The concept of extraordinary circumstances is used to decide whether an airline can be held responsible for a disruption affecting one of their flights or not, and if it is therefore liable to pay compensations or reimbursements. Its definition has evolved over time through court rulings and regulation amendments. Main types of extraordinary circumstances are extreme weather-related issues, strike action, and safety concerns. However, only a subset of specific sub-types fall into the definition, for example lightning strikes and airline crew strikes do not count. Our flight rights page and several other FAQ questions tackle this topic.