Previously, airlines routinely refused to pay out for delays caused by a technical fault, claiming they counted as extraordinary events. Yet last year, two landmark Supreme Court rulings declared that carriers should pay out when a delay was caused by a technical fault.
...
This means that you can challenge the technical faults defence on the basis that an airline should have reasonable expectation that things can go wrong on an aircraft and should have contingency plans in place, ie replacement parts or access to them where it operates.

If your airline tries to claim extraordinary circumstances, challenge them to explain exactly what they were and why they could not have been reasonably avoided. The onus is on them to prove this.

The Sturgeon Ruling of 19 November 2009 was a joined case of passengers against airlines Condor and Air France. It was ruled that despite there not being express provision in Regulation (EC) no 261/2004 to compensate passengers for delays, passengers would now be entitled to the compensation as set out in Article 8 for any delay in excess of three hours, providing the air carrier cannot raise a defence of 'extraordinary circumstances'.
...
It was also ruled that under the definition of 'extraordinary circumstances', technical faults within an aircraft should not be included and therefore an air carrier cannot rely on a technical fault within an aircraft as a defence from a valid claim under the Regulation. Various passenger rights groups reported the case and encouraged passengers to bring claims against airlines in the event of a delay of over three hours.

The Sturgeon ruling was reconfirmed in a ruling of the European Court of Justice on 23 October 2012 in Nelson v Deutsche Lufthansa AG and R (TUI Travel, British Airways, easyjet and IATA) v Civil Aviation Authority.