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What is typically the limiting factor for how large a crosswind component a given plane can be safely landed in?

My impression is that larger planes are less sensitive to crosswind than smaller ones -- for example, many commercial airports seem to be doing quite well with a single or multiple parallel runways for transport-category aircraft, whereas airports catering to light aircraft will be proportionally more likely to have (and routinely use) a system of intersecting runways.

I can imagine one reason for this, namely that a plane with a higher landing speed will need less of a crab angle for a given crosswind component. But is this actually the determining factor?

If yes, then is is safe/legal/practical to land a light aircraft in more crosswind than it can usually handle, by landing faster, assuming there's a nice long runway available to decelerate on? (In other words, can an airport make up for having no crosswind runway by having a longer primary runway instead?).

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    $\begingroup$ This may help understand the difficulty. There is no limit while landing manually, but automated landings (Cat III for instance) have imposed limits. Usually the company requires pilots to remain within crosswinds demonstrated by the manufacturer for certification. $\endgroup$ – mins Feb 8 '15 at 9:55
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    $\begingroup$ Instead of a hard limit, there usually is a maximum demonstrated crosswind component. The question is also related $\endgroup$ – DeltaLima Feb 8 '15 at 10:03
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    $\begingroup$ I am aware that the limit is not hard, but there are situations where a PIC will decide "I won't try to land here because of the crosswind" and divert to somewhere that has a more suitably aligned runway, right? I'm interested in the factors that affect that decision, and why it seems to be necessary less often in airliners than in light aircraft. $\endgroup$ – Henning Makholm Feb 8 '15 at 12:52
  • $\begingroup$ The ability of the pilot's feet to reach the rudder pedals. $\endgroup$ – kevin Feb 8 '15 at 13:55
  • $\begingroup$ The limited deflection of the rudder will limit the ability of the pilot to forward slip and de-crab. But the most important factor maybe the undercarriage capability to sustain lateral constaint. $\endgroup$ – mins Feb 8 '15 at 13:59
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Your question is not easily answered, at least in my opinion, but I'll have a go at it.

Your impression that larger airplanes are less sensitive to xwinds than smaller airplanes is correct, very much so in my experience. I lack the expertise to give you the aerodynamic basis, the equations if you will, but I'm fairly certain that saying an aircraft's sensitivity to a xwind is inversely proportional to it's mass, landing speed, and wing loading is a correct statement. As I remember, the max demonstrated xwind for a Cessna 172 is 15 kts, for a 747 either 25 or 30 kts, and I would much rather be landing a 747 in a 30 kt xwind than a 172 in a 15 kt xwind. In 747 sims we would occasionally crank the xwind up to 40 kts for an interesting but still safe landing.

Using extra speed in a small aircraft to help is not, in my opinion, a good idea. You'd certainly want to add a bit to your approach speed to handle gusting conditions (which typically accompany crosswinds), but what you need to do is transition from flying speed to non-flying speed as quickly as possible rather than extending the time of that transition. You need to get the weight off the wings and on to the wheels as quickly as possible.

There is one exception to what I just said. Back in the 1950s when I first learned to fly, they used to teach what they called 'wheel landings' for handling crosswinds in taildraggers. The problem with a taildragger is that the c.g. is behind the main gear. If you touch down in any kind of a crab, it wants to switch ends. In other words, a ground-loop. The idea of a 'wheel landing' was to get the main wheels on the runway with extra speed (after having slipped to the touchdown) so that the rudder (with the tail still off the ground) had plenty of authority to counteract weathervaning, and of course with the upwind aileron fully up. Then you were supposed to keep the airplane straight while the your speed dissipated to where the tail could no longer fly. I have no idea if they still teach that technique.

So now you're the PIC and having to decide whether to land in a xwind. Following are some of the things I can think of that might enter into your decision making:

  • What's your personal confidence level? If you're nervous about the situation you're probably not going to do as good a job as if you were relaxed.

  • What's your experience level in the airplane? Aircraft, even of the same weight class, vary in their handling qualities in a xwind, and the more you've flown them in xwinds, the better you're going to be in that model of aircraft in a xwind.

  • High wing or low wing? The closer to the ground you get the more the wind decreases, and that few feet of difference can make a difference. Also, I suspect, but do not know, that having most of the wing below the c.g. rather than above the c.g. helps.

  • What is the runway surface, and what is its condition? In other words, how slippery is it going to be. A dry paved runway is, of course, the best. However, a dry paved runway with a lot of reverted rubber in 120 F (49 C) temperatures can be interesting. Ice or wet grass is perhaps the worst. I put a Cessna 150 off the runway in wet grass back in the 1970s.

  • How bad do you need to land there? Some might say that doubts about being able to safely land should always override the need to land, but that's not the real world.

  • For a light aircraft, you may be able to turn that strong xwind into a strong headwind by using a taxiway. I've done that at a uncontrolled field.

A word of caution: Just because the xwind is within the max demonstrated xwind component does not necessarily mean you can safely land, it just means an experienced test pilot did it on a reasonable runway surface. The max demonstrated xwind is a guideline that is sometimes safe to exceed, sometimes not, and sometimes not safe to even get close to.

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    $\begingroup$ Agreed... plus add in recency of experience, if you fly once every month, you're going to be less likely to go to the limits, versus someone who flies daily and knows exactly where their limits are, and how to fly at them. Also, the wind sensors and winds constantly are changing, what worked a minute ago, might not be safe to land in now. $\endgroup$ – slookabill Feb 8 '15 at 23:25
  • $\begingroup$ Thanks for the practical perspective. When I speculated about landing faster I had in mind something to increase the stall speed (such as refraining to extend flaps) so you'd be in "non-flying speed" appropriately faster. But I understand from your explanation that this just isn't done. $\endgroup$ – Henning Makholm Feb 12 '15 at 1:29
  • $\begingroup$ @HenningMakholm Apologies for not having picked up on your intention. You are correct, though, that in a strong xwind it is generally advised to use minimal flaps in some aircraft as you will be 'slipping' into the xwind. As I remember (increasingly a problem at my age) there used to be an admonition in the Cessna 172 POH against slipping with more than 30 degrees of flaps back when you could go to 40 degrees in a 172. I think newer 172s have 30 degrees as their max extension. $\endgroup$ – Terry Feb 12 '15 at 2:50
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For Part 23 airplanes, the answer is:

FAR §23.233:

Directional stability and control.

(a) A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VSO.

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