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https://www.law.cornell.edu/cfr/text/14/25.341

Here it says the turbulence intensity used for certification is 90 feet per second. Before, it was 50 feet per second, that section was added in 1981. So since 1981 the criteria has almost doubled.

But how is it possible that the maximum manoeuvring load factor has remained exactly the same? A transport category plane designed after 1981 still can't pull a nearly 8 G turn.

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  • $\begingroup$ Isn't the turbulence intensity actually expressed as acceleration in fps²? 90 fps² calculates to 2.79G. Earth's gravity acceleration at sea level is about 32 fps². $\endgroup$ Oct 6 at 23:43
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    $\begingroup$ My question boils down to; how is it possible for an aircraft to withstand very high momentary forces, but not very high sustained forces? $\endgroup$ Oct 7 at 0:03
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    $\begingroup$ @ItisTiff_93 it isn't about momentary and sustained forces (that much; flexing smooths the spikes a bit, which helps), but rather about how big aerodynamic forces a sudden wind shear can generate. $\endgroup$
    – Jan Hudec
    Oct 7 at 5:23
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    $\begingroup$ @JanHudec you should answer. The concept is the same as a shock absorber/spring smoothing bumps in the road by flexing (with flexing can cause metal fatigue in mind). So the design criteria could include long and short term loads in any direction (with flutter/resonant frequencies in mind)? $\endgroup$ Oct 7 at 6:50
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    $\begingroup$ @JuanJimenez I don't believe any aircraft ever has been tested and shown to handle wing bending until the wingtips touch. Care to cite a source, I'm certain this is an urban legend? The wings of modern airliner do withstand bending wingtips way above the top of fuselage. $\endgroup$
    – Jpe61
    Oct 7 at 7:04
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Turbulence intensity applies to the air around the aircraft.
The sustained load factor applies to the aircraft itself.
These are apples and oranges, one cannot be directly compared to the other.

In a simple model of a high-G turn, the air remains stationary, and the aircraft increases its lift N times. Thus its wings have to be designed to carry a load of N*W (with fatigue and safety factors).

Gusts or turbulence also produce forces on the wing. But not as much force as would be required to impart the same amount of acceleration to the aircraft as the apparent acceleration of air in the gust. A fan that blows at X fps doesn't make every object in the room move at X fps.

A gust of 90 fps across a stationary flat plate would impart about 12 lb/sqft of pressure. A modern airliner's wing loading is between 100 and 140 lb/sqft. Real numbers will differ a lot, because it's not a static case at all, and manifests as an AoA change, with increased/decreased lift. Still, it's not extra G's of force, as that would require far more velocity.

What makes turbulence dangerous and very perceptible is the rate of change in acceleration, called jerk, not absolute acceleration. Its erratic behavior also contributes to vibration and fatigue. The added force is only a fraction of what the wing normally carries, but its rapid onset and cycling can be dangerous.

Increased design turbulence intensity is a requirement to account for these secondary factors, not to build more static strength into the design.

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  • $\begingroup$ A 90 fps is also ~45 knots, which, when combined with the forward speed, means quite significant change in angle of attack. But that's what the weather penetration speed is about—if the wing stalls before overloading, it isn't a problem. $\endgroup$
    – Jan Hudec
    Oct 11 at 4:50
  • $\begingroup$ I'm not disputing this answer, but I'm having a hard time seeing the difference in gust induced g-loading, and pilot (autopilot) induced g-loading. Having flown in intense weather aboard a plethora of varying acft, gust certainly can induce intense g-loads on planes... $\endgroup$
    – Jpe61
    Oct 11 at 8:42
  • $\begingroup$ @Jpe61 The difference is in the magnitude. Gusts feel intense, but it's their rapid changes in intensity that make it violent. The actual change in G-loading is smaller than what is possible in a turn. $\endgroup$
    – Therac
    Oct 11 at 12:01
  • $\begingroup$ So basically the old requirement of 50ft/s was just out of proportion with the load factor requirement, being too small? Still wondering how a rapidly changing loading is more demanding on the airframe than the same loading induced in a more controlled manner 🤔 $\endgroup$
    – Jpe61
    Oct 11 at 15:32
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    $\begingroup$ @Jpe61 Loads create displacement. The skin bends around the rivets in one direction, then in the opposite one. A cyclic load of just a fraction of design strength can cause local low-cycle fatigue in high-stress areas. As for single high-jerk loads, they can shake loose non-structural parts; under gradually applied loads, they just settle. $\endgroup$
    – Therac
    Oct 11 at 21:30
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My question boils down to: how an aircraft can withstand very high momentary forces but not very high sustained forces.

From your question one can gather the maximum manuvering load factor has remained the same, and the turbulence intensity for design criteria has almost doubled.

How is it possible?

Fiber reinforced composites.

These materials can withstand repeated flexing far better than metals. If one can imagine repeatedly bending a wooden stick and an aluminum tube, both may permanently deform under a similar stress load when broken "over the knee", but the wooden one may be more tolerant of repetitive bending without failing at a specific point due to fatigue.

It is this bending, "damping" or "shock absorption" superiority of more modern materials and building methods which may have lead to an increase in turbulence stress limits in design criteria, along with better understanding of weather phenomena such as microbursts.

One may also consider that a "transport" aircraft would not be designed for extreme manuvering, but would be required to fly in all weather. Higher aspect, more flexible wings (as seen with the 787) would more likely be in the design.

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  • $\begingroup$ Your answer seems to imply the design criteria for gust intensity was increased because materials have advanced. This is causally wrong, but I'm guessing it's not what you meant, so I would change the wording. Planes are still being built without fibre reinforced composites even though the design criteria is more demanding. $\endgroup$
    – Jpe61
    Oct 11 at 8:39
  • $\begingroup$ @Jpe61 the question is "how is it possible". One could also step to another level structurally by adding materials or changing configuration. The last paragraph seems to cover "why", feel free to edit in your input. $\endgroup$ Oct 11 at 12:11
  • $\begingroup$ My point is that rules are changed to ensure safety, extremely rarely (no instances come to mind) because technology advances. Your "how is it possible" part would answer a question about "how is it possible that modern aircraft endure force X". The question here is about "how is it possible that rule Y changed, but a seemingly connected rule Z did not?" $\endgroup$
    – Jpe61
    Oct 11 at 14:53
  • $\begingroup$ @RobertDiGiovanni I'd concur that FRC are only one of many ways to improve turbulence resistance. Others include adhesives, welding, better riveting, better testing. All-metal aircraft comply with the same requirements. Other than the 787, the A350, and the MC-21, most currently-produced airliners are mostly metal. $\endgroup$
    – Therac
    Oct 11 at 21:03

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