[NM]

Technically, a flight should carry enough fuel to get to its planned destination and then still be able to make it to a suitable alternate airport. So even though it often known well before arrival that a diversion is required and a PER-destined flight could divert to Learmonth (if from North) or ADL (if from East), this would not help if PER suddenly became unavailable as the flight was on final approach, such as may be the case if the flight ahead was unable to clear the runway (perhaps due to a wheels-up landing or some other incident).