Watch a Boeing 777X fly and something looks almost alarming: the wings bend. A lot. In gusty air the tips ride metres above where they sit on the ground, flexing like a diving board. Passengers new to it sometimes gasp. But that enormous flex is not a flaw or a sign of stress — it is exactly what Boeing designed the wing to do. And when the 777X taxis to the gate, those same giant wings do something no airliner wing has done before: they fold up.

Why the wing bends so much

The 777X's wing is longer and far more slender than the metal wing it replaces, and it is built almost entirely from carbon-fibre composite. That combination — long, thin, and made of a material that can spring back — is what lets it flex so dramatically. Boeing calls it an "unconstrained" wing, and the flexing is a feature, not a failure.

A stiff wing fights turbulence, transmitting every jolt into the airframe and the passengers. A flexible wing rides it, bending to absorb gusts and then springing back, which smooths the ride and cuts drag and fatigue. The more efficiently a wing can flex with the air, the less fuel the aircraft burns and the gentler the flight feels.

The GE9X engines, the wing and the fuselage of the 777X, explained.

And why it folds

There is a catch to building a wing that big. At nearly 72 metres, the 777X's wingspan is too wide for the standard airport gates that today's 777s use. Rather than force airlines to rebuild their gates, Boeing borrowed a trick from carrier aircraft: hinged, folding wingtips. On the ground, the outer few metres of each wing fold upward, shrinking the span back to around 65 metres so the jet fits the same gates as its predecessor.

The tips fold automatically once the aircraft slows below taxi speed, and they must be locked down and confirmed before take-off — a system regulators studied closely before signing off. It is the first time airline passengers will watch their own wings fold and unfold like a bird settling and stretching.

Tested to the breaking point

If you still find a flexing wing unnerving, consider how hard Boeing pushed it before letting anyone fly. In ground testing, engineers loaded the wing far beyond anything a flight could ever impose — bending it up toward 150 percent of the maximum load the aircraft would ever see in service, until it finally failed. A famous test even ruptured a fuselage door under that extreme strain. The point of breaking one wing on the ground is to be certain none will break in the air.

So the next time you see a 777X wing curve skyward, relax. That is a wing doing precisely what a century of aeronautical engineering has taught us it should: bending, not breaking, and folding itself neatly away when the flying is done.

Why the Boeing 777X has folding wing tips.

The 777X's first flight.

Sources: CNN; Popular Science; Simple Flying; Aerospace Testing International; Boeing.