…..at night at high altitude cruise how do you determine level of icing is enough to switch on W Anti Ice and for how long do you keep it on ?
I'll assume 'high altitude' is >FL300. Up there it is highly unlikely to encounter anything other than very small ice crystals; unless you choose to fly in some horrendous CB. Thus ice accumulation on the upper surface would be unlikely. 2ndly; WAI is more a de-icer than anti-icer. Thus you would allow any ice to build up on the leading edge and then crack it off with a short burst of WAI. Leaving the thing pumping hot air along the LED's could encourage flow back and could lead to severe unwanted consequences.