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The 707 was introduced in the late 50's, the 787 in 2009.

Back when Air France 447 had recently gone missing, many were speculating that the aircraft had broken up in the turbulence associated with the massive thunderstorm it was flying through. Some aviation spokesperson, who I believe were associated with Airbus, said that the planes are designed to withstand the worst storms with a 50% safety margin.

Several airliner models from the 50's and 60's have broke apart in flight due to turbulence associated with thunderstorms (A 720 B, A BAC-1-11, and a F-27 in 1981). There were also breakups involving clear-air turbulence (707, Mt. Fuji, 1966, and a Consolidated Airlines F-27). I checked to see if their G rating was lower than the planes of today and they were the same - 2.5+ G Limit Load, with a 50% safety margin, so 3.75+ G Ultimate Load (beyond which structural failure occurs).

Why is this? Shouldn't the G rating be much higher if modern airliners are more resistant to violent turbulence?

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    $\begingroup$ It may have to do with the manufacturer publishing upto the certification requirements, and not capabilities beyond that. Just a thought. $\endgroup$
    – skipper44
    Dec 26, 2020 at 7:39
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    $\begingroup$ Related question: How come the DC-8 plane can withstand +15G from turbulence, but newer planes can only withstand 20% of that amount? $\endgroup$
    – DeltaLima ♦
    Dec 26, 2020 at 10:27
  • $\begingroup$ Why has maximum safe G load not increased in 70 years of jetliner passenger service? My guess is because with a force of more than 4 G there's a very high potential for the passengers and crew to be killed or injured. Even if the plane can still fly the problem will be the people inside will have so many broken bones and other injuries that the probability that the people will survive long enough for a landing at an airport is quite small. Astronauts will see more G forces but they are strapped tight in special seats, in peak physical condition, and trained on how to react. Just my guess. $\endgroup$
    – MacGuffin
    Dec 27, 2020 at 17:16

3 Answers 3

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Note that the 2.5 g limit is for load due to manoeuvring, not only for turbulence. The certification specification for large aircraft on the subject of turbulence and gusts has changed several times since the certification of the Boeing 707.

Turbulence, and its effects on aircraft, is nowadays much better understood. Different aircraft react differently to the same turbulence. What will result in a high g-load in one aircraft, may be a moderate g-load in another. Therefore the certification regulations now focus on prescribing the type of turbulence that needs to be safely dealt with, instead of the resulting load factor.

In 1964, a formula was introduced describing the gust load that the structure aircraft has to deal with. This was added as FAR 25.341. This section has subsequently been updated in 1990, 1996 and 2015.

Modified from my earlier answer to a related question

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  • $\begingroup$ Correct me if I'm wrong, but it seems as though the gust load limits are very similar, from 56 fps to 66 fps. This does not seem like a big improvement. That's a difference of 7 miles per hour. Am I reading the papers wrong? $\endgroup$
    – Lars Knowles
    Dec 26, 2020 at 19:15
  • $\begingroup$ @LarsKnowles, the original criteria just had a single requirement: handling of discrete vertical gusts. The 2015 update has three: discrete gusts (both vertical and lateral), continuous turbulence, and paired vertical/lateral discrete gusts. $\endgroup$
    – Mark
    Dec 26, 2020 at 19:39
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    $\begingroup$ In addition to Mark's comments, which excellently addresses the main point of my answer, the extra 10 fps in discrete vertical gusts all but insignificant. It's an increase of 18% in speed, and 39% in kinetic energy. $\endgroup$
    – DeltaLima ♦
    Dec 26, 2020 at 19:43
  • $\begingroup$ Thanks for the clarification. However thunderstorms and wind around mountains can produce gusts stronger than 66 fps often, can they not? $\endgroup$
    – Lars Knowles
    Dec 26, 2020 at 19:49
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Resilient to turbulence could also mean that they don’t experience as much roughness when flying through turbulent air. The wing loading of the aircraft has much to do with this. Modern aircraft with more efficient wings and high loading ratios (kg/m2) will be ‘bashed about’ less than older aircraft with lower ratios.

From your example the Fokker F27 has a ratio of 282kg/m2 of wing whereas a Boeing 739 or Airbus A380 have ratios somewhere up in the 689-690kg/m2 region.

Ref

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It depends on the interpretation of ‘resistant’. You’d home that problems like metal fatigue are better understood these days, and modern designs may deal with turbulence without exerting large g-forces. It’s difficult to imagine passengers being too happy with exposure to 2.5G so avoiding or mitigating such events has to be better than just making aircraft stronger.

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    $\begingroup$ Now that we understand metal fatigue, we're building everything out of composites... $\endgroup$
    – user_1818839
    Dec 26, 2020 at 14:12
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    $\begingroup$ Metal fatigue is not a factor in once in a blue moon gust events. It requires long term cyclic loading, within the load limits. A gust event that exceeds yield load limits will leave the airplane bent. If it doesn't exceed yield limits it won't be bent and is good to go. It only comes apart if it exceeds ultimate limits, or if the structure was weakened by fatigue related to normal long term cyclic loading. $\endgroup$
    – John K
    Dec 26, 2020 at 16:18
  • $\begingroup$ You don’t think that pre-existing fatigue might make an airframe more likely to fail when subjected to a gust event? $\endgroup$
    – Frog
    Dec 26, 2020 at 19:20
  • $\begingroup$ @Frog Metal fatigue, if it exists under normal loading conditions, will eventually cause the component to fail completely under normal loading conditions. That is, for example, what causes engines to fail catastrophically once in a while - an undetected manufacturing flaw creates a tiny crack through metal fatigue, which slowly propagates until the fan blade or disc is unable to withstand the forces it is routinely subject to. But there is so much safety margin in main structural parts that metal fatigue is not a factor there. $\endgroup$
    – Chromatix
    Dec 26, 2020 at 21:49
  • $\begingroup$ It follows that at some point between ‘new’ and ‘broken’ there will exist a state where a 2.5G loading would cause failure, does it not? $\endgroup$
    – Frog
    Dec 26, 2020 at 23:14

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