Access: access is difficult and a helicopter is the most appropriate means of extrication.
Number: the number of patients at the scene exceeds the capacity of road resources.
Time: the patient has a time sensitive condition and a helicopter will result in a clinically significant time saving (see table below) in the patient arriving in hospital.
Skill: the patient requires personnel with specific skills and a helicopter will result in a clinically significant time saving in appropriately skilled personnel reaching the patient.