Now, the physics here — Page 413, Lesson 505

Let's start with the question that's sitting right in front of us, because it's a perfect bridge into this chapter. The question asks: when the outside air temperature increases, what happens to the field length limited take-off mass and the climb limited take-off mass? Let me define those two terms first, because they're the heart of this. The field length limited take-off mass is the maximum mass at which you can complete the take-off within the physical runway distance available, accounting for the accelerate-stop distance and the take-off distance. The climb limited take-off mass is the maximum mass at which the aircraft can still meet the required climb gradients after take-off, with one engine inoperative. Now, the physics here. When outside air temperature increases, the air becomes less dense. Less dense air means the wings produce less lift for a given speed, and the engines produce less thrust. So, for a given mass, you need a longer runway to get airborne, and you climb more slowly. So, for the field length limited take-off mass: because the air is less dense, you need more distance to accelerate to the same speed, and the engines are producing less thrust. That means, for a given runway length, you can't be as heavy. So the field length limited take-off mass decreases. For the climb limited take-off mass: with less dense air, the climb gradient you can achieve is reduced. To still meet the minimum required climb gradient, you must be lighter. So the climb limited take-off mass also decreases. So the correct answer is option c: both the field length limited take-off mass and the climb limited take-off mass decrease. Now, this question is actually the last one in the question set for this chapter, and it's a nice lead-in to what we're about to cover. This chapter is about Class A additional take-off procedures. Class A refers to multi-engine aircraft, and these are the non-standard take-off procedures you need to know. Let me give you the roadmap. We're going to cover: non-standard take-off procedures, contaminated runways, take-off with increased V2 speed, take-off with reduced thrust, de-rate, and take-off with anti-skid inoperative. Let's start with non-standard take-off procedures. These are procedures that deviate from the standard, normal take-off. The key idea is that when you change something from the standard configuration or procedure, the performance figures change, and you must account for that. Then we have contaminated runways. This is a big one. A runway is considered contaminated when more than 25% of its surface area is covered by standing water, slush, snow, or ice. The contamination affects braking action and acceleration, so the take-off performance is degraded. We'll look at how you adjust the take-off mass and speeds for contamination. Then, take-off with increased V2 speed. V2 is the take-off safety speed, the speed at which you can climb with one engine inoperative. Sometimes, you increase V2 above the minimum. This gives you more climb capability, but it also increases the take-off distance, so there's a trade-off. Next, take-off with reduced thrust. This is where you don't use full take-off thrust. You reduce it to save engine wear and fuel. But you must ensure you still meet all the performance requirements, like the climb gradients and the accelerate-stop distance. Then, de-rate. This is a specific type of reduced thrust. De-rating is a fixed, pre-determined reduction in take-off thrust, set by the manufacturer. It's not a variable reduction; it's a fixed percentage reduction that's certified. The advantage is that the engine is treated as if it has a lower maximum thrust, so the maintenance intervals are longer. Finally, take-off with anti-skid inoperative. Anti-skid is the system that prevents the wheels from locking during braking. If it's inoperative, your braking effectiveness is reduced, so the accelerate-stop distance increases. You must adjust the take-off mass accordingly. Now, let me show you the procedure for a contaminated runway, because that's the core of this chapter. This figure shows the procedure you follow. You start with the actual take-off mass, and you work through the adjustments for the contamination. The key is that you must reduce the take-off mass to account for the reduced braking and acceleration on the contaminated surface. And here's a sample data table for a runway with 2 mm contamination. This shows you the actual numbers you'd use. For a given runway length and contamination depth, you look up the maximum allowable take-off mass. The contamination depth is measured in millimetres, and the table gives you the mass limit. Now, let me go back to that first question about the certification file. The question listed four options: 3050 m, 3513 m, 2555 m, and 2938 m. This is asking for a specific distance, likely the required runway length for a particular take-off scenario. Without the full question stem, I can't tell you which is correct, but the point is that you're calculating a distance based on the performance data. Let me also show you the first figure. This is Figure 15, which likely shows the relationship between take-off mass and the required distances, or the effect of contamination on performance. So, to summarise what we've covered: we've defined field length limited and climb limited take-off mass, and we've seen how temperature affects them. We've also laid out the structure of this chapter: non-standard procedures, contaminated runways, increased V2, reduced thrust, de-rate, and anti-skid inoperative. The key takeaway is that every deviation from the standard take-off procedure requires you to re-evaluate the performance limits, and the take-off mass is the primary variable you adjust.