At what point is the speed referred to as touchdown speed measured?

Touchdown speed is specifically defined as the speed of the aircraft at the moment it makes contact with the runway during landing. This speed is crucial for ensuring a safe landing, as it is the point when the aircraft transitions from flying to rolling on the ground. At touchdown, pilots assess the aircraft's performance in relation to this speed to make adjustments if necessary.

Measuring the speed at this exact moment is essential because it directly impacts the aircraft's control and stopping distance, making it a critical factor in landing safety. Other points in the landing process, such as crossing the threshold at 50 feet or during the approach segment, do not represent the actual speed at which the aircraft touches down, hence do not accurately reflect touchdown speed. These other options refer to situations that occur before the actual touchdown event, which is why they are not considered correct for defining touchdown speed.