When can the airline refuse on "extraordinary circumstances"?

"Extraordinary circumstances" (Art. 5(3) EU 261/2004) is the one exception airlines may invoke to refuse compensation. In practice it is invoked excessively — hence the ECJ's strict requirements.

Four cumulative requirements (per Wallentin-Hermann C-549/07 + Sturgeon clarification):

1. The event falls outside the normal exercise of air transport activity;
2. The event is beyond the carrier's actual control;
3. The event could not have been avoided by taking all reasonable measures;
4. The delay/cancellation results from the event (causation).

DOES qualify (case law): severe weather making flight impossible, bird strike (Pesková C-315/15), terrorism/bomb threat, third-party strike such as ATC, unauthorised intrusion, sudden passenger medical emergency.

DOES NOT qualify (settled case law): technical faults during normal maintenance (Wallentin-Hermann), staff shortage / pilot illness, in-house strike of carrier's own staff (Krüsemann C-195/17), operational decisions (no reserve aircraft, slot optimisation), normal wear-and-tear damage, foreseen limitations.

Burden of proof: rests on the carrier (Wallentin-Hermann §22). A generic "due to extraordinary circumstances" notice is insufficient — the airline must substantiate in writing what event, when, and which "reasonable measures" were unsuccessfully taken.