Performance - Introduction — Page 148, Lesson 170

Let me walk you through the very start of the Performance section. This is the foundation for everything else, so I want to make sure you understand the three levels of performance we work with in aviation: measured, gross, and net. First, measured performance. This is the performance achieved by the manufacturer under test conditions for certification. The key words there are "test conditions." It uses new aeroplanes and test pilots. Because of that, it is unrepresentative of the performance that will be achieved by an average fleet of aeroplanes. So measured performance is the best-case, factory-fresh, expert-pilot number. It's the certification baseline, but it's not what you should expect in daily line operations. Next, gross performance. Gross performance 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 the realistic average. And here's the relationship: gross performance is measured performance reduced by a set margin to reflect average operating performance. So we take that pristine measured number and knock it down by a defined margin to get what an average, well-maintained fleet should actually do. Then we have net performance. Net performance is the gross performance diminished to allow for various contingencies that cannot be accounted for operationally. Examples given are variations in piloting technique and temporary below-average performance. So even gross performance isn't conservative enough — we reduce it further to cover things we can't predict or control in day-to-day operations. Now here's the important statistical part. Net performance is approximately 5 standard deviations from the average performance or gross performance. That means 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 the chain is: measured is the best, gross is measured reduced for average operations, and net is gross reduced further for contingencies. Net is the number you actually plan against, because it's the one with the built-in safety margin that the authorities require. Now, the chapter then moves into the General Principles of Take-off. The contents list shows the structure: Available Distances, then the Take-off Run Available — TORA, the Take-off Distance Available — TODA, the Accelerate-stop Distance Available or Emergency Distance Available — ASDA/EMDA. Then Required Distances, Forces During Take-off, a Summary of Forces, Take-off Speed, and the Effect of Variable Factors on Take-off Distance, followed by questions and answers. We'll get into all of that shortly. But for now, hold onto this core idea: when we talk about take-off performance, we're always working with these reduced, conservative numbers — net performance — because that's what gives us the safety margin the authorities demand.