How does a UFO crash?
What causes that?
It's a good bet these things don't use petroleum for fuel, so whatever they use might be effected, or might have been effected (before they fixed the problem decades ago) by unplanned contacts between vehicles and or strong radar interference. The gravity cancelling and drive systems are theorized to operate via electromagnetic vortex manipulation (spin freaks anyone). Radar is electromagnetic energy, so that might be a good guess.
The non-scientist, experimenter, John Hutchison has created some very interesting effects that even the CIA wanted a closer look at. We have the dustification of the Twin Towers by some directed energy effect, and there are data stores from many UFO crashes (some of which are most assuredly ours in early development), so if the records are ever opened to the public we may find out lots of things.