beaubo:

Although this is undoubtedly the shortest flight in the world (the airports are about 2 miles apart across open sea,less than the length of Heathrow's main runways, and are clearly visible from each other), this is not an international flight, at least not until the Papa Westray Liberation Army gets quite a bit more organised!

I also think that there are very few trips just ticketed between those two points. It is essentially a triangular operation out of Kirkwall that does one island, then the other, then goes straight back home.

The single fare was £18 the last time I looked, which also makes it the most expensive flight in the world on a per-mile basis.