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On Southwest 1380, initial indications are that a fragment of the engine blade pierced the engine casing, which then hit the airplane window (or perhaps a part of the casing did). According to the BBC,

Mr Sumwalt also said a casing on the engine was meant to contain any parts that come loose but, due to the speed at impact, the metal was able to penetrate the shell.

Is this an reasonably expected containment failure? Or is the failure to contain surprising to industry experts?

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    $\begingroup$ Yes, the engine casing is designed to contain the inner components if they fail. Why it didn't in this case is what the NTSB is actively investigating and nobody here can yet answer, so this question should be closed until the investigation is completed. $\endgroup$ – Ben Apr 21 '18 at 4:52
  • $\begingroup$ Ben's comment is basically the answer. “Yes, you're right to be confused/disturbed.” The BBC line makes it sound as if the event were fully understood. $\endgroup$ – adam.baker Apr 21 '18 at 5:45
  • $\begingroup$ Note that there was already similar accident, Southwest B737 near Pensacola on Aug 27th 2016, uncontained engine failure, but that investigation is also still in progress and nobody seems to have much idea why in the inlet cowling separated. Just that in both cases the initial failure triggered failure of the inlet cowling ahead of the fan and that in turn caused all the other damage. $\endgroup$ – Jan Hudec Apr 22 '18 at 20:56
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We don't yet know how or why the engine casing failed to contain the fan blade on Southwest 1380.

But yes: Not only are the engines designed to contain blade failures at fan red-line speeds, but they are also required by 14 CFR 33.94 to confirm they do, typically via destructive "blade-off testing." (There are a number of good videos of this testing online: Search "turbine blade fail test" or "blade-out engine test.")

As noted here "fan blade-out" failures in the real world are virtually unseen. (Turbine blade failures are relatively common and can even go unnoticed until major service teardowns.)

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    $\begingroup$ I should be noted that uncontained engine failures still do occur at rate of several a year. Quick search shows at least 5 in 2017 and it's similar for preceding years. Usually this is because a larger fragment breaks loose—the requirement is to contain a single blade only. $\endgroup$ – Jan Hudec Apr 22 '18 at 20:18

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