Miles Man,

You need to consult the rules because that's exactly what they are. The airlines are required to adhere to the Contract of Carriage. There is no option, though I'm sure they'd prefer you not know this.

Certainly in the case of mechanical you are well within your rights to demand Rule 240 treatment. Up to and including full refund of that portion of the flight. It's only in the case of Force Majeure (ie: weather) events when things become optional.

Most airlines define non-Force Majeure delays as "schedule irregularities". In these instances, even for the smallest delay you have the right to invoke the Contract of Carriage. Please, Miles Man, do a little research before you post erroneous information.