Case C-549/07: Judgment of the Court (Fourth Chamber) of 22 December 2008 (reference for a preliminary ruling from the Handelsgericht Wien — Austria) — Friederike Wallentin-Hermann v Alitalia — Linee Aeree Italiane SpA (Carriage by air — Regulation (EC) No 261/2004 — Article 5 — Compensation and assistance to passengers in the event of cancellation of flights — Exemption from the obligation to pay compensation — Cancellation due to extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken)