General Principles - Landing — Page 284, Lesson 352

Let's pick up with the landing phase and look at two more factors that shape the landing distance: density and wind. First, density. Air density affects two things: the true airspeed for a given indicated airspeed, and the thrust or power of the engine. Now, during landing, thrust is small, so the main effect comes through the TAS. Low density — which means high temperature, low pressure, or high humidity — gives a higher TAS for the same indicated airspeed. That higher TAS means the aeroplane is moving faster over the ground, so the landing distance increases. But note the qualifier: this effect is to a lesser degree than for take-off distance. So density matters for landing, just not as much as it does on take-off. Now wind. You'll recall that a headwind decreases the true ground speed for any given indicated airspeed. So during a headwind, the forward speed over the landing surface is much less, and as a result the distance required to bring the aeroplane to rest is decreased. A tailwind has the opposite effect — it increases the ground speed for a given indicated airspeed, so the forward speed over the landing surface is much greater, and the distance required to stop is increased. Here's a critical operational point. When you examine the effects of wind on landing performance, you should not use the actual wind that's given. Why? Just in case the wind changes to a worse condition than the one you planned for. So when calculating actual landing distances, the recommendation is to assume only 50% of the headwind component, and 150% of the tailwind component. That's the safety factor built into the planning. Most performance graphs that calculate landing distance already have the 50% headwind and 150% tailwind recommendations applied — so you don't apply them again on top of the graph; they're baked in. And one important note: there is no allowance for crosswinds, and therefore no safety factor for crosswinds. Crosswinds present an additional complication because of the effect of potentially crossed controls. The wind will tend to push the aeroplane off the centre line, so you'd have to apply crossed controls to counteract that — and that's why crosswinds get no safety factor in the landing distance calculation. So to summarise the picture: density affects landing distance through TAS, but less than for take-off. Headwind reduces landing distance, tailwind increases it, and you plan with only half the headwind and 150% of the tailwind as a safety margin. Crosswinds get no allowance at all.