Minus 40 AI
All correct about -40 as the lower limit to use it due to very low prob of icing occurring.....
HOWEVER..... Many recomendations now include using EAI when transiting areas of CBs ... Why ... Because it can be -40 one minute and - 35 for example the next with lots of super cool drops ready to,stick to moving bits.
Thus the cocncept of anti ice is to prevent ice build up, so unless you continually watch the SAT/TAT gauge, there is a chance you might encounter ice in these circumstances...
And then there is that other thing ( bulletin?) that applies to some 777s with RR engines if i remember correctly.... about seeing moisture on the windscreen sort of like rain ... That occurs in colder air usually around ITCZ s etc ... It is actually icecrystals melting on the windscreen I think ,...but it should lead you to throw on the anti ice. Think the temps could be below -40 in that case too...
Not a met scientist, but used to fly 146s too and they used to flame out near anything resembling ice so I have a healthy respect for using it earlier rather than later... Now if it was minus 60 I might think ice pretty well zero chance...
MW