There is another critical point on an approach where a last-minute grade on the approach needs to be made. On a precision approach, where the glideslope will take you to a touchdown point about 1,000 feet down the runway, the threshold crossing height (TCH) is usually 50 feet. Why is that critical? Well, it's where an important characteristic of the airplane starts to change. Once the airplane is in ground effect, which is in full effect at an altitude half the span of the airplane, the airplane decelerates more slowly, about half as rapidly, than it decelerates out of ground effect. This characteristic is especially marked on airplanes that sit low to the ground, as do the 500 series Citations and Mooneys to name a couple.