I recently got a Schengen visa, valid from my date of arrival and departure as per my tickets which are 77 days apart. Duration of the visa shows 78 days. Is this visa valid for 90 days? What can I do in case of unforeseen flight delay etc.?
No, the visa is valid only for the dates that are stated on it. No, not every Schengen visa is 90 days visa. Many visas are valid only for the days of your planned journey. And it can be even less than one week!
If your flight is cancelled you'll have to provide confirming documents at the border control.