The Ultimate Element Crossword Puzzle Key This Element Caused The Two Worst Aviation Accidents In U.S. History, Resulted From Reaction With Oxygen?

This element caused the two worst aviation accidents in U.S. history, resulted from reaction with oxygen? - the ultimate element crossword puzzle key

so this thing is on a crossword puzzle that in doing.

Here is the key length:
You are number one in our books and in the universe, but the ultimate fuel, and a base, do not try, however, the true nature of explosives, two of the worst plane crash in U.S. history the following hiding their reaction with oxygen.

It is 8 characters long,
the third letter "D" and the seventh "E"
Please help:)