enter image description here

The positive and negative limit load factors in this diagram are around 4.5 and -1.8, which means the aircraft experiences structural damage when the aircraft is flown beyond these limit load factors, and beyond the positive and negative ultimate load factors, which are around 6.6 and -2.8 respectively, the aircraft would experience structural failure. These excessive loads typically occur when the aircraft is maneuvered abruptly as in steep dives and steep turns or when the aircraft is flown in turbulent air.

I understand it's excessive loads that cause structural damage or failure when an aircraft is flown beyond the positive or negative limit load factors. But what induces structural damage or failure when the same aircraft is flown within the limit load factors?

My best guess is the parasite drag, but the fact that the normal operating range (green) within the limit load factors jumps to structural failure (red) without going through structural damage (orange) leaves me at a loss, because if the cause is the parasite drag, I don't see the reason structural failure should occur without the structural damage being first experienced.

  • Edited in response to Simon and kevin's advice
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    $\begingroup$ My guess is you're asking about the jump from "Caution range" region to "Structural failure" region, correct? $\endgroup$ – kevin Mar 4 '17 at 13:21
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    $\begingroup$ @kevin Correct! That was the gist of my question. $\endgroup$ – lemonincider Mar 4 '17 at 13:34
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    $\begingroup$ At its current state, it took a bit of guess work on my part to arrive at that conclusion. You may want to edit the question to make it more clear to other readers. $\endgroup$ – kevin Mar 4 '17 at 13:51

Flying way too fast is an example as shown by the graph. For example descending with full power while maintaining 1 G load factor. Or with a powerful enough engine, exceeding the speed limit in level flight.

The air impacts dynamic pressure on the aircraft, the faster the plane, the higher this pressure—up to the design limitations.

What causes structural failure when the aircraft is merely flown fast?

High speed induces vibrations, flutters, etc., all would likely cause loss of control and/or structural damage/failure.

Flutter: At its mildest this can appear as a "buzz" in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or lead to the destruction of the aircraft.

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(Source) Flutter.

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  • $\begingroup$ Thank you for your answer but how would you explain that structural failure is incurred without structural damage being first experienced? $\endgroup$ – lemonincider Mar 4 '17 at 11:37
  • $\begingroup$ What I'm trying to ask is why an aircraft directly experiences structural failure (without experiencing structural damage first) when it's flown way too fast unlike when it's flown beyond limit load factors. $\endgroup$ – lemonincider Mar 4 '17 at 11:45
  • $\begingroup$ Actually the PHAK distinguishes between the two. Structural damage refers to a bending of the aircraft structure and structural failure its breakage. But if flutters are the cause, I think I have the answer now. Flutters do not entail bending and go right into breakage, I guess. $\endgroup$ – lemonincider Mar 4 '17 at 11:50
  • $\begingroup$ I guess I'm wrong here. Flutters seem to cause bendings of the aircraft structure as well based on this video: youtube.com/watch?v=qpJBvQXQC2M $\endgroup$ – lemonincider Mar 4 '17 at 12:00
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    $\begingroup$ @lemonincider, the thing with flutter is that the frequency at which the aerodynamic forces induce it increases with speed (in this case it is true airspeed, not dynamic pressure). While that frequency is lower than the eigenfrequency of the structure, the oscillations are damped and nothing is happening. But when the frequency increases to match the eigenfrequency, the oscillation start to grow until the structure breaks. And this transition happens quickly, with just a couple of knots increase in speed. $\endgroup$ – Jan Hudec Mar 5 '17 at 12:17

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