And on-topic, I suppose airports are limited in how they can build their perimeter fences.
I think we all agree that you cannot turn an airfield into a bunker, as that would pose lots of problems in normal operations. What we really need is a system where unlawful entry is immediately detected and an appropriate and effective response (i.e. armed) is initiated without undue delay. This is obviously not the case in Bruxelles and god knows how many other major fields.
