fpc552

I've worked for several air ambulance services, both helicopter and fixed wing. In the long-haul fixed-wing market, almost every service offers national service, since the planes are easily relocated. This market is very competitive because of this, as well as the fact that this sort of service is seldom covered by health insurance, so it is very often cash up front.
If you are choosing the service, the most important thing is to insist upon a CAMTS or EURAMI accredited service. CAMTS publishes a list of all accredited services on their website.
If you think that it is likely that you will need such a service, you should look into obtaining specific insurance that covers long-haul, not medically necessary air medical transport. This is often referred to as "repatriation" and is seldom covered by traditional health insurance. Typically, the travel/adventure insurance carrier will arrange the transport using their network of air ambulance services.
Another option to consider is that some air ambulance services offer a "membership" program where you pay them a small membership fee annually, in exchange for a guarantee of no balance billing when you use their service (they accept whatever your insurance pays as payment in full). These are typically for helicopter (short-haul/regional) services. I don't know if any long-haul services have this available.