Performance - Introduction — Page 140, Lesson 163

We’re starting the Performance section of your ATPL studies, and I want to walk you through the opening definitions. These are the building blocks for everything we’ll do with take-off and landing calculations, so let’s take them one at a time. First, a turboprop. This is an aircraft having a jet engine in which the energy of the jet operates a turbine that drives the propeller. So the core is a gas turbine, but instead of using all that exhaust energy for pure jet thrust, it spins a turbine, and that turbine turns the propeller. Turboprops are often used on regional and business aircraft because of their relative efficiency at speeds slower than, and altitudes lower than, those of a typical jet. So if you’re not flying fast or high, the turboprop is the efficient choice. Next, variable pitch propellers. This is a propeller, the pitch setting of which changes or can be changed, when the propeller is rotating or stationary. Pitch is basically the angle of the blades. So a variable pitch propeller lets you adjust that blade angle, whether the prop is spinning or not. That’s what lets you optimise the propeller for different phases of flight. Now we get into the take-off speeds, and these are critical. VEF is the calibrated airspeed at which the critical engine is assumed to fail, and it is used for the purpose of performance calculations. It is never less than VMCG. So when we do our take-off performance maths, we assume the worst engine fails at exactly VEF. And VMCG is the minimum control speed on the ground — the slowest speed at which you can still maintain directional control after an engine failure. So VEF can’t be below that, because below VMCG you simply can’t control the aeroplane on the ground. Then V1, which is referred to as the decision speed. This is the big one. Engine failure prior to V1 demands that the pilot must reject the take-off, because there is insufficient distance remaining to enable the aircraft to safely continue the take-off. Engine failure at or faster than V1 demands that the pilot must continue the take-off, because there is insufficient distance remaining to safely bring the aircraft to a stop. So V1 is the point of no return — before it, you stop; at or after it, you go. It’s a balance between stopping distance and continuing distance. Let me show you that graphically. Here’s Figure 14.1, a graph showing the ideal position of V1. You can see that a V1 at the intersection point of the graph is the ideal — that’s where the accelerate-stop and accelerate-go distances balance. And there’s a constraint on V1 too. Figure 14.2 shows the relationship of V1 with VMCG and VMBE. V1 cannot be allowed to be less than VMCG, because engine failure below VMCG means the aeroplane can’t be controlled on the ground. VMBE is the maximum brake energy speed — the fastest speed at which the brakes can absorb the energy of a rejected take-off without overheating. So V1 sits in a window between those limits. Now, a wet runway. A runway is considered wet when the runway surface is covered with water, or equivalent moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water. So it’s that sheen of moisture — reflective but not puddled. That matters because it changes your braking performance. Figure 6.7 shows the effect of contamination on landing distance — typically on a dry runway the braking coefficient of friction is high, but contamination reduces it, and that extends your landing distance. Next, windshear. This is a localized change in wind speed and/or direction over a short distance, resulting in a tearing or shearing effect that can cause a sudden change of airspeed with occasionally disastrous results if encountered when taking off or landing. So the wind isn’t steady — it changes abruptly over a short space, and that can suddenly change your airspeed right when you’re most vulnerable, near the ground. Then yaw. This is motion of an aeroplane about its normal axis. The normal axis runs vertically through the aeroplane, so yaw is the nose swinging left or right. Think of it as the rotation you’d get from pressing the rudder pedals. Finally, zero flap speed. This is the minimum safe manoeuvring speed with zero flap selected. So with the flaps fully retracted, this is the slowest speed at which it’s still safe to manoeuvre the aeroplane. And at the end of the page we have the start of an abbreviations list — AC, which we’ll pick up as we go through the chapter. That’s the opening set of definitions for Performance.