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Wikipedia says:

As existing regulations do not cover the folding wingtips, the FAA issued special conditions, including proving their load-carrying limits, demonstrating their handling qualities in a crosswind when raised

Why? Especially when this question says:

The 777X family (777-8/-9/-10) features wingtips that fold up on the ground to let the aircraft fit in tight spaces. These have to be extended and locked in place before a revenue flight can take place,

So, what is the point of certifying the 777X for wingtip up flight? I haven't found a reason for this in Wikipedia either. Is it for ferry flights or something?

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    $\begingroup$ I'm unclear on what "raised" means, is that retracted or extended? The wingtips on the 777X shouldn't ever be locked in a "raised" position AFAIK. $\endgroup$ – zymhan Sep 8 at 15:49
  • $\begingroup$ @zymhan And I'm not sure what you mean by "retracted or extended"? I'm not a native English speaker, but to me these words don't make sense when talking about the 777X wingtip. Raised and lowered seem more intuitive to me. $\endgroup$ – Bianfable Sep 8 at 15:52
  • $\begingroup$ @zymhan actually yes, it does sort of mean retracted, though the term would misleading in this case (I think). Anyway, pretty much anyone who's familiar with the 777X would know that a raised tip is a folded tip $\endgroup$ – Abdullah Sep 8 at 15:54
  • $\begingroup$ Boeing also has to demonstrate “acceptable” handling qualities during crosswind conditions, even if one wingtip fails to completely fold. This would indicate that they are not simply looking at Open vs Closed.flightglobal.com/faa-sets-certification-rules-for-777x-wingtip/… $\endgroup$ – zymhan Sep 8 at 16:16
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    $\begingroup$ @zymhan: Some parts of that article sound almost like a threat in hindsight, now we know what would happen only 4 months later: "As with any critical airplane systems, like flight controls, […] Boeing uses a hazard class determination for certain extremely improbable airplane level failure events to drive the appropriate high-integrity system architectures, design redundancies, and safety features to preclude such events,” Boeing says. “This same safety and certification methodology was used with our new folding wing tip […]" Oh really, they applied the same safety methodology as for MCAS? $\endgroup$ – Jörg W Mittag Sep 8 at 21:58
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If you read the details in the Federal Register, you can see that this refers to crosswind handling on the ground, not in the air:

The folding wingtips and their operating mechanism must be designed for 65 knot, horizontal, ground-gust conditions in any direction as specified in § 25.415(a). Relevant design conditions must be defined using combinations of steady wind and taxi speeds determined by rational analysis utilizing airport wind data.

And:

The airplane must demonstrate acceptable handling qualities during rollout in a crosswind environment, as wingtips transition from the flight-deployed to folded position, as well as during the unlikely event of asymmetric wingtip folding.

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  • $\begingroup$ Why is asymmetric wingtip folding "unlikely?" I don't think there is any mechanical connection between the two wingtips going right along the wing spars and through the fuselage which is strong enough to make it impossible that one moves and the other does not! $\endgroup$ – alephzero Sep 9 at 17:56
  • $\begingroup$ If they got it right this time, failure to raise or lower a wingtip will require more than one failure in the system responsible. (Barring extreme conditions like total absence of the wingtip) Which ought to be unlikely. Even in that condition the aircraft should be safe on the ground : hopefully they won't try to fly it like that. $\endgroup$ – Brian Drummond Sep 9 at 20:29
  • $\begingroup$ @alephzero, there is no mechanical connection between flaps either, but the system still must prevent asymmetric deployment. This is similar. $\endgroup$ – Jan Hudec Sep 11 at 7:02

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