Aviation does not "forbid" the use of either "ascend" or "descend", or for that matter other similar-sounding words; there's no central authority that's going to issue a citation to a pilot who uses non-standard phraseology. (That said, if using non-standard phraseology might cause or even contributed to flight safety issues, then the individual in question might have to re-take training and a test to help prevent a recurrence.) In aviation, the concern is with clarity between people who sometimes share no language besides Aviationish, but also with getting information across at all. This results in normally highly structured statements where each word carries specific meaning, and even who is saying something itself carries meaning; but also an attitude similar to "if you can't think of the exact phraseology, just say it in plain language and we'll figure it out together".

That makes aviation radio communications very different from when you are writing a book.

For an example of where Aviationish differs from English, if you watch reenactments of aviation accidents or even read transcripts of radio communications in aviation accident investigation reports, there will be times where air traffic controllers ask the pilots for "number of souls on board". They don't phrase it that way because they or the people who came up with these phrases are necessarily spiritual; they phrase it that way because it's the standard phraseology for asking for the total number of persons on board, including passengers, crew, and others. Yes, it could be spelled out each time, but it's easier to have a phrase that means that and exactly that and that everyone agrees on the meaning of, even if the meaning differs somewhat from what one could take it to mean in plain language.