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

Let’s start with the concept that ties this whole chapter together: the critical engine. In a multi-engine aeroplane, the critical engine is the one whose failure would have the most adverse effect on the aircraft’s performance and handling — typically the one that produces the greatest yawing moment. Everything we talk about now — VEF, V1, VGO — is built around the assumption that this critical engine fails at the worst possible moment during take-off. Now, the first speed we meet is VEF. The full name is engine failure speed, and the certification definition is precise: VEF is the calibrated airspeed at which the critical engine is assumed to fail. It is used for the purpose of performance calculations. And there’s a hard limit attached to it — VEF is never less than VMCG, which is the minimum control speed on the ground, the speed below which you cannot maintain directional control with the critical engine failed. So VEF sits at or above that. Here’s the strange part about VEF, and I want you to really follow this because it’s the heart of the logic. VEF is not a speed you fly at or a speed you decide at. It is a speed selected by the aeroplane manufacturer for certification testing. The manufacturer picks it for two main reasons: first, to establish the range of speeds from which V1 may be selected, and second, to help determine the accelerate-stop distance — that’s the distance required to accelerate and then stop safely if you reject the take-off. So why does the engine have to be assumed to fail at VEF, and not at V1? Let’s look at 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 if you want to recognize the failure at V1, the engine must have actually failed about 1 second before V1. That earlier speed, the speed at which the critical engine fails so that it may be recognized at V1, is exactly what we call VEF. So think of it as a timeline: the engine fails at VEF, about one second passes while you perceive and recognize the failure, and by the time you act, you’re at V1. That one-second recognition lag is the entire reason VEF exists as a separate speed. Now we come to the most important speed in the take-off for Class A aeroplanes — V1, the decision speed. It’s called the decision speed because it determines the outcome of a critical decision that must be made following an engine failure or other major critical systems failure. V1 has a dual definition, and both halves matter. First, V1 is the maximum speed at which the pilot must take the first action in order to stop the aeroplane within the remaining accelerate-stop distance. Second, V1 is also the minimum speed following engine failure at which the pilot is able to continue the take-off within the remaining take-off distance. So V1 is the balancing point — below it, you can still stop safely; at or above it, you can still get airborne and clear the required distance. It’s the speed where the decision to stop or go is made. And that brings us to VGO — the excerpt cuts off mid-word, but I can tell you the concept it introduces: VGO is the take-off go speed, the speed at which, following an engine failure, you can continue the take-off and still meet the required climb and distance performance. It’s the "go" side of the decision, paired against the "stop" side that V1’s first definition covers. Let me show you how these fit together on a graph, because the relationship between VEF, V1, and the distances is visual. That figure shows the ideal position of V1 — you can see the intersection point on the graph where the accelerate-stop distance and the accelerate-go distance balance. That intersection is where V1 sits, and VEF sits about one second before it on the speed axis. So to recap the chain: the critical engine fails at VEF, you take about one second to recognize it, and by then you’re at V1 — the decision speed. Below V1 you stop, at or above V1 you go, and VGO is the speed that defines the "go" path. That’s the skeleton of Class A take-off performance.