Class A - Additional Take-off Procedures — Page 421, Lesson 518

Let's start with the core idea of Class A additional take-off procedures. When you're dealing with a Class A aeroplane — that's a multi-engine transport category aircraft — you often can't just use the full rated thrust for take-off. There are limits, and the whole point of these procedures is to find a reduced thrust setting that still gives you a safe take-off. The very first step is to determine the most limiting performance condition. Here's the clever part: the only common parameter that lets you compare all the different performance limits is temperature. So, for your actual take-off mass, you calculate the maximum permissible temperature from each of four different graphs. Let me name them: the field limit graph, the climb limit graph, the tyre speed limit graph, and the obstacle limit graph. From these four temperatures, you select the lowest one. That lowest temperature is your assumed temperature — but you must check it doesn't exceed the environmental limit. If it does, then the environmental limit itself becomes the assumed temperature. So the assumed temperature is essentially the highest temperature you can "pretend" it is, for thrust-setting purposes, without violating any of those four limits. Now, the procedure to actually calculate this comes from CAP 698, which is the UK CAA performance document. Let me walk you through the steps, a) through e). Step a): Calculate the maximum assumed temperature from CAP 698 Figure 4.17a or 4.17b, as appropriate. You enter the left column with the actual ambient temperature — that's the real outside air temperature — and you read the maximum temperature in the column appropriate to the aerodrome pressure altitude. So pressure altitude is the altitude in the standard atmosphere, and you pick the right column for your airfield. Step b): From CAP 698 Figure 4.17c, on the bottom line, you determine the minimum assumed temperature for the aerodrome pressure altitude. So now you have a maximum from step a) and a minimum from step b). Step c): From that same table, for the assumed temperature you're going to use, you determine the maximum take-off % N1. N1 is the low-pressure compressor speed, expressed as a percentage. And here's a key detail: you add 1.0% N1 if the air conditioning packs are off. The assumed temperature you use must neither exceed the maximum from paragraph a) nor be below the minimum from paragraph b). So you're bracketed between those two. Step d): Now enter the left column of CAP 698 Figure 4.17d with the assumed temperature minus the ambient temperature. That's the difference. Travel right along the line to the column appropriate to the ambient temperature, interpolating if necessary. Read the % N1 adjustment. Interpolating just means estimating a value between two listed ones. Step e): Subtract the value determined at paragraph d) from that at paragraph c) to determine the % N1 to be set at take-off. So you take your maximum N1 from step c), subtract the adjustment from step d), and that final number is the N1 you actually set on the throttles for take-off. Now let's move to a different procedure: De-rate. Both Airbus and Boeing use De-rated thrust, which reduces engine thrust by a fixed percentage. For example, De-rate 1 will reduce thrust by 4%, and De-rate 2 by 10%. So it's a fixed, pre-set reduction. Here's why it matters: because the thrust reduction is fixed, the minimum control speeds on the ground and in the air — VMCG and VMCA — can also be reduced. VMCG is the minimum control speed on the ground, and VMCA is the minimum control speed in the air. Reducing those can help increase take-off mass on a short runway, because you have more margin before you hit those control speed limits. But there's a critical limitation: once De-rate is selected, thrust cannot be increased until the aeroplane is accelerated during flap retraction. So you're locked into that reduced thrust for the early part of the take-off, and you can't just push the levers up if something goes wrong. Finally, the last additional take-off procedure is for when the anti-skid system is inoperative, using the simplified method. You might think anti-skid doesn't matter for take-off, but it does. Class A aeroplanes have to demonstrate that in the event of an engine failure, the aeroplane can stop within the confines of the runway. So the accelerate-stop distance required must be less than or equal to the field available. Accelerate-stop distance is the distance needed to accelerate to V1, then abort and stop. If the anti-skid system doesn't work, your stopping ability is severely reduced, and that causes the accelerate-stop distance to increase dramatically. To solve the problem, V1 is reduced. And you may recall that reducing V1 decreases the accelerate-stop distance — because you're committing to stop at a lower speed, so you have more runway left to stop in. So to tie it all together: you've got the assumed temperature method for finding your reduced N1, the De-rate method for a fixed thrust cut, and the anti-skid inoperative procedure where you lower V1 to keep the accelerate-stop distance within the field available. Each one is a tool to manage the take-off under different constraints.