At the risk of squandering what little credibility I have by posting on this thread, I need to explain something to some of you....
The fact that someone can make a "Test flight" and return the aircraft in one piece after exposure to some level of volcanic ash proves absolutely nothing. Detailed examination of the hot section of the engine is required, probably right down to the microscopic level and including sectioning of first stage turbine blades and nozzle guide vanes to check the cooling passages for contamination.
I'm not sure what turbine blade life is these days, but it must be of the order of at least 6000 hours plus, and probably well over 10,000 engine hours. If volcanic ash contamination compromises the figures for blade life in the slightest then airlines cannot fly because they are buying themselves a simply massive maintenance cost increase in the future. Furthermore, there would not be enough blade and vane manufacturing capacity available to satisfy demand.
To put it another way, I can take the air filter off my car and still run it up and down the road today and nothing will happen, however I would be wrong to conclude from that experiment that the provision of air filters by the car manufacturer was unnecessary overkill designed merely to boost profits.
To put it another way, If the engines will get their lives shortened by dust contamination, then the aircraft cannot fly, at least not at todays ticket prices anyway.
Having experienced the disruption caused by Fog at Heathrow for a few days years ago, I have a rough idea of the scale of disruption this event is causing. My Son is in Spain at the moment and I have just had to explain to him that there is no way he is getting to England until this clears up, as I imagine that train/bus/ferry systems are overloaded and will remain so. I've also had to explain that if this continues his plans for summer employment in holiday related industry are similarly moot.
..As for those conspiracy theorists and critics of NATS, especially those wondering why there is no "Plan B" to get them to their holiday destinations, I just shake my head in bemusement.
I think that if this continues for another week, even if a few flights are possible, there is going to be a need to ration available seats to essential travel only, and prioritise it to first return stranded people to their homes. I also suspect that some form of Government welfare support is going to be needed for the stranded because I don't think travel insurance covers it (force majeur?) and people must be fast running out of money and credit.
God knows what this will do to the tourism industry around the world, let alone the airlines.