The system determines at what altitude can a plane fly in a certain airspace
or
The system determines at what altitude can a plane fly in certain airspace
Word complains when using "a certain". Yes, I know I should be guiding myself after Word, but it often led me to insightful answers. In this case, I'm curious not only why it complains but also why to me the 1st version sounds better. I'm not a native speaker :)
My rule of thumb: when in doubt, get rid of extraneous bits:
The system determines at what altitude can a plane fly in an airspace
or
The system determines at what altitude can a plane fly in airspace
And then, if certain words (ha! See what I did there?) throw you (as airspace does to me,) swap 'em out for something else:
The system determines at what altitude can a plane fly in a hangar
or
The system determines at what altitude can a plane fly in hangar
Clearly, at this point, the correct choice would be a hangar, but is hangar the same sort of word as airspace? Hangar is clearly a countable noun; I'll admit I'm not sure if airspace is. (Can you have multiple airspaces?) Space, on the other hand, (in the context of outer space) is an uncountable noun.
Unfortunately, my lack of knowledge about airplanes (I can count the number of round trips I've flown on one hand) prevents me from giving a definitive answer, if you can determine whether or not airspace is a countable or uncountable noun, then the answer is easy -- countable nouns need the article; uncountable ones don't. (And, if that weren't confusing enough, some words can be both: I love coffee; go fetch me a coffee.)
