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For any low-wing aircraft, particularly the MD-80 or some of the private jets, CRJ's, the wings appear about 5ft or less above the ground. It seems that just few degrees of roll before touchdown would cause a wing strike on the runway and a disaster.

How do pilots and aircraft manufacturers avoid this ? It seems like with the thousands of daily flights - in all sorts of inclement weather conditions - this (fortunately) isn't a problem. (I'm not saying it's never happened, just that it seems exceptionally rare.)

So my question is, why isn't this a serious problem? I would expect turburlence and wind gusts to do all sorts of unpleasant things to aircraft, one of which might be disrupting the roll a couple degrees and planting the wingtip into the runway. What steps do pilots and aircraft designers take to mitigate this?

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    $\begingroup$ Pilots? If the crosswind/gusts are too much to handle, you use another runway or go to another airport. $\endgroup$ – jamesqf Mar 25 at 18:31
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    $\begingroup$ Remember that while for regulation the wings are bended "downwards", and the bending needs to be taken into account. However while it is landing the wings are bended "upwards" as lift is more than gravity of the wing itself, only after full touchdown with all wheels the lift becomes zero. $\endgroup$ – paul23 Mar 26 at 1:40
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It's not how high the wing is off the ground, but the angle between the main landing gear when compressed and the tip of the wing, which with the wing dihedral makes it even higher off the ground – on the MD-80 that's 2.6 m (8'7"). This will be the roll angle limit.

enter image description here

It's for that reason, unlike smaller high-wing general aviation planes, big jet-liners don't decrab as much in a crosswind landing, whereas decrabbing (going one-wing-low) would be a problem. From an MD-80 flight manual:

Crosswind Landing

On final approach, establish a crab angle into the wind to hold the aircraft on the extended centerline of the runway. Maintain the crab angle until just before touchdown, then use the rudder to align the aircraft with the runway. The touchdown is made with cross-controls as necessary to track straight down the runway. Keep wings as level as possible as the wingtip will touch at an 8° bank angle. Do not hold off downwind wheels; prompt and firm runway contact will greatly assist in rollout stabilization.

So the prevention is respecting the limits, good training, and knowing when the weather is too gusty to just abort the landing.

It does remain a concern though. Wikipedia has an article on it, and the issue would also affect the engine when wing mounted, but with the engine typically forward of the wing, it's given better clearance when the nose is still higher for flaring. The A320 accident mentioned on Wikipedia seems to have been the result of both pilots controlling the plane during landing.

In 1997 a 747 hit the runway with the outer engine when landing in Kai Tak:

enter image description here
(flickr.com)

A more recent one is a Bombardier Global 6000 in 2014.

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    $\begingroup$ Thank you for the data. I attempted to look up the MD-80 "wing height" from google before posting and I couldn't find it, so I guessed. Also thank you for mentioning decrabbing. I knew of the concept and mentioning it in my question would have made my intent clearer. Because if a pilot were to decrab, that further lowers the error margin on that side's wing.Is the situation any worse for private jets such as a Citation? I would expect them to get blown around much more due to having less mass. $\endgroup$ – mike Mar 25 at 16:44
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    $\begingroup$ @mike The effect of crosswind also varies with fuselage side area, which is generally going to be lower if the mass is lower. If you decrab late enough, inertia will keep you on the centerline just long enough to touch down before the crosswind accelerates you sideways. The stronger the crosswind, the later you have to decrab and thus the more skill required. If you decrab too early, you have to sideslip, which risks an expensive wing tip or engine strike. $\endgroup$ – StephenS Mar 25 at 17:34
  • $\begingroup$ Kai Tak was a graveyard for engine and wing strikes... even on a good day it was a nightmare. When a suitably performant aircraft might legitimately consider a split-s as a reasonable approach pattern, you know there's going to be some excitement. $\endgroup$ – J... Mar 25 at 23:30
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    $\begingroup$ @dotancohen No, I was being facetious. Still, it doesn't seem much more crazy than flying directly at a mountain until the last minute before a hard bank, drying the line hung laundry of the HK suburbs, and sliding sideways onto the tarmac. $\endgroup$ – J... Mar 26 at 11:10
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    $\begingroup$ @J... As the pax watch the TVs in the apartments bordering the airport. $\endgroup$ – dotancohen Mar 26 at 13:04
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On heavies, as ymb1's answer shows, the wings level crab and kick method (basically air-skidding into the touchdown) is used because the mass of the airplane means you don't have the roll rate necessary to screw around safely with the wing tips close to the surface. On the other hand, the mass gives a good delay between initiating the skid into the crosswind and the airplane starting to drift laterally (because you are in effect in a skidding turn and eventually you will start turning, which is perceived as a downwind drift) so it favours this technique.

On lighter jets like Regionals and corporate, you use light aircraft side slip technique, wing down with rudder to align with the runway. On the CRJ IIRC, you have about 8-10 degrees of roll available before wingtip contact, which is actually quite a bit. Side slipping to correct for a crosswind when you are at 125-135 kt rarely requires more than, say, 5 degrees of bank.

If a crosswind is strong enough to require more than 5 degrees, you usually end up touching down with a bit of crab in, or you may use an extra shot of rudder to kick it straight without lowering the wing any more, and lower the nose more aggressively to plant the plane right away before it starts to drift because you have more rudder than necessary for the bank (and live with a firm touchdown).

That being said, on the CRJ fleet there are usually several wing tip strikes a year during crosswind landings in gusty conditions, where a gust dropped the wing more than expected. The result is usually just a bit of wing tip road rash. Winglet repair schemes are a recurring event at the OEM's support organization.

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  • $\begingroup$ "wing tip road rash". Holy moly, that's terrifying but reassuring at the same time, knowing the aircraft can take that kind of abuse if absolutely necessary. $\endgroup$ – mike Mar 25 at 20:37
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    $\begingroup$ By the time the tip touches the pilot flying already has counteracting aileron applied, just not enough soon enough, and airplanes like the RJ have really powerful roll control, so the tip is off the ground again right away. $\endgroup$ – John K Mar 25 at 21:08
  • $\begingroup$ Those wings are strong! I remember watching a documentary which showed a 777 having its wings tested to destruction (for model certification purposes). I couldn't believe how far they bent before finally disintegrating. I can quite believe that scraping a winglet is not very serious. $\endgroup$ – nigel222 Mar 26 at 9:48

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