Whitebeard
Senior Member.
I know I'm talking about the UK here, but in my area we have two air ambulances covering a large area of south west England, and whilst there is always one on stand by, they are NEVER auto launched, they are called in by the area emergency services controller as they deem fit on the advice of the first respondent on the scene. Factors that determine their use include, the availability of the aircraft, the ease of access for ground vehicles, proximity of safe landing sites to the incident area, condition of casualty and the urgency for their transportation and distance to the nearest suitable medical facility, etc