Yes it happens. However, Vienna isn't a major hub like Toronto and handles approx. 22million pax per yr. Toronto handles 35million. As such, it would be expected that Toronto would have backup systems and a redundancy to support the volume of pax and the importance of Toronto as the primary national hub.. Obviously, YYZ does not have a reliable backup system. Just another failure to add to the long list of deficiencies. Why Canadians are willing to tolerate such deficiencies and poor planning is beyond me. All we get are excuses and never an action plan to deal with the problems.