely complete its landing without the risk of encountering the wake turbulence created by a preceding aircraft.

This separation is crucial because wake turbulence can create significant hazards, especially during the landing phase when an aircraft is at lower speeds and altitudes. By ensuring that the following aircraft maintains separation until the leading aircraft has touched down, air traffic control minimizes the risk of the following aircraft losing lift or control due to turbulence.

In contrast, the other options do not offer the same level of safety. Takeoff does not guarantee that the aircraft has safely landed and is no longer generating wake turbulence. Landing is an ambiguous term and may not signify the same cessation of turbulence impact as touchdown does. Crossing the threshold occurs when the aircraft is still in the air and may still be affected by wake turbulence until it has landed completely.