Originally Posted by
lomapaseo
Again the original OP never defined the operating conditions regarding flight e.g. are we ramming air (in-flight) or sucking air (on taxi out or in.) and where is the ice assumed to be forming (on the inlet surfaces or on the engine surfaces). There are different levels of protection involved.
Exactly - the risk of inlet icing is dependent on not only the air temperature, but on the airspeed and the engine power setting. At low speed, the air going in the inlet speeds up and cools, while at higher airspeeds the air slows down and warms passing through the inlet, with the amount of that heating or cooling related to the engine power setting. +10 deg. C protects for most foreseeable conditions where water/supercooled droplets could cause icing, without requiring the pilots to look up on a complex chart to see if there is a risk of icing.
In flight, (at least on Boeing), selecting engine anti-ice increases the idle speed to insure the bleed air used for anti-ice is warm enough, with enough pressure to do the anti-ice job.