Originally Posted by
Miniguppy
Why do we have to turn the engine anti ice on during descent even if the SAT is below -40 degrees?
Because it's probably going to warm up as you descend, and unless someone is paying close attention you can get a significant engine ice accumulation before noticing and turning on engine AI.
As noted above, wing AI is basically a deicer, but engine is intended to prevent ice. If there is significant ice accumulation on the inlet prior to AI selection, the resultant ice shed can cause considerable engine damage. In a severe case it can cause fan damage, but even relatively small accumulations can play havoc with the fan duct acoustic panels when the fan slings it outward at high velocity.