Class A Aircraft - Take-off — Page 387, Lesson 482

Let’s start with the big picture. When we analyse take-off performance for a Class A aeroplane, we’re really asking one question: how much runway does this aircraft need, and what is the heaviest it can be for the runway we have? The full, complete analysis would have to account for any stopway and clearway available. But that is time-consuming, and it will often give a maximum permissible take-off mass that is higher than we actually need. So, in practice, we often use simplified data to get a rapid assessment of the take-off mass. One method of doing this is to use balanced field data. Let me define the balanced field carefully, because everything else hangs off it. A balanced field exists if the take-off distance is equal to the accelerate-stop distance. Now, an aerodrome which has no stopway and no clearway has a balanced field. So think of a plain runway with nothing extra at either end. On that runway, the two critical distances are the take-off distance and the accelerate-stop distance. Here’s the key relationship. For an aeroplane taking off, if an engine failure occurs, the later the engine fails, the greater will be the accelerate-stop distance required, but the less will be the take-off distance required. Let me unpack that. If the engine fails very early, you have plenty of runway ahead, so you can stop easily — the accelerate-stop distance is short. But you’re slow, so you can’t climb away, so the take-off distance is long. If the engine fails very late, you’re fast, so you can climb away easily — the take-off distance is short. But you’re fast and near the end of the runway, so stopping is hard — the accelerate-stop distance is long. So one distance goes up as the other goes down. At some speed, the two distances will be equal. That speed is the balanced field V1, and that equality is the balanced field. Figure 14.6 shows the variation of these distances graphically. On that graph, the point labelled A is the balanced field length required for the prevailing conditions. And here’s the important part: it represents the maximum distance required for those conditions. Why? Because at whatever speed the engine fails, the distance is adequate — either to stop if the failure occurs before V1, or to complete the take-off if the failure occurs after V1. So the balanced field length is the worst-case distance you must plan for. Now, for a given weight and conditions, the balanced field V1 gives the optimum performance, because the take-off distance required, the TODR, and the accelerate-stop distance required, the ASDR, are equal. But in some circumstances, this balanced field V1 will not be acceptable, because V1 must lie within the limits of VMCG, VR, and VMBE. Let me name those three limits, because they drive everything that follows. VMCG is the minimum control speed on the ground — below that speed you cannot maintain directional control with the critical engine failed. VR is the rotation speed — the speed at which you rotate the aircraft to lift off. And VMBE is the maximum brake energy speed — the speed above which the brakes cannot absorb the energy of a rejected take-off without overheating. So the balanced field V1 is only valid if it sits between those limits. When it doesn’t, we get an unbalanced field. There are three situations that give an unbalanced field, and I want to walk you through each one. First, V1 less than VMCG. At low weights and altitudes, the balanced field V1 may come out below VMCG. That’s not acceptable, because below VMCG you can’t control the aircraft on the ground with an engine failed. So V1 would have to be increased up to VMCG. Now think about what that does. A higher V1 means you commit to take-off later, so the take-off distance required, the TODR, would be less, and the accelerate-stop distance required, the ASDR, would be greater than the balanced field length. The field length required would then be equal to the ASDR at VMCG. So in this case, the accelerate-stop distance at VMCG is the governing distance. Second, V1 greater than VMBE. This happens at high weight, high altitude, and high temperature. The balanced field V1 may exceed VMBE, which is the maximum brake energy speed. That’s not acceptable either, because the brakes can’t handle the energy. So V1 would have to be reduced down to VMBE. That gives a TODR which is greater, and an ASDR which is less, than the balanced field length. The field length required would then be equal to the TODR at VMBE. So here, the take-off distance at VMBE governs. Third, V1 greater than VR. For aircraft with good braking capabilities, the stopping distance will be short, which gives a high balanced field V1 speed. If that V1 exceeds VR for the weight, then V1 has to be reduced down to VR. And the field length required will be equal to the TODR at VR. So again, the take-off distance at VR governs. So the pattern across all three cases is the same. The balanced field V1 is the ideal, but it must respect the limits of VMCG, VR, and VMBE. When it can’t, we shift V1 to the limiting speed, and the field length we need becomes whichever of the two distances — TODR or ASDR — is the larger at that new V1. That’s the unbalanced field.