Some of this analytic terminology changes so often that it's impossible to give a definitive answer without knowing the methodology taken by your textbook - or whatever you're using as your guide. "before building a new runway" sounds like a (subordinate) clause to me! "before (PRO they) build (ing) a new runway" Taking a stab at "have to do", I would say have [ Clause [NP PRO (airport)] [VP to do [NP a cost-benefit analysis]] endClause] No guarantees that this is what you need!
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