They aren't indestructible RainyDay, but they are robust. They have a few layers to them and more solid innards than you would like in a plane! They are also armoured with steel plate which would make for a rather heavy and expensive plane and are placed in the tail to provide further protection ( as impacts almost always happen to the front of the plane first you could think of the whole fuselage as a crumple zone for the flight data recorder and the cockpit voice recorder). Basically you wouldn't want to fly inside one but you wouldn't want to fly without one either.