Kallio

As far as I understand, not really. It is probably a breach of collective bargaining agreement but if we are talking a typical strike that is after the agreement has expired it wouldn't matter either.

But I very much doubt using strike breakers would work. First it would make FAs' union people quite mad and make negotiations much harder. Also if the strike breaking actually keeps planes flying, other unions would probably start supporting labor actions and ground the planes. Supporting strikes could be illegal but Finnish labor law doesn't allow punishing employees taking part of any kind of organized strike or similar labor action, legal or illegal. The organizing union would get a fine, but they are relatively small.