In the case of Article 5 of the NATO Treaty (an attack on one is an attack on all) the member states can still decide what if any level of assistance they wish to provide. Therefore European nations can individually say "no thanks, guv." If Turkey wants to start dropping Russian aircraft that are posing no threat to its own territory then they can take the consequences. It wouldn't be the end of NATO but might possibly be the end of Turkey in NATO. As Ankara is a direct sponsor of Al Qaeda that would be no bad thing, imo.