One of the absolute requirements of an aircraft turbine engine (usually some sort of turbofan or turboprop) installation is that, in the event of a destructive failure of the engine, the engine cowling must be able to contain any and all fragments released in the process. In layman’s terms: engine blow up, engine parts stay in cowling. The cowling’s ability to contain an engine disintegration must be demonstrated in testing. All of this has been the case, without exception, for decades.
And, yet, uncontained engine failures continue to occur. As recently as October 2016, a 767’s engine exploded during takeoff. (Yes, I know about the one with the 737 earlier this year, but that one’s still under investigation, and, as such, off-topic until the NTSB releases their final report.)
Why is this? It can’t be for lack of testing capability, as the engine manufacturers can and do – indeed, are required by law to – blow up engines in their test stands to verify their inability to escape their cowlings, and causing an engine failure for such a purpose is ridiculously easy: wrap some detcord around a fan or turbine blade (to test against the engine throwing a blade), tie it to a fan or turbine disk (to test against one of the rotors seeing fit to come apart in flight), or wrap it around the engine shaft (to test against a shaft separation and consequent turbine overspeed and disintegration, LOT 007-style), run the engine up to full blast, and push the button. So why do engine cowlings still sometimes fail to contain rapid unplanned engine disassemblies?