Airth is the posh name of the Central Scotland Airport opened in May 1939 and became RAF Grangemouth in September 1939.
It remained open until the mid-1950s when there was murmurings of making it an airport for Central Scotland. Nothing ever came of it and Grangemouth oil refinery was built on the land occupied by the airfield. I can vaguely remember seeing pictures of a KLM aircraft at Grangemouth, in fact here it is:
It would have been the perfect place to build an airport if you resite the oil refinery and power station on the other side of the Forth; it has rail links and motorway links very close by and is about as central as you can get, from a population point of view, for Scotland. It will never happen of course, at least not in my lifetime.
Prestwick
can operate in a supporting role for both GLA and EDI handling freight, MRO and diversion traffic, but I won't be holding my breath to see any significant changes at PIK. There are too many vested interests and PIK is likely to be used a political football. Being totally reliant on Ryanair is a dangerous game to play, as they will make decisions that will be in their own interests, as many marginal airports have found to their terminal cost all around Europe.
If Ryanair get made a better offer by GLA, you can bet they will move up the A77 before you can say Route Development Fund.