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

Let's start with the contaminated runway definition, because it sets the threshold for everything else. 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 surface water more than 3 mm deep, or by slush or loose snow equivalent to more than 3 mm of water. So the key numbers: 25% of the area, and 3 mm depth. If less than that, it's not contaminated by this definition. Now, why does contamination matter? Slush, loose snow, or standing water on the runway affects both the take-off distance required and the accelerate-stop distance required. The take-off distance required increases because of two drag sources: additional wheel drag and impingement drag. The accelerate-stop distance increases for two reasons as well: the increased distance to accelerate, and the increased distance to stop resulting from the reduced runway coefficient of braking friction. So for given distances available, the maximum take-off mass and V1 will be reduced compared to the dry runway. And here's an important relationship: the greater the depth of contamination, the greater the mass reduction, and the less the V1 reduction. So deeper contamination hits your mass harder than it hits your V1. The supplementary performance information required by EU-OPS 1 should include accelerate-stop distance, take-off distance, and take-off run, appropriate to the relevant contaminant, derived in a similar manner to those distances with a wet runway. Now, the acceleration distance should take account of three things: the additional drag due to gear displacement drag, the spray impingement drag, and the decrease of drag which occurs above the aquaplaning speed. For rotating tyres, or tyres going from a dry surface to a flooded surface, the hydroplaning speed, VP, is calculated using the formula in Figure 15.1. So the hydroplaning speed is the speed at which the tyre loses contact with the runway surface due to water pressure. That's the point where drag actually decreases, which is why it matters for the acceleration distance calculation. Now, the broader context: this chapter covers additional take-off procedures beyond the normal ones. The normal procedure for determining take-off mass, take-off speeds, and thrust settings is in the previous chapter. Chapter 15 adds procedures for: take-off with contaminated runway, take-off with increased V2 speed, take-off with reduced thrust, and take-off with anti-skid inoperative. Most of these procedures are in CAP 698, pages 24 to 34 of section 4. You don't need to memorise too much — CAP 698 details both the theory and the methodology of each procedure. The contaminated runway procedure specifically is on page 24 of section 4. So the takeaway: contamination is defined by that 25% area and 3 mm depth threshold, it increases both take-off distance and accelerate-stop distance through drag and reduced braking friction, it reduces your maximum take-off mass and V1, and the hydroplaning speed VP is the point where drag behaviour changes.