Such a delay does not, however, entitle passengers to compensation if the air carrier can prove that the long delay was caused by extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken, namely circumstances beyond the actual control of the air carrier.
and
a technical problem in an aircraft which leads to the cancellation or delay of a flight is not covered by the concept of ‘extraordinary circumstances' within the meaning of that provision, unless that problem stems from events which, by their nature or origin, are not inherent in the normal exercise of the activity of the air carrier concerned and are beyond its actual control.
There is no automatic right and if the carrier satisfies those that the circumstances where beyond its control it doesn't need to pay out.
Such a clause is only logical. It should cover genuine breakdowns of aircraft it couldn't have know about (things do break. How often have you turned the lights on your car on and found the bulb has blown - easy to fix on a car not so easy on an airplane!)
Mark