When is a claim for flight delay compensation not a claim for flight delay compensation?
The answer: where an airline can prove an 'exceptional circumstance' caused the flight delay problem.
What is an exceptional circumstance? Well the obvious ones are things like bad weather or a volcanic ash cloud. These external events are beyond the control of the airline and therefore, if a flight is delayed as a result, you would not be able to claim delayed flight compensation.
Other exceptional circumstances include: strikes by air traffic controllers or other airport staff; bomb scares at an airport; passenger incidents e.g. someone being taken ill or being disruptive on a flight; strikes by airline staff (but only for the first 24 hours); lightning strikes on planes and bird strikes away from the airport.
In the past, airlines tried to claim that a technical fault with an aircraft should be treated as an exceptional circumstance but the European Courts have ruled that such faults are not to be treated as exceptional. They are within the airline's control through maintenance and inspections and so the airlines still have to pay delayed flight compensation even if there was an unexpected problem with the aircraft.