The EU Regulation 261/2004 provided consumers with a possibility to claim compensation in case their flight was cancelled (pursuant to Article 7 of the Regulation) or delayed (confirmed in ECJ case: Sturgeon C-402/07. However, in order to protect the interests of the airline industry, an exception had been made for situations when the flight was cancelled or delayed due to extraordinary circumstances (i.e. they could not have been avoided even if all reasonable measures had been taken), Rec. 14 and Article 5 Sec. 3 of the Regulation. In recent ECJ case law (e.g. Sturgeon) it has been confirmed that these extraordinary circumstances should be understood narrowly and that the burden of proof that such an extraordinary circumstance had happened laid on the airline.This means that, in general, if your flight is cancelled you have a RIGHT to claim compensation. If your airline refuses to pay it - they have to justify that refusal. If their justification is not on the grounds of extraordinary circumstances: it is not valid and you should proceed with your claim.
What happens in practice, however? When a consumer approaches the airlines to claim compensation after his flight was cancelled or delayed he hears that no compensation is due to him since the reason for the given cancellation or delay was 'extraordinary circumstances' (an example may be found in a recent Guardian post). Usually, no explanation follows, which means that consumers may not question the true 'extraordinary' nature of these circumstances. Unfortunately, this tactic works since many consumers take it at a face value that there were indeed extraordinary circumstances and that as the result they had lost their right to claim compensation. Hereby, I would like to make an appeal for more consumers to actually question the airlines. Ask what the nature of the 'extraordinary circumstances' was. Ask for a written statement/explanation/denial of your compensation claim. If it does not sound convincing or if it does not give you any sufficient justification: demand more and demand your compensation. If that does not work - go to court to protect your rights and rights of other consumers!