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The outer 11.4 feet (3.8 meters) of the wings of the Boeing 777X are designed to fold up when on the ground. Why aren't they designed to fold down?

It seems to me that folding down would be more "fail safe" if the locking mechanism failed in the air. The wings are generating lift, so they would hold themselves up and continue to contribute to keeping the plane aloft.

Up-folding wings could lift themselves if the locking mechanism failed in flight and destabilize the plane. The folding bit is less than 12 feet while the diameter of the engine nacelle is greater than 14 feet, so it doesn't seem like proximity to the ground is the issue.

What are the biggest reasons for folding up instead of down?

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    $\begingroup$ Navy aircraft have folded their wings up for decades, so presumably they have the mechanisms pretty well figured out. $\endgroup$
    – fooot
    Commented Feb 6, 2018 at 16:35
  • $\begingroup$ With folding-down wingtips, you're restricted (w.r.t. the length of the folding part) by the groud clearance. There is plenty of space in the upwards direction. $\endgroup$
    – Dohn Joe
    Commented Feb 25, 2019 at 8:43

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The FAA has decided/proposed (Nov 2017) that the folding mechanism must comply with certain standards, of which:

The wingtips must have means to safeguard against unlocking from the extended, flight-deployed position in flight, as a result of failures, including the failure of any single structural element.

All sources of airplane power that could initiate unlocking of the wingtips must be automatically isolated from the wingtip-fold operating system (including the latching and locking system) prior to flight, and it must not be possible to restore power to the system during flight.

The wingtip latching and locking mechanisms must be designed so that, under all airplane flight-load conditions, no force or torque can unlatch or unlock the mechanisms. The latching system must include a means to secure the latches in the latched position, independent of the locking system.

It must not be possible to position the lock in the locked position if the latches and the latching mechanisms are not in the latched position, and it must not be possible to unlatch the latches with the locks in the locked position.

Source: federalregister.gov (broke it down into paragraphs for easy reading)

As to why they chose up and not down, only the responsible engineering team at Boeing can answer that. My guess is that they trust their design over the drivers around the plane when it's parked.

This is echoed by airinsight.com:

(...) Boeing could point out that the upward folding wing offers considerably less opportunity for "ramp rash". It is, unfortunately, true that vehicle drivers around aircraft are often clumsy and hit aircraft.

Interestingly, the folding wingtip was envisioned 28 years ago for the 767-X (what eventually became the 777):

enter image description here
(flightglobal.com)

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    $\begingroup$ "Ramp rash" was my first though, too... $\endgroup$
    – RonJohn
    Commented Feb 6, 2018 at 12:42
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    $\begingroup$ From pictures, it looks as though the folding part might be long enough to touch the ground if it folded down. $\endgroup$
    – jamesqf
    Commented Feb 6, 2018 at 20:48
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    $\begingroup$ @jamesqf I don't think it would quite touch the ground, but it would definitely be uncomfortably close, especially when the wing tanks were fully fueled. The 777 wings are quite a bit higher off the ground than most, due to needing to mount those massive GE90s under the wings. The last thing you want, though, is some careless baggage cart driver running a ULD into the wingtip. That would be a very, very expensive mistake. $\endgroup$
    – reirab
    Commented Feb 7, 2018 at 8:09
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    $\begingroup$ Is there an argument in that during flight, start, landing or taxi, you'd rather the tip went upwards if the mechanism holding it straight failed than downwards. So that only being able to go up would be the safer failure than the tip going down? $\endgroup$
    – Bent
    Commented Feb 7, 2018 at 11:21
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    $\begingroup$ Ramp rash could be avoided by folding the wing tip all the way flush with the bottom of the wing, or at least close to flush. The motors and additional equipment may be heavier to accomplish this compared to the 90 degree fold though... $\endgroup$
    – Owen
    Commented Feb 9, 2018 at 7:08
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It's not that big a deal. It's fairly easy to make a latching mechanism that is incapable of operating under load, a great example is extension ladders or an over-center dog mechanism on a ship door.

Even mechanisms which aren't actually intended for that, are very hard to release under load simply because they bind. Try to release one of these under tension!

enter image description here

And this one's not even trying. It would be easy to make a mechanism which maximizes this binding force.

In the case of wingtips, wings have significant force bending the joint upward in flight (lift), and significant force bending the joint downward in taxi (the weight of the wing). That makes it all the easier to design such a mechanism, that is passively unable to release in lift.

Simply design the latch so there is considerable binding force when in lift. Size the actuators to be far too weak to operate it against that binding force. And/or put an intentional weakness in the operating lever where it'll shear off well before force becomes large enough.

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    $\begingroup$ _ It's fairly easy to make a latching mechanism that is incapable of operating under load_ - yes, and it's actually quite a tough job to design a mechanism that can be released under load that is comparable in load capacity and reliability to those mechanisms which can't. $\endgroup$
    – Pavel
    Commented May 13, 2019 at 6:49

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