Class A Aircraft - Take-off — Page 387, Lesson 484

Let’s pick this up right where the take-off story gets practical. We’ve already talked about balanced fields and the idea that V1 is chosen so the accelerate-stop and the one-engine-inoperative take-off both fit inside the same distance. Now I want to walk you through what happens when the field is not balanced, and then we’ll move into the real business of this chapter: calculating the limiting masses for a Class A aeroplane. First, the V1 range. If the balanced field available is greater than the balanced field required for the take-off mass and conditions, you get a window of speeds where V1 can be chosen. That window is bounded by two speeds. VGO is the first speed at which the take-off can be completed within the distance available. VSTOP is the last speed at which the accelerate-stop could be completed within the distance. So V1 can be chosen anywhere between VGO and VSTOP. That’s the range of V1. Now, take-off from an unbalanced field. If the aerodrome is not a balanced field, you can still use the balanced field data by assuming a balanced field equal to the lesser of the Take-off Distance Available and the Accelerate-stop Distance Available. That assumed distance may exceed the Take-off Run Available, unless the TORA becomes limiting. The take-off mass you get from this method will be less than what you could get by properly accounting for stopway and clearway. But if that mass is sufficient for the flight, you don’t need to go into a more detailed analysis. Now let’s shift to the heart of this section: the field limit brake release mass, also called the field limit mass. This is the maximum mass that will allow the aeroplane to meet its field length requirements at the airfield concerned. If you’re heavier than the field limit mass, then either the one-engine-inoperative or the all-engine-operative take-off run, take-off distance, or accelerate-stop distance exceeds the available distance at the airfield. Here’s the key difference between Class A and Class B aeroplanes. Class B aeroplane data for take-off shows what length of runway would be used for any given mass. Class A aeroplane data shows what maximum mass could be taken for a given runway length. That makes sense because Class A aeroplanes are used commercially, and airlines want to carry the maximum payload possible. So most performance graphs or tables give a mass as their outcome. Now, since airfields can have different lengths of take-off run, take-off distance, and accelerate-stop distance available, you might expect many mass graphs — one for the take-off run required within the take-off run available, one for the take-off distance required within the take-off distance available, and one for the accelerate-stop distance required within the accelerate-stop distance available. But there is only one graph and one assumed value. That’s the field limit brake release mass.