Class A - Additional Take-off Procedures — Page 421, Lesson 516

Let's pick up with the tail end of the Class A additional take-off procedures, then move into the third one, which is the big one. So, we just finished the improved climb performance procedure. The last few steps there: after you've done the tyre speed limit graph, you take the lower of the two mass increases you found, and that's the one you use, along with its associated speed increases. You add that mass increase to the normal climb mass limit, then you determine the V speeds for this increased mass, apply the speed increases to the appropriate speeds, and finally check VMBE — that's the maximum brake energy speed, the speed beyond which the brakes can't absorb all the energy if you abort. Now, the third additional type of procedure — and this is probably the most common one you'll see in practice — is the reduced thrust take-off. It goes by many names: variable thrust take-off, assumed temperature take-off, and Airbus calls it the flexible take-off. All of these refer to the same thing. Why do we do it? Two main reasons: to preserve engine life and to help reduce noise. The principle is that you can use it any time your actual take-off mass is less than the maximum permissible take-off mass, and the available runway distance greatly exceeds what's required. In other words, you have performance to spare, so you don't need full thrust. Now, there's a hard limit here: the maximum reduction in thrust from the full rated take-off thrust value is 25%. You can't cut it back more than that. But — and this is critical — reduced thrust take-off is not permitted under certain conditions. Let me list them: icy or very slippery runways, contaminated runways, anti-skid inoperative, reverse thrust inoperative, an increased V2 procedure, and PMC off. PMC is the performance management computer — if that's off, you can't do it. Also, it's not recommended if potential windshear conditions exist. Now, how does the procedure actually work? Essentially, it assumes that the temperature is a lot hotter than it actually is. Here's the logic: imagine the outside air temperature continually increasing. As temperature goes up, the thrust produced by the engines continually decreases. There will eventually be a temperature beyond which there's insufficient thrust to complete a take-off. That temperature is then used as the assumed temperature, and the thrust equating to that temperature is set as the take-off thrust. So you're essentially telling the engines, "pretend it's hotter out there than it really is," which means they produce less thrust — but still enough to complete the take-off safely at your actual mass. That's the whole trick. The full procedure is detailed on page 31 of section 4 in CAP 698, so you don't need to memorise every step — you'll use that page as your reference. One thing to keep straight: this is different from the previous two procedures. The improved climb and tyre speed procedures were about increasing your mass limits. This one is about reducing thrust while keeping the take-off safe. Different goal, different method.