Originally Posted by jimmyg
I am always amazed with the twits I fly with that think every cloud is icing conditions and ask for engine anti ice
Of course not every single cloud presents icing conditions, but Boeing says (737):
Engine anti-ice must be ON during all flight operations when icing conditions exist or are anticipated, except during climb and cruise when the temperature is below -40°C SAT. Engine anti-ice must be ON before, and during descent in all icing conditions, including temperature below -40° SAT.
When operating in areas of possible icing, activate engine anti-ice before entering icing conditions.
In addition, it gives an important warning, which is why most pilots turn on engine anti-ice in clouds 10°C TAT or below:
WARNING: Do not rely on airframe visual icing cues before activating engine anti-ice. Use the temperature and visible moisture criteria because late activation of engine anti-ice may allow excessive ingestion of ice and result in engine damage or failure.
My guess is that the preson who wrote this knows much more of how ice forms on the engine intake than me, so I'll take Boeing's word and use engine anti-ice as per FCOM/FCTM.