Performance - Introduction — Page 140, Lesson 163

Right, let's get into the Performance section of your ATPL studies. This is where all the aerodynamics and systems knowledge starts to come together into the operational numbers you'll actually use. We're starting with the definitions. These aren't just dictionary entries — they're the precise, legal meanings that underpin every performance calculation you'll do. Let's take them in order. 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 all the thrust coming from the jet exhaust, the turbine extracts most of the energy to spin a propeller. That's why turboprops are often used on regional and business aircraft — they're relatively efficient at speeds slower than, and altitudes lower than, those of a typical jet. So think of it as the bridge between a pure jet and a piston-propeller aircraft. 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. The pitch is essentially the angle of the blades. Being able to change it lets you optimise the propeller's angle for different phases of flight — fine pitch for take-off, coarse pitch for cruise. And note the definition includes the ability to change it even when stationary, which is relevant for feathering and ground checks. Now we get to the critical take-off speeds. VEF is the calibrated airspeed at which the critical engine is assumed to fail and is used for the purpose of performance calculations. It is never less than VMCG. So VEF is a defined point in your calculations — the speed at which you plan for the worst engine to quit. And it's constrained: it can never be lower than VMCG, which is the minimum control speed 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 balancing point — before it, you can stop safely; at or after it, you must keep going and get airborne. It's the last point at which you can abort. Let's look at the 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 distance to stop and the distance to continue balance out. And here's the relationship with the other speeds. V1 cannot be allowed to be less than VMCG because engine failure below VMCG means the aircraft can't be controlled on the ground. So V1 is bounded by these constraints. Next, 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 that makes the surface look reflective, but not puddles. This matters because it changes your braking performance and therefore your landing and take-off distances. You can see the effect of contamination on landing distance in this graph. Typically on a dry runway the braking coefficient of friction is high, but on a wet runway it drops significantly, which extends your landing distance. Then 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 it's a rapid change in the wind — either speed or direction — over a short distance. It shears the airflow, and that can suddenly change your airspeed. If you hit it at low altitude on take-off or landing, it can be catastrophic because you might lose airspeed just when you need it most. Yaw is the motion of an aeroplane about its normal axis. So imagine the vertical axis through the aircraft — yaw is the nose swinging left or right around that axis. That's the motion controlled by the rudder. Finally, Zero Flap Speed. This is the minimum safe manoeuvring speed with zero flap selected. So it's the slowest speed at which you can safely manoeuvre the aircraft when the flaps are fully retracted. Below that speed, with no flaps, you don't have enough margin above the stall for safe manoeuvring. And we're starting the abbreviations list with AC — that's the first one, and we'll build on that as we go through the chapter. So to tie it together: these definitions give you the language of performance. VEF and V1 are the decision points for engine failure. Wet runway and windshear are the environmental factors that change your numbers. And yaw, zero flap speed, and the propeller definitions are the aircraft characteristics you're working with.