Class A - Additional Take-off Procedures — Page 413, Lesson 509

Let’s start with the big picture. In the previous chapter you learned the normal take-off procedure — how you determine take-off mass, take-off speeds, and thrust settings for a clean, dry runway. Chapter 15 now adds what we call the non-standard take-off procedures. These cover four special cases: take-off with a contaminated runway, take-off with an increased V2 speed, take-off with reduced thrust, and take-off with anti-skid inoperative. Now, here’s a practical note that will save you stress: most of these procedures are actually published in CAP 698, in section 4, pages 24 to 34. So you don’t need to memorise every detail of the theory and methodology — CAP 698 gives you both the theory and the step-by-step method for each procedure. Your job is to know where to find it and how to apply it. Let’s focus on the first and most important one: contaminated runways. This is detailed on page 24 of section 4 of CAP 698. First, the definition — and this is a precise regulatory definition you must know exactly. A runway is considered contaminated when more than 25% of the runway surface area — whether in isolated areas or not — within the required length and width being used, is covered by either: - surface water more than 3 mm deep, or - slush or loose snow equivalent to more than 3 mm of water. So the threshold is 25% of the area, and the depth threshold is 3 mm — whether it’s actual water, or slush or loose snow that has the water-equivalent of more than 3 mm. Now, why does contamination matter? Because slush, loose snow, or standing water on the runway affects two critical distances: the take-off distance required and the accelerate-stop distance required. Let me explain each. The take-off distance required increases because of two types of drag: additional wheel drag and impingement drag. Wheel drag comes from the tyres pushing through the contaminant. Impingement drag is the drag from the spray of water or slush hitting the aircraft structure. The accelerate-stop distance also increases — and this is for two reasons. First, the distance to accelerate increases, because the drag slows you down during the take-off roll. Second, the distance to stop increases, because the runway coefficient of braking friction is reduced — the brakes simply can’t grip as well on a wet or slushy surface. Now here’s the key operational consequence. For a given runway distance available, the maximum take-off mass and V1 will both be reduced compared to a dry runway. And there’s an important relationship to note: the greater the depth of contamination, the greater the mass reduction — but the less the V1 reduction. So deeper contamination hits your take-off mass harder than it hits your V1. There’s also a regulatory requirement here. The supplementary performance information required by EU-OPS 1 should include the accelerate-stop distance, take-off distance, and take-off run, appropriate to the relevant contaminant. And these should be derived in a similar manner to the distances for a wet runway. Now, when you calculate the acceleration distance, you must account for three things: the additional drag due to gear displacement drag — that’s the drag from the landing gear displacing the water or slush; the spray impingement drag — the drag from spray hitting the aircraft; and the decrease of drag which occurs above the aquaplaning speed. And that brings us to a specific formula. For rotating tyres, or for tyres going from a dry surface to a flooded surface, the hydroplaning speed, denoted VP, is calculated using a formula shown in Figure 15.1. Let me make sure you understand what hydroplaning speed means. It’s the speed at which the tyre begins to ride on a film of water rather than on the runway surface — and above that speed, the drag characteristics change, which is why the formula matters for your acceleration distance calculation. So to summarise the key points you need to carry: the 25% area threshold, the 3 mm depth threshold, the two drag types that increase take-off distance, the two reasons accelerate-stop distance increases, the reduced braking friction, the mass and V1 reductions and how they relate to contamination depth, the EU-OPS 1 requirement for the three distances, and the hydroplaning speed VP formula. That’s the contaminated runway procedure in full.