Class A Aircraft - Take-off — Page 379, Lesson 462

All right, let's get into the heart of Class A take-off performance. We're going to start with the speeds that define the critical decisions a pilot makes on the runway. And the first one I want to introduce is a strange one, just like the book says — it's called VEF. VEF stands for the speed at which the critical engine is assumed to fail. It's a calibrated airspeed, and it's used for the purpose of performance calculations. Now, the critical engine is the one whose failure would have the most adverse effect on the aeroplane — the one that makes handling and performance worst. So VEF is the speed at which we assume that engine fails during take-off, for all our calculations. And there's a hard limit on it: VEF is never less than VMCG. VMCG is the minimum control speed on the ground — the speed below which you can't maintain directional control with the critical engine failed. So the assumed failure speed can never be lower than that. Now, why is VEF so strange? Because it's not a speed the pilot actually sees or acts on. It's selected by the aeroplane manufacturer for certification testing. The manufacturer picks VEF for two primary reasons: first, to establish the range of speeds from which V1 may be selected, and second, to help determine the accelerate-stop distance required — that's the distance needed to accelerate and then stop safely. Let me explain what VEF is really all about, because it hinges on the definition of V1. V1 is the speed at which, if the failure of the critical engine was recognized, there is sufficient distance remaining to either reject the take-off or continue the take-off. But here's the catch — recognizing that the engine has failed takes time. In fact, it takes about 1 second. So to recognize the engine failure at V1, the engine must have actually failed about 1 second before V1. The speed at which the critical engine fails, so that it may be recognized at V1, is called VEF. So think of it as a timing offset. VEF is the actual failure point, V1 is the recognition point, and there's roughly one second of recognition lag between them. That's the whole purpose of VEF — it's the assumed failure speed that makes the V1 decision mathematically possible. Now, the big one. V1 — Decision Speed. This is by far the most important speed in the take-off for Class A aeroplanes. It's called the decision speed because V1 determines the outcome of a critical decision that must be made following an engine failure or other major critical systems failure. Here's the precise definition, and I want you to hold both halves of it. V1 is defined as being the maximum speed at which the pilot must take the first action in order to stop the aeroplane within the remaining accelerate-stop distance. And V1 is also the minimum speed following engine failure that the pilot is able to continue the take-off within the remaining take-off distance. So you see, V1 is a balancing point. Below V1, your primary option is to stop — you can still bring the aeroplane to a halt within the runway remaining. Above V1, your primary option is to continue — you can still get airborne and clear the required distance. At V1 itself, both options are equally valid, and that's why it's the decision speed. The pilot must decide at that moment: stop, or go. And that's where VEF fits in — the engine is assumed to have failed one second before V1, so that by the time you reach V1, you've recognized the failure and can make that decision. Let's look at the graph that shows the ideal position of V1. That's the core of it — VEF as the assumed failure point, and V1 as the decision point. Now, the excerpt cuts off mid-sentence at "VGO is the low…" — that's the next speed we'll pick up, the VGO, which is the take-off decision speed for continuing. But for now, make sure you've got VEF and V1 firmly in your mind, because everything else in take-off performance builds on these two.