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'Extreme' turbulence is a derived vertical gust velocity of >3,000 fpm (50 fps) that causes a vertical G-load of >3. The limit turbulence intensity for a modern day airliner is 90 fps. (there is likely a 50% safety factor, so the plane is capable of withstanding more).

Lateral turbulence is the type that puts most stress on things like engine pylons and the vertical tail, rather than the wings. An accident involving lateral turbulence is BOAC Flight 911 and the B-52 crashes (Savage Mountain and Elephant Mountain).

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  • $\begingroup$ The only thing I can think of that would impose extreme lateral gusts like that would be, maybe, wake turbulence from really big airliner hitting a smaller one behind it, or maybe hitting some kind of rotor just so. Maybe running into mountain wave rotor while parallel to the wave? Or CAT on the boundary of a jet? The risk of breaking the fin off from excessive rudder use is the bigger problem, like what happened to that Airbus that crashed into NYC just after 9-11 from rudder overcontrolling by the pilot. $\endgroup$
    – John K
    Feb 21, 2021 at 18:41
  • $\begingroup$ @PeterKämpf do you mean lateral or vertical are more dangerous? There appears to be a typo at the end of your sentence. $\endgroup$
    – Lars Knowles
    Feb 21, 2021 at 19:46
  • $\begingroup$ The lateral lift curve slope (better: Sideforce slope) is much lower than the vertical one. A lateral gust that does damage needs to be much stronger than an equivalent vertical one. $\endgroup$
    – Peter Kämpf
    Feb 21, 2021 at 20:22
  • $\begingroup$ Hi Lars, thank you for pointing out my mistake. I deleted the comment and replaced it with a hopefully better one. $\endgroup$
    – Peter Kämpf
    Feb 21, 2021 at 20:22

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Yes they are. FAR 25.427 has the requirements for lateral gust loads.

I don't know if there was FDR information for the BOAC flight, but it may very well have been a similar event to AA Flight 587, the one that lost its vertical fin over NYC when the pilot got on the pedals in reaction to a wake turbulence encounter that induced a large yaw, and made rudder input loads that the fin is not designed to handle.

In that case the fin broke off not because of the gust, but because of the control response. You will likely be out of phase in the attempt, and your inputs become "doublets"; amplifying inputs that make the yawing worse. You don't normally touch the rudder pedals on an airliner between departure and landing unless an engine quits; you just let the yaw damper take care of yaw excursions like that and it'll have no problem staying in phase to dampen the motion. I'll bet that the BOAC incident was that sort of case, where a gust reaction was followed by an input, at high speed.

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  • $\begingroup$ Thanks for that. I'm a little rusty, but what gust strength (in fps or mph) would be required to cause the vertical stabilizer to snap off, like in BOAC 911? I remember reading about the accident saying they estimated the gust strength to be 140 miles per hour, hitting the vertical tail broadside. $\endgroup$
    – Lars Knowles
    Feb 21, 2021 at 21:52
  • $\begingroup$ I have no idea what the numbers are. Peter might know, or know where to find it. I added some extra context to my answer. $\endgroup$
    – John K
    Feb 21, 2021 at 23:17
  • $\begingroup$ A gust would be unlikely to snap the tail, because the gust is pushing on the fuselage as well as the tail. The problem in cases like AA587 is that the first rudder input had everything moving in one direction, then the next input pushed the tail hard in the opposite direction. Normally, having the rudder push the tail simply moves the rest of the plane with it, but in that case the momentum and the forces involved got out of hand. A gust pushing on the whole aircraft together would have neither the effects of momentum nor the push-on-the-tail-only that broke the American Airbus that day. $\endgroup$
    – Ralph J ♦
    Feb 22, 2021 at 0:08
  • $\begingroup$ I just described exactly that in my answer. That's what a "doublet" is. When testing the yaw damper system during production test, you disconnect the YDs and perform a very mild doublet on the pedals to get a dutch roll going, then reengage a YD channel. You do not try to correct any yaw excursions on a swept wing jet by trying to counteract with rudder. You will be out of phase and just get a dutch roll PIO going. $\endgroup$
    – John K
    Feb 22, 2021 at 1:45
  • $\begingroup$ FDR information was not available for BOAC 911. (The aircraft had an FDR, but it was destroyed by the postcrash fire.) $\endgroup$
    – Vikki
    Mar 19 at 1:36

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