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

I want to walk you through the additional take-off procedures for a Class A aeroplane. This is the part of performance work where we deliberately use a higher assumed temperature than the actual outside air temperature, to reduce the thrust we set for take-off. Let me explain why we do that, and then we'll go through the exact steps. First, the core idea. Before we can use any of these procedures, we have to determine the most limiting performance condition. The only common parameter that lets us compare all the different limits is temperature. So we calculate the maximum permissible temperature for the actual take-off mass from each of four graphs: the field limit graph, the climb limit graph, the tyre speed limit graph, and the obstacle limit graph. From those four temperatures, we select the lowest one. Then we make sure that lowest temperature does not exceed the environmental limit. If it does exceed it, then the environmental limit itself becomes the assumed temperature. Now, the assumed temperature is the key concept here. We are pretending the air is hotter than it actually is, which tells the engine computers to produce less thrust. Let me walk you through the calculation steps, which are labelled 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, and read the maximum temperature in the column appropriate to the aerodrome pressure altitude. Step b): From CAP 698 Figure 4.17c, on the bottom line, you determine the minimum assumed temperature for the aerodrome pressure altitude. Step c): From that same table, for the assumed temperature to be used, you determine the maximum take-off % N1. N1 is the low-pressure compressor speed, expressed as a percentage. You add 1.0% N1 if the air conditioning packs are off. And remember the constraint: the assumed temperature used must neither exceed the maximum from paragraph a) nor be below the minimum from paragraph b). Step d): Enter the left column of CAP 698 Figure 4.17d with the assumed temperature minus the ambient temperature. Travel right along the line to the column appropriate to the ambient temperature, interpolating if necessary, and read the % N1 adjustment. 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 the whole process is: find your assumed temperature limits, get the maximum % N1, adjust it, and subtract the correction to get the final N1 setting. Now let's look at 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%. Because the reduction is fixed, 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 these can help increase take-off mass on a short runway. But there's a critical limitation: once De-rate is selected, thrust cannot be increased until the aeroplane is accelerated during flap retraction. 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 isn't important for take-off, but remember: Class A aeroplanes have to demonstrate that in the event of engine failure, the aeroplane is able to stop within the confines of the runway. Therefore, the accelerate-stop distance required must be less than or equal to the field available. If the anti-skid system does not work, stopping ability is severely reduced, and the accelerate-stop distance increases dramatically. To solve the problem, V1 is reduced. Reducing V1 decreases the accelerate-stop distance, which is exactly what we need when stopping performance is degraded. Let me show you the procedure visually. This is Figure 15.2, the procedure. And is Figure 15.3, sample data for a runway with 2 mm contamination. So to summarise what we've covered: we determine the most limiting temperature from the four graphs, we calculate the assumed temperature and the % N1 to set, we understand De-rate as a fixed thrust reduction that also reduces VMCG and VMCA, and we know that with anti-skid inoperative we reduce V1 to keep the accelerate-stop distance within the field available.