General Principles - Landing — Page 273, Lesson 340

We're starting a new topic now: the landing stage of flight, and specifically landing distance. Let me walk you through the definitions and the physics that govern it. First, the landing stage itself. It's defined as that stage of flight commencing from 50 ft above the landing threshold and terminating when the aeroplane comes to a complete stop. That 50 ft point is sometimes referred to as the landing screen height. Now, an important contrast here: the landing screen height is fixed at 50 ft for all classes of aeroplane. That's unlike the take-off screen height, which is 35 ft for Class A aeroplanes and 50 ft for Class B aeroplanes. So for landing, everyone uses 50 ft; for take-off, it varies by class. From the approach down to that landing screen height, the aeroplane must have attained the landing reference speed, known as VREF. This is a critical speed. For Class A aeroplanes, VREF must be no less than the greater of two values: 1.23 times the stall reference speed in the landing configuration, which we write as 1.23 VSR0, and the velocity of minimum control in the landing configuration, which is VMCL. So you take whichever of those two is greater, and VREF must be at least that. For all other classes of aeroplane, VREF must be no less than 1.3 times the stall speed in the landing configuration, written 1.3 VS0. Why is VREF so important? Because the landing distances in the aeroplane flight manual are based on aeroplanes flying at VREF. So if a landing aeroplane is not at VREF, the landing distance given by the manual will not be achieved by the pilot. A landing carried out at a speed other than VREF could seriously jeopardize the safety of the landing. So VREF is the reference speed that the entire published landing performance is built around. Now, the landing itself can be divided into two parts. We call these the airborne section and the ground run, or landing roll. The first part, the airborne section, starts from the landing screen height of 50 ft and ends when the aeroplane's main wheels touch the landing surface. The airborne section is usually given as being about 1000 ft in length. Within the airborne section, certain criteria apply — and that's where we'll pick up next.