Forget about the physical damage to a building
what if his REAL target had been a sizable public gathering, say at a sporting event, several kilometers away?
Or what if his real objective had been not to crash the plane but to spread something nasty throughout Frankfort's business district (i.e. a terrorist attack on an economic target)?
In either case I'd think the wisest course of action would be to blow him out of the sky at the earliest opportunity, hopefully well before he reached his intended target.