Performance - Introduction — Page 148, Lesson 170

Right, we're starting the Performance section of your Mass and Balance and Performance book. This is the very beginning, the introduction, and it's all about one crucial idea: the difference between the performance a manufacturer measures and the performance you can actually rely on in the real world. Let's start with the first concept: Measured Performance. This is the performance achieved by the manufacturer under test conditions for certification. Think about what that means. It's a brand-new aeroplane, flown by a test pilot, in perfect conditions. The book is very clear that this is unrepresentative of the performance that will be achieved by an average fleet of aeroplanes. So, it's the best-case scenario, the headline number, but not what you should plan your take-off around. Next, we have Gross Performance. This is the average performance that a fleet of aeroplanes should achieve if satisfactorily maintained and flown in accordance with the techniques described in the manual. So, it's a more realistic number. The key relationship here is that gross performance is measured performance reduced by a set margin to reflect that average operating performance. So we take that perfect test number and knock a bit off it to get a figure a normal, well-maintained fleet can expect. Now, the most important one for you as a pilot: Net Performance. This is the gross performance diminished to allow for various contingencies that cannot be accounted for operationally. Examples the book gives are variations in piloting technique, or temporary below-average performance. So, we're layering on another safety margin on top of the gross performance. The book makes a very powerful statistical point here. It says this level of performance is approximately 5 standard deviations from the average performance or gross performance. Let me unpack that. In statistics, a standard deviation is a measure of how spread out a set of numbers is. Being 5 standard deviations away from the average is an enormous margin. It means that 99.99994% of the time, the aeroplane will achieve net performance or better. In other words, there is less than one chance in a million that the aeroplane will not achieve the net performance. This is the safety standard which the aviation authorities aim to achieve. So when you're doing your performance calculations, you are always working with net performance. You are planning on the assumption that the aeroplane will perform at this very conservative, statistically guaranteed level. It's your safety net. So, to summarise that chain: Measured performance is the test pilot's perfect run. Gross performance is the realistic average for a good fleet. And net performance is the rock-bottom, statistically guaranteed figure you actually use, with less than a one-in-a-million chance of not achieving it. Now, the rest of this excerpt is just the contents list for the next chapter, which is called "General Principles - Take-off". It's a roadmap of what we're about to cover. We're going to look at Available Distances, which includes the Take-off Run Available, or TORA, the Take-off Distance Available, TODA, and the Accelerate-stop Distance Available, or ASDA, which is also called the Emergency Distance Available, EMDA. Then we'll move on to Required Distances, the Forces During Take-off, a Summary of those Forces, Take-off Speed, and the Effect of Variable Factors on Take-off Distance. That's the structure of what's coming. But for now, the core takeaway from this introduction is that hierarchy of performance: measured, gross, and net, and why net performance is the one that keeps you safe.