Answer:
You need some experience working as a ground paramedic ( about 7 or 8 years, also air ambulance paramedics tend to work on response cars before they get selected to work on the HEMS unit (Helicopter Emergency Medical Service).
I worked on an Air ambulance for 6 months of last year and should be starting again in the summer. Paramedics and Doctors don't only work on the air ambulance, they take it in turns with other paramedics and when its not their turn to work in the air, they work on the ground in normal ambulances or cars.