Engineering for damage control is an unfortunate necessity, but sometimes things go wrong. Reliability doesn’t always mean avoiding failure. It can be just as important to contain it.
As reported by CBS News, an Airbus A319 operated by Spirit Airlines suffered an "uncontained" failure of one of its engines. Passengers reported an explosion which resulted in debris being ejected from the engine casing. No one was injured and the flight landed safely. This begs the question, “Can an exploding engine be considered a 'contained' failure?”
The answer comes in the form of definition. As long as debris is not thrown through the outside engine enclosure, it is considered contained. If debris exits that enclosure it can then go on to damage the wing, fuselage, or other critical components; even passengers. It is critical that a failure be contained and kept from cascading into a major disaster.
Jet engines are tested in a variety of ways. One of the most spectacular is the “blade off” test. Jet engines essentially operate like fans, albeit about a million times more complex. If you pop a blade off of your ceiling fan and turn it on high, it is going to be ugly. Now imagine it spinning 1000 times faster.
Although there are other considerations of mass distribution and support in a jet engine, it is still a very bad thing. Manufacturers spend significant time and resources to ensure the destruction stays inside the casing. To test the design suitability, a blade is rigged with a small explosive charge that dislodges it and throws the engine off-balance. As the misbalance forces the assembly off center, the remaining blades contact the wall, are destroyed and the engine eventually grinds to a halt due to the failure.
Although the engine is ruined, the damage is contained. Uncontained failures are rare, but there can be significant repercussions if it happens. In the Spirit flight, the reported explosion occurred shortly after takeoff. Although no official report has been filed, the circumstances may be quite different from a blade off situation. Doubtless, however, is that “routine” engineering design precautions reduced the severity of the failure.
A video depicting blade off testing is below. The force generated during such failures is tremendous and a testament to the requirements put on the engine casing.