Let me pick up where we were — Page 284, Lesson 353

Let me pick up where we were. We'd just finished talking about crosswind limits — the idea that a crosswind pushes the aeroplane off the centre line, so the pilot has to apply crossed controls to counteract it, and that's why every aeroplane type has a maximum crosswind limit published. Now I want to move on to the next factor that affects landing distance: flap setting. Flaps are devices on the wing that increase its camber. Camber is the curvature of the wing's upper surface — the more curved it is, the more lift it generates at a given speed. So by increasing camber, flaps generate more lift, and that extra lift allows the aeroplane to fly slower on approach. That's the primary purpose: reduce the landing speed. But here's the trade-off. Using a lot of flap dramatically increases aerodynamic drag as well. Now, in most phases of flight, drag is something you fight against — it slows you down and costs you fuel. But in landing, drag is actually a benefit. It helps slow the aeroplane down during the landing roll. So flaps give you two things at once: more lift to fly slower, and more drag to help you decelerate once you're on the ground. Let me walk you through the three flap settings and how each one affects landing distance. With no flap selected, the aeroplane has a much faster approach speed. It also has very little aerodynamic drag. The result is that the landing distance will be large — you're coming in fast and you're not getting much help from drag to slow down. With some flap selected — a partial setting — the approach speed is less, and there's more aerodynamic drag. Consequently, the landing distance is smaller than with no flap. With full flap, the approach speed is minimized and the aerodynamic drag is maximized. Therefore, the landing distance is least compared to the other flap settings. So full flap gives you the shortest landing distance. Now, there's a practical consequence of that. Having full flap selected for landing allows the aeroplane to maximize its landing mass. Think about it this way: the shorter your landing distance, the more weight you can carry and still stop within the runway available. So full flap is what you want for a normal landing. But there is a disadvantage to full flap, and it shows up in a specific situation: if the aeroplane needs to abort the landing and go around for another attempt. You'll recall from our earlier discussion of climb performance that large flap angles greatly deteriorate climb performance compared to no flaps. The flaps create so much drag that the aeroplane can't climb as steeply or as quickly. 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 point — the retraction isn't done arbitrarily; it's done according to the specific procedure in the flight manual for that aeroplane type. So to summarise the relationship: no flap means fast approach, little drag, long landing distance. Some flap means slower approach, more drag, shorter landing distance. Full flap means minimum approach speed, maximum drag, shortest landing distance — and that's what lets you carry maximum landing mass. But full flap hurts your climb performance, so in a go-around you retract the flaps as soon as the climb is established, following the flight manual procedure. That figure shows you the variation in drag through the landing roll — you can see how the drag forces change as the aeroplane decelerates on the runway.