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. $\endgroup$– JScarryCommented Jul 5, 2017 at 1:35
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1$\begingroup$ 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! $\endgroup$– user18035Commented Oct 30, 2017 at 7:51
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3 Answers
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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... $\endgroup$– xxavierCommented Jul 4, 2017 at 14:53
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1$\begingroup$ @xxavier, what is your source for that? $\endgroup$– acpilotCommented Jul 4, 2017 at 16:49
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2$\begingroup$ @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.$\endgroup$– kevinCommented Jul 4, 2017 at 16:53 -
3$\begingroup$ @kevin: This is valid for approach, but not landing. $\endgroup$ Commented Jul 4, 2017 at 17:08
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1$\begingroup$ @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? $\endgroup$– jamesqfCommented Jul 5, 2017 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.
answered Jul 4, 2017 at 14:41
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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... $\endgroup$– xxavierCommented Jul 4, 2017 at 16:03
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1$\begingroup$ @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. $\endgroup$ Commented Jul 4, 2017 at 17:18
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$\begingroup$ @alephzero. Yes, it takes precisely that small time, that is the same at any landing speed... $\endgroup$– xxavierCommented Jul 4, 2017 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).
