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Most commercial airliners are rated to take +2.5g and -1.0g. Why is this? Is it designed that way on purpose? To clarify, this would mean that the cost difference between -1g and -2.5 g is minuscule, so price (and thus engineering difficulty, which drives price) isn't really a factor.

Or, are there in fact significant difficulties with creating a frame that can handle a lot of negative Gs? (And if there are, what are they?)

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    $\begingroup$ The gravity of Earth is +1g. So if the +2.5g and -1.0g ratings are correct, planes can take an extra positive 1.5g (2.5g-1.0g) but 2.0g lower than normal 1.0g-(-1.0g). Hope this makes any sense. $\endgroup$
    – collector
    Jun 24 '15 at 17:24
  • $\begingroup$ @collector It had occurred to me. It almost makes it sound like they add a little structural reinforcement for negative G's since the difference from mean gravity is roughly neutral, with it being skewed towards negative Gs. This works really well when the plane is right side up, but I've seen similar figures for fighter jets, which are designed to go upside down as well... Anyway, you might spin that comment into an answer, maybe the community will agree with you :). $\endgroup$
    – Jay Carr
    Jun 24 '15 at 17:28
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It's partly pragmatic (you start from +1.0 G, so why not optimize for that???) and partly because (most) aircraft are designed to carry human beings. Humans as well as a lot of other things are FAR more tolerant of positive G-forces than they are negative G-forces.

There's very little technical reason that you couldn't design an aircraft wing and other structures to handle high negative G loads. But adding that strength also means adding weight, complexity, and cost. And since the benefit of being able to pull high negative G loads is pretty small, very few designers choose to do this.

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