Icing Conditions
Shytorque mentioned that "you might encounter an engine icing situation although the airframe remains ice free" ... the following definitions appear in one of my company's limitations documents:
"Rotor and airframe icing conditions are defined as cloud or fog in the temperature range 0ºC to –30ºC when the visibility is less than 1,000 metres" (where –30ºC is the bottom end of the aircraft environmental envelope). However ... "Engine icing conditions are defined as cloud or fog with visibility less than 1,000 metres, or rain, all in the temperature range of +10ºC to –30ºC". (The increased upper limit (+10ºC) recognises the cooling effect of the engine intakes ...)
The same document also includes the following comment: "Due account must be taken of the limit of accuracy of the OAT indication, which is ±1ºC".
CY