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So for example if I am flying the Boeing 737-800, the FMC gives me three flap settings and approach speeds.

Why would a pilot want to fly with anything less than full flaps where possible?

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  • $\begingroup$ If flying a GA aircraft and you encounter icing, the recommended practice is to land without extending the flaps unless you are sure the wing surfaces are clear of ice. $\endgroup$ – JScarry Jul 5 '17 at 1:35
  • $\begingroup$ If you are below the glide-slope during short-final, you landing speed has to be higher than usual, or else you'll stall inches above ground - it might end up being fatal! $\endgroup$ – ClobberXD Oct 30 '17 at 7:51
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Less flaps gives a faster approach speed. In some scenarios, a faster approach speed is better than a slow one, for example:

  • Strong crosswind
  • Possible wind shear
  • One-engine failure

A higher approach speed provides a better stall margin and higher control authority in challenging situations. For example, the Boeing 737 QRH calls for a flaps 15 landing in "One Engine Inoperative" and "Stabilizer Trim Inoperative" scenarios.

In the crosswind scenario, let's assume your approach speed is 100 knots, and the crosswind is 20 knots. The crosswind is therefore 1/5 of the approach speed, and you need a larger crab angle to keep the airplane aligned with the runway. If you increase your approach speed to 120 knots, then the crosswind is only 1/6 of the approach speed, and the crab angle is smaller, making landing easier.

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  • $\begingroup$ Besides, it's advisable to use little or no flaps when landing in turbulent conditions, since a higher speed ensures a more uniform lift when flying in turbulent air... $\endgroup$ – xxavier Jul 4 '17 at 14:53
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    $\begingroup$ @xxavier, what is your source for that? $\endgroup$ – acpilot Jul 4 '17 at 16:49
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    $\begingroup$ @acpilot not sure about the aerodynamics part, but it is stated at least in the 737 FCTM: Flap extension in an area of known turbulence should be delayed as long as possible because the airplane can withstand higher gust loads with the flaps up. $\endgroup$ – kevin Jul 4 '17 at 16:53
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    $\begingroup$ @kevin: This is valid for approach, but not landing. $\endgroup$ – Peter Kämpf Jul 4 '17 at 17:08
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    $\begingroup$ @xxavier: Does the exact terminology matter more than the fact that in either case the airplane is likely to impact the ground in an uncontrolled fashion? $\endgroup$ – jamesqf Jul 5 '17 at 5:57
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In strong crosswind-conditions you want to consider not using full flaps to give the wind less attack area. So you have a slightly higher approach and landing speed, but you aren't blown as much to the side as you would be with full flaps.

In a Cessna 172, for example, this is normal practice.

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  • $\begingroup$ Well, if you're landing in a crosswind of X m/s, you'll be 'blown' sideways at exactly X m/s, no matter the landing speed you have... Unless you crab, of course, in which case you won't be blown sideways at all. The crab angle with flaps will be higher, but that's the only difference. And when you align with the runway, at the last moment, you'll be blown sideways at X m/s, irrespective of your speed at that moment... $\endgroup$ – xxavier Jul 4 '17 at 16:03
  • $\begingroup$ @xxavier No, you won't be instantly be blown sideways at X m/s "at the last moment", because the plane has mass, and therefore takes time to accelerate sideways up to X m/s. $\endgroup$ – alephzero Jul 4 '17 at 17:18
  • $\begingroup$ @alephzero. Yes, it takes precisely that small time, that is the same at any landing speed... $\endgroup$ – xxavier Jul 4 '17 at 20:49
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In a Cessna 150 for example, you want to be on the downwind going 70-75 knots, 65-70 on base, and 55-65 on final, depending on wind gusts. A pilot in a small plane might tend to put in 10° of flaps on the downwind, 10° on base, and adjustments or anything extra on final to allow them to get their speed to where they need it. It is nice to be in the middle where you can add or remove some if necessary (assuming you're not too close to the ground).

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