Almost every airline with more than ten aircraft actually does have a replacement aircraft sitting around the place. In the case of Arann, they have 14 aircraft so it would be very hard to see the logic in flying all 14 without having one in reserve - all it would take is one minor snag and the whole schedule goes pop. If you look at their first flight each day, they have four starting from Galway, three in Cork, one in Luton, two on the Isle Of Man (one of which is operating for euromanx), and one each in Kerry, Knock and Donegal - that makes 13, so number 14 must be a spare.