A technical fault in itself does not exempt airlines from paying out
The EU's top court says passengers must be compensated if an airline cancels a flight for technical reasons, unless "extraordinary" events are to blame.
The European Court of Justice says it is up to the airline to prove that the circumstances are "extraordinary".
Such circumstances would include a defect revealed by the manufacturer or acts of sabotage or terrorism, the court in Luxembourg said.
It was ruling on a claim against Alitalia by an Austrian family.
An engine defect on an Alitalia plane meant that the Austrian couple were unable to take their scheduled flight from Vienna to Brindisi via Rome, and the Italian carrier transferred them to an Austrian Airlines flight instead.
The Alitalia flight was cancelled five minutes before the scheduled departure time, and the couple arrived in Brindisi nearly four hours late. The Alitalia plane's defect had been discovered the day before.
Alitalia refused to pay the plaintiff, Friederike Wallentin-Hermann, compensation of 250 euros (£235; $350) and 10 euros for telephone charges, a court statement said on Monday.
The plaintiff took Alitalia to court in Vienna, and the Austrian Commercial Court asked the European Court to define the concept of "extraordinary circumstances" that could exempt an airline from paying compensation.
The European Court, whose rulings are binding on EU member states, said "technical problems which come to light during maintenance of aircraft or on account of failure to carry out such maintenance do not constitute, in themselves, 'extraordinary circumstances'".
The court statement continued: "It is not ruled out that technical problems are covered by 'exceptional circumstances' to the extent that they stem from events which are not inherent in the normal exercise of the activity of the air carrier concerned and are beyond its actual control."