I usually see fatal plane crashes end with a huge explosion, what causes the fuel to ignite?
2$\begingroup$ It is possible worth clarifying that plans very rarely 'explode' since that has a specific meaning with respect to the burn front en.wikipedia.org/wiki/Deflagration. Aircraft instead normally have a rapid burnoff of fuel, potentially making a rising fire ball. This looks a lot like what films call 'an explosion' since burning fuel is cheap and impressive, but does change the precautions/prevention engineering. End result for the people on board is not meaningfully changed between 'explosion' and 'massive fireball' so does not impact the answer. $\endgroup$– GremlinWrangerFeb 4 at 0:03
$\begingroup$ Hollywood special effects, maybe? $\endgroup$– Peter KämpfFeb 4 at 1:07
$\begingroup$ @PeterKämpf To be fair, most videos of airshow disasters do show planes deflagrating (instead of detonating) which most lay people would call "explode" even though it's just burning quickly instead of actually going "boom". The main cause is quite obvious - when you have lots of metal moving at high speed hitting other bits of metal moving at high speed you get a log of sparks which will cause fuel to burn $\endgroup$– slebetmanFeb 5 at 16:29
In the case of jet or turbo prop aircraft, the combustion section of the engine is continuously lit, and is not contained by a cylinder and valves like in a piston engine. When fuel tanks rupture you have fuel and an open flame in close proximity that could ignite fumes.
And in a high speed impact, there is a chance that the engine case would also rupture, presenting an even greater opportunity for spilt fuel and an open flame to combine in a spectacular manner.