It's inflated（使充气） with helium, unlike the original, which was filled with hydrogen and, unbeknownst （未知的adj）to the pilot, potentially explosive.
上面这句话存在省略现象
1如果还原是什么样
2上面这句话有那些省略现象。
谢谢您
分析一下结构，也许可以从另一个角度帮你解决问题：开始说it充装了helium，与original的那种不一样; 后面的部分都是在说明original那种到底是什么样的——它filled with hydrogen,它potentially explosive，而且这种explosive是pilot所不知道的。
It means tire.It's inflated with helium, unlike the original (filled/aired tire), which was filled with hydrogen and, unbeknown to the pilot, (it is) potentially explosive.
