Low percentage/probability. Weather forecasts are probability based so you will get the unmentioned 5% conditions occasionally, e.g. 25kts crosswind instead of 15kts ruining LAHSO. Stronger wind than forecast requiring more time spacing in the sequence (slower down final equals more time between arrivals).
Adverse conditions arrive early and you lose 10 slots - those aircraft have to fit somewhere.
Equipment failure or staffing issues meaning you can't run PRM approaches. For Melbourne when conditions deteriorate we have to include Essendon traffic in the sequence. MEDEVAC traffic.
Events occasionally conspire to ruin the best laid plans is what I'm saying. If pilots are expecting things to be set in concrete or even jelly hours before they arrive every single time they're going to be sorely disappointed. The world doesn't work that way.
I think you'll find "the pilot in command of an aircraft arriving at a destination without sufficient fuel for actual traffic holding will not be accorded a priority approach unless the pilot declares an emergency" has been ever thus.
Last edited by le Pingouin; 27th Aug 2014 at 14:45.
Reason: comment about priority