Translation:Airplanes also fly in the sky.
In addition to marking the direct object of a verb, を can also have the meaning "through": So in my head I parse this as "fly through the sky". It seems that if an alternative particle has to be used, に would have been preferred over で. I'm unfortunately not really able to explain why, though.
It's a bird! It's a plane! It's SUPERMAN! ...wait, there's a plane up there too, I guess.
Both of those are variations on the verb 飛ぶ (とぶ). 「とび」is the polite form (also known as the masu stem) and とんで (which is what I assume you meant) is the te-form (slightly misleading in this case because it's actually "de"). The te-form is used in a variety of ways, but the most common usage to chain verbs (ie. 「とんでおちました」"I flew and fell").
I put "airplanes also fly the sky," and it marked me wrong, obviously, because that obviously doesn't make any sense in English, but that is literally what the sentence is saying. Airplanes also fly the sky! Once again, を is not a preposition particle, right?!