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There’s a point on a runway during take-o that a plane reaches V1 speed. Once it passes V1, it has reached the point of no return. The point where take-o cannot be aborted. It has to take o . Or crash. In order to determine its V1 speed every plane will factor in its weight, wind-speed, weather conditions, slope, length of runway etc. So although there’s not a physical line drawn on each runway, it’s there.
