Some of you are probably aware of this but the standard weather service methodology and terminology used to characterize and classify the icing environment was developed from in-flight icing tests conducted on a Douglas DC4 and DC6 type aircraft. Thus labels such as trace icing, light icing, moderate icing and heavy icing related to the rate of ice accretion on a probe on a DC6, which does little to ascertain or predict the rate of ice accretion on a rotor system.
As an example, light icing is defined as an accumulation of one half inch of ice on a probe after 40 miles of flight. The rate of accretion is sufficient to create a hazard if flight is prolonged in these conditions, but insufficient to require a diversion.
The prior definition may well fit a large aircraft however there is no assurance that the rotating surfaces of a helicopter will accumulate only one half inch of ice over the same 40 miles, indeed there are many more factors in play with ice accretion to a rotor surface as opposed to a single non rotating airfoil that make it a much larger problem in much less time. Further while one half inch of ice on the wing of an airplane may correctly and appropriately be called “light icing” there is every reason to believe from testing that one half inch of ice on the leading edge of most helicopter rotor systems could result in tragic consequences if autorotation became necessary.