For a twin engined helicopter, it is determined by how the operator chooses to operate it. Of course there may be some operational regulations requiring operation in one of those classes, depending on the rules applicable in the locale. See for example JAR-OPS 3. The rules are a bit too complex to briefly explain in a post.
A single engine heli can only operate in PC 3.
In simplistic terms, with PC1, in the event of an engine failure you can maintain safe flight or land on the helipad/ runway. PC 2 is the same except that there is a brief period during takeoff and landing when you may have to make a forced landing off the helipad / runway if an engine fails
PC3 you may have to make a forced landing any time an engine fails.
Last edited by HeliComparator; 24th November 2012 at 07:50.