How do we know that "upward as high as an aircraft can fly" is an adverbial phrase and not an "adjective". I thought "upward" can act as adjective, and so can modify a "noun", ie "extent".
Example: The upward trend in revenue this year looks promising.
=> "upward", which is adjective, modifies "trend", which is a noun.
Similarly, in the given sentence, "upward" should be able to modify the noun "extent",
...
Example: The upward trend in revenue this year looks promising.
=> "upward", which is adjective, modifies "trend", which is a noun.
Similarly, in the given sentence, "upward" should be able to modify the noun "extent",
...
