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My understanding as a RR 767-300ER driver is that the Eng anti-ice is on at all time where there is visible moisture below +10 OAT, except when climbing and cruising when SAT <-40. This heats the cowl intake, turns on the ignitors and prevents the ice from building up, then breaking off to disapear down the intake removing fan blades. Run back not really an issue here.
For wings, it was described to me that it should be operated a bit like pneumatic anti-ice boots. When ice is detected (using the high tech wiper arm) switch on the anti-ice for 1 minute. Then off for 1 minute, repeat until you leave icing condition or at 500' on approach 'cos you need the power for the go-around. Because it is only the leading edge (and then only outboard of the engines), run back is an issue. Operating it using cycles is supposed to dislodge the ice build up without run back occuring.
We do not have the "auto" function on our models, so not sure wether the auto function uses continuous heat or cycle method.
apologies if this is teaching you to suck eggs....hope it helps.
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