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

Let’s start with the big picture. When we talk about take-off performance for a Class A aircraft, we are really asking one question: how much runway does this aeroplane need, and what is the heaviest it can be for that runway? The answer depends on what happens if an engine fails at the worst possible moment. Now, a complete analysis would have to account for any stopway and clearway available. A stopway is the area beyond the runway that is strong enough to support the aeroplane if it has to abort the take-off and stop. A clearway is an area beyond the runway, over which the aeroplane can climb initially, that is free of obstacles. Taking both into account is time-consuming, and it often gives a maximum permissible take-off mass that is higher than you actually need. So, to allow a rapid assessment, we often use simplified data. One method is the balanced field. Here is the definition you must know precisely. A balanced field exists if the take-off distance is equal to the accelerate-stop distance. An aerodrome which has no stopway or clearway has a balanced field. So, if the runway is just a runway, with no extra stopway and no clearway, the field is balanced by definition. Let me unpack the two distances. The take-off distance is the distance needed to accelerate, become airborne, and climb to a specified height. The accelerate-stop distance is the distance needed to accelerate and then, if you decide to abort, brake to a complete stop. Now, here is the key relationship. For an aeroplane taking off, if an engine fails, the later the engine fails, the greater will be the accelerate-stop distance required, but the less will be the take-off distance required. Think about why. If the engine fails early, you are still slow, so you can stop in a short distance, but you have a long way to go to reach take-off speed and climb. If the engine fails late, you are already fast, so you are close to being able to take off, but you are moving so fast that stopping takes a long distance. At some speed, the two distances will be equal. That speed is the balanced field V1. Figure 14.6 shows the variation of these distances graphically. The distance at point A in that figure is the balanced field length required for the prevailing conditions. It represents the maximum distance required for those conditions, 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, and it is the length you must have available. Now, for a given weight and conditions, the balanced field V1 gives the optimum performance, because the take-off distance required and the accelerate-stop distance required are equal. But in some circumstances, this V1 will not be acceptable, because V1 must lie within the limits of VMCG, VR, and VMBE. Let me define those three speeds, because they are the constraints. VMCG is the minimum control speed on the ground. It is the speed at which, when the critical engine fails, you can maintain directional control using only the rudder, with the nose wheel steering ineffective. VR is the rotation speed, the speed at which you rotate the aeroplane to lift the nose wheel off the ground. VMBE is the maximum brake energy speed, the speed at which, if you abort, the brakes can absorb the energy without overheating and failing. So V1 must be at or above VMCG, at or below VR, and at or below VMBE. If the balanced field V1 falls outside these limits, you have an unbalanced field. There are three situations that give an unbalanced field. Let me walk through each. First, V1 less than VMCG. At low weights and altitudes, the balanced field V1 may be less than VMCG. In that case, V1 would have to be increased to VMCG. Now, what happens to the distances? If you increase V1, you commit to taking off at a higher speed, so the take-off distance required becomes less, and the accelerate-stop distance becomes greater than the balanced field length. The field length required would then be equal to the accelerate-stop distance at VMCG. So you need a longer field, because the stopping distance is now the governing factor. Second, V1 greater than VMBE. At high weight, altitude, and temperature, the balanced field V1 may exceed VMBE. Here, V1 would have to be reduced to VMBE. Reducing V1 gives a take-off distance required that is greater, and an accelerate-stop distance that is less, than the balanced field length. The field length required would be equal to the take-off distance at VMBE. So now the take-off distance governs. Third, V1 greater than VR. For aircraft with good braking capabilities, the stopping distance will be short, giving a high balanced field V1 speed. If this exceeds VR for the weight, V1 will have to be reduced to VR, and the field length required will be equal to the take-off distance at VR. So the pattern is clear. The balanced field is the ideal, but V1 is constrained by VMCG below and by VR and VMBE above. When you have to move V1, one of the two distances grows, and that larger distance becomes the field length you must have. That is the unbalanced field. Let me make sure the contrast is crisp. In a balanced field, take-off distance equals accelerate-stop distance, and the field length is the maximum you need. In an unbalanced field, you have shifted V1 to satisfy a control or brake limit, so one distance is longer than the other, and the longer one dictates the required field length. That is the core of balanced and unbalanced field take-off performance.