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Current crew alerting system messages, I believe, are displayed basing on the software detection of the failures basing on the sensor information.

  1. What kind of structural failures can occur to an aircraft in flight? (I know one case an engine got separated and communicated to pilot as just 'engine fail' instead of 'engine broken or something that might indicate a physical disconnect'). The had me wondering what other structural failures are possible when in flight. When I say structural failure, I am talking about a physical disconnect, bent, etc. Is there a (remote) chance that a structural failure can be caused in an airplane.

  2. If at all a structural failure occurs in the middle of flight, is there any mechanism to let pilot know about it.

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  • $\begingroup$ Engine-off alerts are somewhat discussed here. There are indicators for the hydraulic system that tells if they have a problem or not. $\endgroup$ – SMS von der Tann Jul 23 '16 at 18:37
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    $\begingroup$ "What kind of structural failures can occur to an aircraft in flight": Normally none. They happen only if some unexpected destructive event or fatigue occurs. The consequences of an unexpected cause are indeed nearly impossible to anticipate and difficult to sense. On the other hand whatever happens in an engine will likely create vibrations, so engine vibrations level is displayed to the crew with an alert threshold. $\endgroup$ – mins Jul 23 '16 at 21:12
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    $\begingroup$ For #2. The aircraft starting to tumble out of control might be a sign. But there isn't much point to building in an alerting system when no possible pilot action can alter the outcome. $\endgroup$ – Ralph J Jul 24 '16 at 0:29
  • $\begingroup$ so if a part of wing is severed, I guess it is impossible to detect $\endgroup$ – user2927392 Jul 24 '16 at 8:00
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    $\begingroup$ Not impossible but pointless displaying a warning message saying "wing missing, suggest search ocean floor" $\endgroup$ – Notts90 Jul 24 '16 at 20:00
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First of all, the design is analyzed to ensure that no normal conditions will cause the structure to fail. In reality, any structure can fail, so any kind of structural failure is possible. Of course, we like our airplanes to not fall apart in flight, so many things are done to prevent that from happening.

When considering failure of structure, two main things are considered: what is the consequence of failure, and how easy is it to detect?

When the consequence of failure is high, of course the part is designed as much as possible to not fail. Parts are designed such that any damage will be contained and not become a widespread failure. Another approach is fail safe structure, such that if part of it fails, other parts will still carry the load until an inspection will find it.

Aircraft are inspected regularly, in detail ranging from a regular walkaround by the crew before/after a flight, all the way up to a D check where the whole airplane is basically taken apart, inspected, and put back together. These inspections are designed to find structural issues and fix them.

Some failures are obvious, such as large parts departing the airplane. As Carlo mentioned, there may be many clues if this happens. A failure of the fuselage pressure vessel will result in depressurization. Structural failure may cause other systems to be damaged, such as hydraulics, control surfaces, or electrical systems, which will trigger warnings in the cockpit. In these cases, the parts are designed to allow the plane to safely land, after which it is assumed the damage will be repaired. Of course, this doesn't always happen, but structure is designed to handle much more load than most flights ever experience.

Other failures may not be apparent at all. In these cases, special inspections may be required to check for issues, or the failure must not compromise the structure until a regular inspection will find the issue.

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At the risk of sounding like a smartass, the alert is generally in the form of a loud BANG! or CRACK! followed, at best by different handling characteristics and increased wind noise/vibration, at worse, departure from controlled flight.

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  • $\begingroup$ That reminded me of Aloha flight 243. $\endgroup$ – orique Jul 24 '16 at 8:34
  • $\begingroup$ If you add that there's no electronic warning as it's usually in the form of a Big Bang I'd say your answer should be accepted! $\endgroup$ – Notts90 Jul 24 '16 at 19:57

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