Expert: Ray Wilkinson - 9/14/2006

Question
If an airplane engine exploded in midair, what could be the best cause for it?

Answer
Hi Kevin

It's almost unknown for an engine actually to explode, but there are many cases of engines breaking up in dramatic ways. It's generally known as an uncontained disc failure. Assuming we are talking about a jet engine, there are two or three large components inside that rotate at high speed - as fast as 80,000 rpm for small engines and perhaps 8,000 rpm for the largest. These are the compressor and turbine spools that (respectively) compress the incoming air before it enters the combustion chamber and extract energy from the combustion gases to turn the compressor and fan or propeller. They have high mass, and hence a lot of stored energy when they're spinning. If anything solid (for instance a large bird or a piece of structure) enters the fan or compressor and hits the blades, they may become unbalanced and cause a lot of vibration, which can cause the whole thing to break up. Also, if one compressor blade breaks free it may cause damage to the next row, which can result in a 'domino' effect right through the engine.
Normally, the casing is strong enough to contain the failure, but in a major break-up the bits may not be contained and the casing may burst. For this reason, it is mandatory to test engines for bird-strike resistance - one of the gorier bits of the test programme, but very necessary.

I hope this answers your question.

Regards, Ray.