First, crosswind — Page 284, Lesson 353

Let me walk you through the effect of crosswind and flap settings on landing performance — this is a key part of understanding why we set the aeroplane up the way we do for landing. First, crosswind. When the wind is blowing across the runway, it will tend to push the aeroplane off the centre line. To counteract that, the pilot has to apply crossed controls — that is, a combination of aileron and rudder working against each other to keep the aeroplane tracking straight down the runway. The problem is that these crossed controls have to be held throughout the approach and landing, and there's a limit to how much control authority you have. That's why aeroplanes are given maximum crosswind limits — a published figure, in knots, beyond which you must not land. It's a hard limit, set by the manufacturer, because beyond it you simply cannot guarantee you'll keep the aeroplane on the centre line. Now let's look at flap setting. Flaps are devices used to increase the camber of the wing — that is, the curvature of the aerofoil section — and that generates more lift, which in turn reduces the landing speed. But here's the trade-off: using a lot of flap dramatically increases aerodynamic drag as well. And in landing, that drag is actually a benefit — it helps slow the aeroplane down. Let's trace through the three cases. With no flap, the aircraft has a much faster approach speed and very little aerodynamic drag, so the landing distance will be large. With some flap selected, the approach speed is less, there's more aerodynamic drag, and consequently the landing distance is smaller. With full flap, the approach speed is minimized and the aerodynamic drag is maximized, so the landing distance is least compared to the other flap settings. There's also a mass benefit: having full flap selected for landing allows the aeroplane to maximize its landing mass. That's because the lower approach speed and higher drag mean you can carry more weight and still stop within the runway available. But there's a disadvantage to full flap, and it concerns the go-around. If you need to abort the landing and go around for another attempt, you'll recall that large flap angles greatly deteriorate climb performance compared to no flaps. So in a go-around, after the climb has been established, you retract the flaps as soon as possible, in the manner prescribed in the aeroplane flight manual. That's the key sequence: establish the climb first, then clean the flaps up — because retracting them too early would sacrifice the lift you need to climb away safely. So the picture is: full flap gives you the shortest landing distance and the highest landing mass, but it costs you climb performance if you have to go around — and that's why the go-around procedure is so specific about when and how you retract the flaps.