Turbine D & tubby linton,
Back on the question
"Why pressing the pb ON for Probe/Window HEAT, when those heaters are already working in automatic mode?"
At least, I have found this part on the procedure in the FCOM 3.02.34.
Now, I'm sure that pressing the pb ON (in flight) will do something, and that my first explanation (from memory) could also be the right one: super-heating.
