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This is the official documentation that we found regarding current commercial airplane design criteria; https://www.law.cornell.edu/cfr/text/14/25.341

It says the criteria for continuous turbulence is 90 feet per second, or 'fps'. Is this a big improvement from the criteria of the 1950's and 1960's, which saw several commercial airplanes lost due to turbulence-induced structural failure?

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  • $\begingroup$ 90fps approximates to 120kph or 75mph, so winds of that strength aren’t uncommon. The question of turbulence is rather different because it presumably refers to a localised airspeed relative to the airframe. Like many test criteria this is easy to set up in a wind tunnel but it’s difficult to predict what real-world atmospheric conditions would result in that rate of air movement. $\endgroup$
    – Frog
    Jun 21 at 5:54
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    $\begingroup$ Thunderstorm, bad day in the mountains, strong frontal boundary, microburst. $\endgroup$ Jun 21 at 6:56
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90 feet per second is 5,400 feet per minute. Per the PHAK, Chapter 12 (p.12-11), a microburst can produce those conditions:

A typical microburst has a horizontal diameter of 1–2 miles and a nominal depth of 1,000 feet. The lifespan of a microburst is about 5–15 minutes during which time it can produce downdrafts of up to 6,000 feet per minute (fpm) and headwind losses of 30–90 knots, seriously degrading performance.

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