EC 261/2004: wear and tear or 'extraordinary circumstances'?

[S2ACR]

A friend of mine is engaged in a dispute with a European airline over whether the delay to her flight from the UK to Spain is the result of mechanical failure caused by normal wear and tear or, as the airline asserts, extraordinary circumstances. The CAA has already come down in my friend's favour, though the matter has dragged on for some months.

The reason provided for the delay was a problem with magnetic chip detectors in one engine. Whilst normally inspected weekly, the detectors were then placed on a daily watch. Problems were subsequently discovered with similar detectors on the same engine's gearbox, resulting in a replacement of the transfer gearbox, thereby grounding the plane.

European rules appear to place what occurred firmly in the category of mechanical wear and tear, though the airline is understandably adamant. Could this matter ever reasonably be attributed to extraordinary circumstances?

UPDATE:

Folks, I'd like to thank you all for replying to my post. Since last week, a settlement has been reached in my friend's case. Apologies for not responding sooner, but this is the first chance I have had to log in since posting.